Can we make telescope mirrors from something else for Christ's sake?
Jan R. Sugalski
mention have fatal drawbacks for astronomical mirrors. In most cases
the hardness or thermal expansion of the material is not suitable. Look
up the thermal expansion of aluminum. Furthermore when you buy a
telescope mirror you are not just paying for the glass. There are
other possible glass-like materials which can be used but I guarantee
they will not save you money.
Regards, Jan
On 28 Dec 1995,
............................... wrote:
> Date: 28 DEC 1995 02:16:23 GMT
> From: ............................... <yio...@ix.netcom.com>
>
> I am perplexed as to why the machining companies and telescope
> companies have not started making mirrors from other materials instead
> of Glass. The cad-cam machine shop tollerences are so incredible that I
> Does anyone see my point here ??
> Please help me out maybe I'am missing something.
>
>
>
>
>
Martin Cohen
yio...@ix.netcom.com(............................... ) writes:
>
>I am perplexed as to why the machining companies and telescope
>companies have not started making mirrors from other materials instead
>of Glass. The cad-cam machine shop tollerences are so incredible that
I
>machine and have them machine me a 17" mirror.
>
>These mirrors would make it easier to make Schmidt-Cassegrains by
>making it easier to make a donut mirror.
>
>Am I dreaming here ? Or is this somekind of conspiracy to keep
>telescope prices up.?
>$15000 for a Meade LX comon!!!!!!
>
>I Want a mirror made from Fiberclass.. Or maybe A Stamped one out of
>tin
>Does anyone see my point here ??
>Please help me out maybe I'am missing something.
>this high priced nonesence.
>
>Yiorgos
>
>
>
>
>
Make them out of tele-adics. That way we would get
infinite precision in both directions!
William Sommerwerck
very fine tolerance. And it has to have a low coefficient of expansion
so that its shape won't change (much) with temperature. Metals are not
suitable materials, nor is fiberglass (!).
By the way, the $15,000 price for the 16" LX-200 is a very LOW price
for a 16" SCT.
Bryan Greer
yio...@ix.netcom.com(............................... ) wrote:
>I am perplexed as to why the machining companies and telescope
>companies have not started making mirrors from other materials instead
>of Glass. The cad-cam machine shop tollerences are so incredible that I
>am tempted to bring in an aluminum blank put an equation into their
>machine and have them machine me a 17" mirror.
>
Nice thought (I wondered the same long ago :->), but CAM technology won't
even get you close when it comes to most optical surfaces. Texereau
("How to Make a Telescope") puts it well with his description of the high
surface accuracy required. If a typical amateur mirror (6-8") were
enlarged to a diameter of 1 mile, then deviations could not exceed the
thickness of a playing card! This works out to scant millionths of an
inch accuracy and doesn't even take into consideration other "micro"
effects like surface roughness and ripple.
However, most commercial outfits do use CAM mills and diamond tools to
"rough out" the glass. It can be a time saver, especially for large
and/or deep mirrors.
Also, solid aluminum is not a good mirror material due to its thermal
expansion properties (unstable dimensionally) and it's too soft
(difficult to work the shape without deforming as you do it) though it
has been tried!
Bryan Greer
Alec Muffett
>You see I know what you are talking about. However, what I,d like to
>know is that someone has actually tried different materials, different
>methods such as machined or stamped molded etc. I am willing to bet
>that a precision stamped or machined mirror lets say made of steel
>would hold its shape pretty well.
Check out the book "Unusual Telescopes" - recent publication, can't
remember the author's name off the top of my head, I think the
publisher is Cambridge University Press.
There's an interesting section on basalt/granite mirrors, liquid
mirrors, vacuum-formed mylar mirrors, etc...
(followups trimmed to something sensible)
- alec
--
#!/usr/local/bin/perl -- -pwcracker-in-3-lines-of-perl-plus-disclaimer
$u{$p[1]}=$p[0] while(@p=getpwent);while(<>){chop;foreach $c (keys %u)
{printf "u=%s p=%s\n",$u{$c},$_ if(crypt($_,$c) eq $c);}} # Use: pwc dict ...
# Alec.M...@UK.Sun.COM. Not speaking for my employers. Not my fault.
Alec Muffett
>Check out the book "Unusual Telescopes" - recent publication, can't
>remember the author's name off the top of my head, I think the
>publisher is Cambridge University Press.
Got it - info from books.com:
Unusual Telescopes
Manly, Peter L.
PUBLISHER: Cambridge U Pr
CATEGORY: Science/Mathematics
PUB DATE: 03/95
ISBN: 052148393X
BINDING: Paperback
PRICE U.S.: $19.95
Link URL = http://www.books.com/scripts/view.exe?isbn~052148393X
Doug McDonald
William Sommerwerck <will...@nwlink.com> wrote:
> Any mirror material has to be hard enough to "take a curve" with a
> very fine tolerance. And it has to have a low coefficient of expansion
> so that its shape won't change (much) with temperature. Metals are not
> suitable materials, nor is fiberglass (!).
Metals certainly are suitable materials. They just are hard to polish to
a good finish. Aluminum, beryllium, molybdenum, and copper are often used.
For infrared use, they are not polished but rather just machined with laser
interferometer controlled milling machines or lathes.
The expansion problem is not really a problem in many applications, especially
ones that only use a pinhole of light on-axis. The high thermal conductivity
means that they will hold their shape well while changing temperature (unlike glass),
and designs using them simply have the mechanical mounting made such that
the focus stays right as the temperature changes. That is, the struts that hold
the pinhole atthe focus are made of material with the same expansion coefficient.
Just sit down before asking the price, though, except for flat ones up to three
inches, which are cheap due to big sales.
Doug McDonald
mcdo...@aries.scs.uiuc.edu
Doug McDonald
Bryan Greer <74503...@compuserve.com> wrote:
>
> Nice thought (I wondered the same long ago :->), but CAM technology won't
> even get you close when it comes to most optical surfaces.
Again, that is not true. Interferometer controlled machining can get
accuracy to 1/20 wave of visible light.
What it can't do is get you a surface that has microbumps smaller than the
surface figure. 1/20 wave of microbumps is really hazy looking.
>
> Bryan Greer
Doug McDonald
mcdo...@aries.scs.uiuc.edu
Bradley D. Tice
could be wrong.
In article <4bsuln$5...@ixnews8.ix.netcom.com>,
grey_havner
pool of mercury is spun, precisely and slowly. It forms a perfect parabola.
The only problem is you can only look straight up.
__________________________________
Grey Havner
Ft. Worth, Texas
gr...@fastlane.net
----------------------------------
Greg Granville
>In article <4bu9d8$k...@ns.concourse.com>
>Bryan Greer <74503...@compuserve.com> wrote:
>> Nice thought (I wondered the same long ago :->), but CAM technology won't
>> even get you close when it comes to most optical surfaces.
>Again, that is not true. Interferometer controlled machining can get
>accuracy to 1/20 wave of visible light.
>What it can't do is get you a surface that has microbumps smaller than the
>surface figure. 1/20 wave of microbumps is really hazy looking.
So if it scatters light that badly, it can't be of much value as an optical
surface, can it?
I'm not familiar with "interferometer controlled machining", but I am quite
familiar with conventional CNC machining technology. The positional feedback
used in most system only has an accuracy of a tenth (.0001). Even this is
difficult to achieve in real life due to mechanical backlash consistency,
and spindle growth factors related to temperature changes. Cutter vibration
is always a major limiting factor on the quality of the surface finish also.
I suspect that this special interferometer machining you refer to would be
vasty more expensive that the "Rube Goldberg" techniques that have been used
hundreds of years... And which can easily attain a smooth surface with
accuracies of several millionths of an inch.
---
Greg Granville - Applied Research Lab @ PSU
ga...@psu.edu (work) - gr...@penn.com (home)
http://users.aol.com/greggran15/index.html
Mel Bartels
> very fine tolerance. And it has to have a low coefficient of expansion
> so that its shape won't change (much) with temperature. Metals are not
> suitable materials, nor is fiberglass (!).
Actually, metal mirrors are suitable and in use as we talk. Typical metal
mirror is made from aluminum with several thou layer of electroless nickel
that is polished and figured, then aluminized. The problem is that aluminum
mirrors are not any cheaper than glass mirrors to make.
Low coefficient of expansion only matters when the temp of mirror and
surrounding air don't agree --- scope is useless anyhow until mirror cools
down, then it doesn't matter what the expansion is.
By the way, why all the newsgroup cross postings? I am limiting this
response to sci.astro.amateur and sci.astro...
--
Regards, Mel Bartels programmer/technical analyst
mailto:mbar...@efn.org
homepage http://www.efn.org/~mbartels/
How To Computerize a Dobsonian Telescope:
http://zebu.uoregon.edu/~mbartels/altaz/altaz.html
Dan Prener
> I am perplexed as to why the machining companies and telescope
> companies have not started making mirrors from other materials instead
> of Glass. The cad-cam machine shop tollerences are so incredible that I
> am tempted to bring in an aluminum blank put an equation into their
> machine and have them machine me a 17" mirror.
>
> These mirrors would make it easier to make Schmidt-Cassegrains by
> making it easier to make a donut mirror.
>
> Am I dreaming here ? Or is this somekind of conspiracy to keep
> telescope prices up.?
> $15000 for a Meade LX comon!!!!!!
>
> I Want a mirror made from Fiberclass.. Or maybe A Stamped one out of
> tin
> Does anyone see my point here ??
> Please help me out maybe I'am missing something.
> Anyone on the planet can reply to this. Please do Lets put an end to
> this high priced nonesence.
There was a newspaper article a few weeks ago about telescopes made
by spinning a dish of mercury. Evidently, this allows very large
telescopes, but they must point straight up. Also, I suspect that
the sophisticated suspension, to minimize vibration, keeps this from
being inexpensive.
--
Dan Prener (pre...@watson.ibm.com)
Mike Zorn
were made of metal (speculum metal [I can't remember the
complosition]). They made some pretty big ones: Lord Rosse's [can't
remember the size]. But glass has the delightful property of being
able to be polished to 1/10 or 1/20 of a wavelength of light
(conversion to inches or cm. left as an exercize to the reader) across
its entire surface. And this by an average dedicated workman, in his
own garage.
They may have tried lead - it's so easy to shape.
Glass mirrors, silvered [this is the process of putting a thin film
of aluminum on the glass (they used to use silver - you can still find
the formulas in Ingalls), will reflect way over 90% of the light
falling on them. The best metal mirrors ran about 40-60%. They gave
up there because glass came along.
One thing amateurs don't seem to be able to make easily is the
eyepiece. [I'd be glad to hear from anyone who has, though.] A Really
Good eyepiece can run about $200 or so.
As long as we're talking about metal, don't forget mercury: a
rotating pool assumes a nice paraboloid shape (though the axis has to
be vertical). And when the night's over, you just pour it back in the
bottle.
Mike Zorn oz...@kaiwan.com
http://www.kaiwan.com/~ozma/
Mel Bartels
beam 'polishing and figuring'. This is a very slow and painful job, but can
and does yield very accurate results.
Doug McDonald
gr...@penn.com (Greg Granville) wrote:
> In article <4budhk$r...@vixen.cso.uiuc.edu> mcdo...@aries.scs.uiuc.edu (Doug McDonald) writes:
>
>>In article <4bu9d8$k...@ns.concourse.com>
>>Bryan Greer <74503...@compuserve.com> wrote:
>
>>> Nice thought (I wondered the same long ago :->), but CAM technology won't
>>> even get you close when it comes to most optical surfaces.
>
>>Again, that is not true. Interferometer controlled machining can get
>>accuracy to 1/20 wave of visible light.
>
>>What it can't do is get you a surface that has microbumps smaller than the
>>surface figure. 1/20 wave of microbumps is really hazy looking.
>
> So if it scatters light that badly, it can't be of much value as an optical
> surface, can it?
Sure can, especially in the infrared, where the wavelength gets longer and the
scattering less. It also does not matter much if you are just focusing
one spot to another. You don't lose much light, just contrast.
>
> I'm not familiar with "interferometer controlled machining", but I am quite
> familiar with conventional CNC machining technology. The positional feedback
> used in most system only has an accuracy of a tenth (.0001). Even this is
> difficult to achieve in real life due to mechanical backlash consistency,
> and spindle growth factors related to temperature changes. Cutter vibration
> is always a major limiting factor on the quality of the surface finish also.
>
> I suspect that this special interferometer machining you refer to would be
> vasty more expensive that the "Rube Goldberg" techniques that have been used
> hundreds of years... And which can easily attain a smooth surface with
> accuracies of several millionths of an inch.
\
It is vastly more expensive if you want flats or spheres. It's not more expensive
if you want aspherics. Especially things like torics.
Doug McDonald
mcdo...@aries.scs.uiuc.edu
C. Wayne Parker
>In <4bu87o$q...@texas.nwlink.com> William Sommerwerck
><will...@nwlink.com> writes:
>>
>>Any mirror material has to be hard enough to "take a curve" with a
>>very fine tolerance. And it has to have a low coefficient of expansion
>>so that its shape won't change (much) with temperature. Metals are not
>>suitable materials, nor is fiberglass (!).
>>
>>for a 16" SCT.
>
>
>called a good deal. Hmmmm?
>
>
William Sommerwerck
>> By the way, the $15,000 price for the 16" LX-200 is a very LOW price
>> for a 16" SCT.
> What planet do you come from where a hobby telescope for $15,000 is
> called a good deal. Hmmmm?
It's called "Earth."
The 16" LX-200 is not a Dobsonian. It's a Schmidt-Cassegrain, with an
elaborate drive system. In terms of features and construction, it is
inexpensive. I doubt you can find a comparable 'scope anywhere else
for less than twice the $15,000 price.
What do you mean by a "hobby" telescope? Do you mean a "mass-produced"
product, as opposed to an Astro-Physics or Ceravolo? Is Arthur C.
Clarke to be looked down upon because he owns a Celestron? Or Levy
because he uses a Meade?
Just because most of the people in this group (myself included) can't
afford a 16" LX-200, doesn't mean it isn't a good value. How much did
you (or the other people in this group) pay for your last car? If you
paid more than $9,000 or $10,000, don't complain that the 16" LX-200
costs $15,000.
William Sommerwerck
C. Wayne Parker
C. Wayne Parker
C. Wayne Parker
>To all the News That were the recipient of my question about Telescope
>Mirrors I apologise I will keep this thread going In Sci.Astro and Sci
>Astro.Amateur.
>
>It was an accident. I happened to be reading some Ridiculus crossposted
>Article from ARCHIMEDES.PLUTONIUM about the compatibillity of his IQ to
>that of an ape, and about him crossbreeding with primates, when I
>decided to do a follow up post by just changing the subject to mirrors.
>Not realizing How many groups this crossbreeding post was crossposted
>in I wrote my article and send it.
>
>Sorry
>
>Yiorgos
>
Jonathan Silverlight
metal foam, described in the November 1988 "Astronomy Now". A team
led by Dan Vukobratovich at the University of Arizona made a 30cm
mirror weighing 5 kg, and anticipated a market for thousands of
them.
_ _ _ _ _ MARK W. LUND _ _ _ _ _
> In article <4bsuln$5...@ixnews8.ix.netcom.com>, ............................... (yio...@ix.netcom.com) writes:
>>I am perplexed as to why the machining companies and telescope
>>companies have not started making mirrors from other materials instead
>>of Glass. The cad-cam machine shop tollerences are so incredible that I
>>am tempted to bring in an aluminum blank put an equation into their
>>machine and have them machine me a 17" mirror.
>>
>>These mirrors would make it easier to make Schmidt-Cassegrains by
>>making it easier to make a donut mirror.
>>
The tolerances and finishes for diamond turned metal optics have been
good enough for visual optics for some time now, especially if super-polished
after turning. Last time I had some optics turned I considered
carefully the posibilities of making a telescope this way--particularly
an off-axis one. It was still too expensive, but that was six or eight
years ago. As for donut, remember that the Schmidt corrector is at
the center of curvature of the spherical mirror, which is what makes
a donut the right shape for the corrector. There have been reflective
correctors for exotic applications, but that requires a beam splitter
(throw away 75% of the light) or some horrendous obscuraton. Still
an off-axis all-reflective Schmidt is a possibili!
best regards
mark
--
Mark W. Lund, PhD
Director
MOXTEK, Inc.
Orem UT 84057
801-225-0930
FAX 801-221-1121
lu...@xray.byu.edu
Mel Bartels
> For example, what happened to the plans for making mirrors from
> metal foam, described in the November 1988 "Astronomy Now". A team
> led by Dan Vukobratovich at the University of Arizona made a 30cm
> mirror weighing 5 kg, and anticipated a market for thousands of
> them.
About the same thing that happened to plans of building giant refractors for
pennies by utilizing liquid lenses and building CCD detectors from CMOS...
Gerald L. Hurst
>
>I don't know whats going on but it smells. Jesus, I could take some
>fossilized DINO DOO-DOO and make an optical surface out of it. Almost
>everything can become an otical surface even water and air. But then
>you have people talking about thermal expansion nonsense. Most
>telescopes are useless anyway untill the mirrors reach ambient
>temperatures. I will try my hardest to make an optical metal mirror no
>matter what the cost. Research always cost more anyway.
>
>We'll see. I hope one day every father can afford a telescope where the
>planets come in nice and clear, so the children can see the universe in
>a better light.
>
>Best regards
>
>Yiorgos
>
Yiorgos, I heartily agree with your philosophy. I for one will
be very much surprised if you do not succeed with your project
providing you pursue it diligently. People always know dozens
of reasons why something new is not going to work. An aluminum
mirror sounds like a fine idea.
Jerry
William Sommerwerck
they were ignored. Some *were* great, others were wrong-headed. I think
Yiorgis's suggestion needs some cold water thrown on it.
The bottom line is this -- the optically critical and expensive part
in making any mirror is the final figuring. I do not see where metal
offers any particular advantage over glass. If anything, its relative
deformability and high temperature coefficient seem to be obstacles.
I don't see how metal can be easily machined to the sub-wavelength
tolerances required for a high-quality mirror.
I would also like to bring up a point which does not seem to have been
considered. In a Schmidt-Cassegrain (or Maksutov) telescope, there is
the little problem of the corrector plate. How does one make a *metal*
corrector plate? Transparent aluminum?
[A note for Star Trek fans. In Star Trek IV, Scotty tells the plastics
engineer how to make transparent aluminum. Well, transparent aluminum
ain't aluminum. Metals are metals because they have high concentrations
of free electrons -- which makes them excellent reflectors.]
The whole point of an S-C is that the main mirror is spherical ^, with
the corrector plate fixing the errors. Unfortunately for Yiorgis's
suggestion, spherical mirrors are relatively easy to grind and finish.
The expensive part of an S-C is the corrector plate and its sub-mirror
-- not the main mirror. And you can't make a corrector from metal.
The readers in this group may not be aware of the advances in ion-beam
machining. Kodak has a proprietary system which can figure glass
surfaces to a fraction of the wavelength of light. And since the beam
heats the glass to a fairly high temperature,(besides blowing away the
"excess" material), the final surface is optically "smooth."
Such a system offers the potential of higher-quality optics, possibly
at a lower price. It migh even make schiffspieglers practical, at a
reasonable price. Unfortunately, no telescope maker I'm aware of is
using ion-beam milling.
One other point. I am not a religious person. However, the repeated
use of "For Christ's sake!" is unnecessary and offensive to some.
^ If this is wrong, would someone please correct me? What would be the
point of a compound telescope with a paraboloidal mirror?
R. Edward Nather
>> I will try my hardest to make an optical metal mirror no
>>matter what the cost. Research always cost more anyway.
>>Yiorgos
>>
> An aluminum
>mirror sounds like a fine idea.
>
>Jerry
Aluminum mirrors are such a cool idea they were tried many years ago, by
no less than Harold Johnson, who developed the UBV photometry system still in
widespread use in professional astronomy. Why haven't you heard about them?
Because they didn't work for sour apples, that's why. After being temperature
cycled in observatory domes for a year or two, the metal crept enough so the
optical surface was destroyed. The last of them (there were many) was retired
from service [read: replaced with a decent glass-based mirror] just a few years
ago.
A lot of people lost a lot of money and a lot of time on this idea for cheap
mirrors. "Those who do not read history are condemned to repeat it."
ed
Doug McDonald
And some can learn from it.
I point out once again that "metal mirrors don't work" is a myth. They work fine.
Lots and lots of very high quality, very precise, metal mirrors are used
by the military. I've used metal mirrors, including precise ones. Properly made
ones work just fine. Don't let anybody tell you otherwise. Don't let anybody tell you
"X made a metal mirror, and it failed due to metal creep after Y years" and
think that that means you can't make excellent, long lasting metal mirrors. You can,
IF you spend as much time and money as the military does. Try SORL, 508-256-4511.
It's true that I don't see many people claiming to use aluminum for such mirrors.
Aluminum is common for condensor grade mirrors, however.
Doug McDonald
mcdo...@aries.scs.uiuc.edu
Alain Maury
>What planet do you come from where a hobby telescope for $15,000 is
>called a good deal. Hmmmm?
>
The street price of most commercial items ( including cars, hifi
equipment, cameras, you name it ) is usually ten times higher than
what it costed for them to be manufactured. This is a rule of thumb
of course, but this is the ballpark.
So a 8inch Celestron on a superpolaris mount which you can buy for
$1,000 or so, costed $100 of material to make. It is very likely, eventhough
I don't expect any official confirmations, that a $15,000 telescope
cost only $1,500 for the parts. And I don't think anybody going
to some professional makers ( of optics, of metal castings, of electronics,
of software, etc.. ) will get a 16" SCT as fancy as a LX200 for $1,500 of
parts. Then, if you count the labor price, plus all the other things that
add up to the price from the electricity that powered the machines, the
cost of the machine itself, the benefit for the manufacturer, the dealer's
share, $15,000 is not a good deal, it is a miracle.
OK, an expensive one, but still a miracle :-)
Take also into account that this "top of the line" telescope is very likely not
going to be sold at more than 100 units/year, contrarily to the 8" which is
likely to be one of the best seller on the current market.
It is just a pity that this miracle didn't materialise in my chimney
last monday ( Dec. 25th ) :-(
The price of telescopes is growing up exponentially with diameter, and
I think you can look at several places to get a telescope of such and such
diameter, and they almost all follow the same rule.
If you consider your work to be free, have all the machines needed,
can get your hands on "unused parts" at your work place ( :-) ), of
course you will be able to build yourself a 16" SC telescope for less than
$800 because you are a smart person. Probability laws, as dicted by a
fellow by the name of Murphy tell me that most people embarking in
this type of project spend a lot of time to create something good enough
to be rebuilt after the first tests. Also tell that most people building
telescopes, even for cheap, do not use them on the sky much anyway.
It is important though to find new ways of lowering optics prices.
My point of view is that if some companies are still in the business,
it is because they have long solved the problem of making good enough
optics for a dirt cheap price. You would be surprised at the "real"
price of an 8" commercial optical set.
Just my 2 cents.
Happy new year with lots of inches of aperture, nanometers of precision
and thousands of dollars saved.... :-)
Alain
Mel Bartels
> machining. Kodak has a proprietary system which can figure glass
> surfaces to a fraction of the wavelength of light. And since the beam
> heats the glass to a fairly high temperature,(besides blowing away the
> "excess" material), the final surface is optically "smooth."
>
> Such a system offers the potential of higher-quality optics, possibly
> at a lower price. It migh even make schiffspieglers practical, at a
> reasonable price. Unfortunately, no telescope maker I'm aware of is
> using ion-beam milling.
Yes, we mirror makers in the amateur community are aware of this technology.
But the word on the street is that this is a very expensive and time
consuming method to figure mirrors. Think of the cost of the vacuum chamber,
the ion generator and controller and mirror test apparatus...and the cost of
running the equipment over long periods of time for a single mirror.
I would venture an opinion that, while technologies such as ion beam removal
of glass ought to be looked at, state of the art is technology that UA is
using with computer controlled flex polishers and testers. They are able to
achieve remarkable surface smoothness and figure control at a reasonable cost
and time.
It is remarkable that even in this age of computer controlled equipment, that
one can, with relatively little sophistication and cost, grind an optical
mirror by hand to an excellent figure.
William Sommerwerck
industry, the figure has traditionally been 5x. As integrated circuits
have reduced the need for labor, the ratio has probably come down to
something more like 4x or even 3x. How else do you think a company can
sell a cassette player/FM radio for $25?
I assure you, a $2000 computer-controlled SCT has a *lot* more than
$200 worth of parts in it. (Probably closer to $500 or $600.) A good
chunk of the cost of such a product is the labor required to produce
the mirror and corrector plate. (The metal castings aren't cheap,
either.)
If the Meade 16" LX-200 had only $1500 worth of parts in it, Meade
could easily sell it for $7500.
William Sommerwerck
> I point out once again that "metal mirrors don't work" is a myth.
> Lots and lots of very high quality, very precise, metal mirrors are
> used by the military. I've used metal mirrors, including precise ones.
But what *type* of mirrors are you talking about? There's an *enormous*
difference between mirrors made simply to reflect or collimate light,
and mirrors for astronomical observation -- which must achieve and
maintain a degree of correction hundreds of times greater.
William Sommerwerck
dynamics of manufacturing and selling products.
You are also "guilty" of an attitude common in the United States --
"If I can't afford something, it is either overpriced or not worth
the money." This is just as snobbish as the opposite view: "If it's
expensive, it must be good, and better than anything that costs less."
You ought to look at the prices for camera lenses. Complex zoom
lenses can cost over $1000 -- even after discount. You might also want
to ask Zeiss why the Biogon lens for the G-1 camera costs $3,000.
I am not defending Meade or Celestron. I am simply pointing out that
their products -- in the current market -- really represent value for
the money.
Doug McDonald
William Sommerwerck <will...@nwlink.com> wrote:
I am talking both types. Can't you read? I clearly said "1/20 wave". The metal,
machined mirrors can be good to 1/20 wave 633nm light. And they will keep
this figure. I've already said that. Twice. This is the third time.
I've got metal mirrors, both copper and molybdenum, that are claimed 1/8 wave
surface accuracy at 633 nm (over the inner 85% of the 2 inch aperture). The
diffraction patterns I see after several reflections off these things lead me to
believe that the specs are not lies. The surfaces of these are plenty good
for astro use.
That said, and I'll say it the third time for the understanding-impaired,
there are two kinds of such mirrors. The purely machines ones and the ones that
are machines and then polished. The latter cost more. They are quite good enough
for astronomical use. They are suitable for high power laser use
(i.e. 5000 watts per square cm CW or 10e9 watt per square cm pulsed) which is much
more critical of surface roughness (too rough and it melts.) and makes it
very hard to maintain the figure (they are metal because you can water cool them).
The purely machined ones have the figure, but they scatter too much for
general astronomical use in teh blue or green. They would be OK in the
red or near IR. And they would also be OK in the blue if you just wanted
to look at a star through a pinhole (i.e. a spectrograph). But then, they cost more
than glass except for small (2-3 inch) flat or long radius ones.
Doug McDonald
mcdo...@aries.scs.uiuc.edu
William Sommerwerck
>> But what *type* of mirrors are you talking about? There's an *enormous*
>> difference between mirrors made simply to reflect or collimate light,
>> and mirrors for astronomical observation -- which must achieve and
>> maintain a degree of correction hundreds of times greater.
> Don't you get it? See, even if I can have 50% of the clarity of a
> 16" Meade and 1000x magnification with let's say a 40" metal mirror,
> I'll take a metal mirror any day.
Your ignorance is showing. What do you mean by 50% of the clarity?
What numbers are you talking about -- surface smoothness, wavefront
error?
Mr. Yiorgos, if I'd had your brilliant idea, I would have started by
asking people in the group what they knew about metal mirrors. NOT by
proclaiming that I'd discovered something revolutionary that everyone
else was trying to suppress (either out of stupidity or venality).
In many years of coming up with "brilliant" ideas, I've learned one
basic principle -- DO YOUR HOMEWORK. Find out what is already known
FIRST.
William Sommerwerck
> It isn't *exactly* what the original poster was talking about but there
> are astronomers at the university of british columbia who use a metal
> mirror. I'm a little sketchy on the details but apparently what they
> do is use a continuously rotating pool of mercury as the primary and
> are able to make a quite large primary this way (i.e. no rigid mirror
> is ever generated - the mercury assumes the desired shape during
> observing).
This IS NOT the type of mirror Mr. Yiorgos is talking about. He wants
a stamped-metal (or milled-metal) mirror, which is a completely
different thing.
Jim Van Nuland
->Date: 28 Dec 1995 15:19:29 +0000
[...]
AL->Check out the book "Unusual Telescopes" - recent publication, can't
->remember the author's name off the top of my head, I think the
->publisher is Cambridge University Press.
The book is a few years old. The author is Peter Manly; lives in the
Phoenix, Arizona area. The book is now available in paperback. Peter
took pictures of my own candidate Unusual Telescope (setting circles on a
pipe mount), but it didn't make the book.
___
X SLMR 2.1a X Turn off your lights -- turn on to Astronomy
--
|Fidonet: Jim Van Nuland 1:143/11
|Internet: Jim.Van...@pctie.si-star.com
|
| Standard disclaimer: The views of this user are strictly his own.
David Jacob
slightly different. I tried making a refractor telescope using a hologram
for a lense. I figured it this way: What if you took a hologram of a
lense, and then used the hologram as the lense itself? My physics teacher
at the time said if I took the hologram with the lense in front of a
sheet of white paper, it should work. I was all ready to begin conducting
experiments, but the lousy astronomers were *very* hostile to my idea and
would not let me use their half meter lense. They refused to even let me
touch their $35,000 'baby', even though all I wanted to de was take a
picture of it. I realize now thay really did nt like the idea of using
cheap holograms instead of real glass. It would let too many
'undesirables' into the field.
Anyway, the idea is still valid, and seems better than actually making a
metal lense.
Dave.
P.S. if you use my idea and begin to make millions in excess profit by
not using real lenses, please cut me a check so I may continue to have
more good ideas -- the Shareware concept :)
J. Chapman
com> writes:
:>
:>I've had many "great" ideas in my own time, and was disappointed when
:>they were ignored. Some *were* great, others were wrong-headed. I think
:>Yiorgis's suggestion needs some cold water thrown on it.
It isn't *exactly* what the original poster was talking about but there
are astronomers at the university of british columbia who use a metal
mirror. I'm a little sketchy on the details but apparently what they
do is use a continuously rotating pool of mercury as the primary and
are able to make a quite large primary this way (i.e. no rigid mirror
is ever generated - the mercury assumes the desired shape during
observing).
J. Chapman
Terra Nova Research Ltd.
t...@infomatch.com
Gregory Edwards
Ever buy a bag sand?
They both need to be melted and slowly cooled.
They both need grinding (even with spin casting), figuring and final polish.
They both need a good tube and a better mount.
And as for Pentium computers costing $1500, telescopes will be much
cheaper when 40,000,000+ a year are sold.
I'd also like to point out that the good Pentium computers with much
RAM, disk, tape, 20" monitor, +software cost a little more than $1500.
Also, 50% clarity may work for light buckets, but not at 1000x.
Gregory Edwards
(who has a 16" telescope with a ceramic mirror---much cheaper than Al)
>
>Dont you get it see, Even If I can have fifty percent of the clarity of
>a 16" Meade And 1000x magnification with lets say A 40 inch Metal
>mirror, I'll take a metal mirror anyday.
>
>
>And use the 16" as a spotting scope.
>
> Best Regards
>
>Yiorgos
>
>
Tim Hollebeek
............................... <yio...@ix.netcom.com> wrote:
>In <4c66aa$f...@texas.nwlink.com> William Sommerwerck
><will...@nwlink.com> writes:
>
>>Your ignorance is showing. What do you mean by 50% of the clarity?
>>What numbers are you talking about -- surface smoothness, wavefront
>>error?
>>
>>Mr. Yiorgos, if I'd had your brilliant idea, I would have started by
>>asking people in the group what they knew about metal mirrors. NOT by
>>proclaiming that I'd discovered something revolutionary that everyone
>>else was trying to suppress (either out of stupidity or venality).
>>
>>In many years of coming up with "brilliant" ideas, I've learned one
>>basic principle -- DO YOUR HOMEWORK. Find out what is already known
>>FIRST.
>
>
>You see this is what I want: In a short time I hope to get thousands of
>people with varying experiences, knowledge of materials, knowledge of
>different technologies excited about this.
First you have to show (1) you know what you are talking about, and
(2) what you know is actually worth getting excited about, and (3)
you actually have thought of something noone else has thought of before.
>And I hope that they all
>try to make mirrors from something other than Glass.
People do; mercury mirrors are becoming quite popular lately. The
fact that you think the idea of not using glass is a new idea only
shows how ignorant you are. In fact, polished metal mirrors were used
long before glass ones; people don't use them any more because glass ones
work better ....
>After all that is
>what the INFORMATION SUPERHIGHWAY is all about sharing information,
>accelerating research etc.
The internet was around long before people started babbling about the
information superhighway in all caps.
--
Tim Hollebeek | Everything above is a true statement, for sufficiently
PChem Grad Student | false values of true.
Princeton Univ. | t...@wfn-shop.princeton.edu
-------------------| http://wfn-shop.princeton.edu/~tim
Slc.dennis Bishop
weithed over 300lbs!
Slc.dennis Bishop
NOT a meade, I went with an Orion DSE 12.5 DOB.
Duane Sand
>I tried making a refractor telescope using a hologram
>for a lense. I figured it this way: What if you took a hologram of a
>lense, and then used the hologram as the lense itself? My physics teacher
>at the time said if I took the hologram with the lense in front of a
>sheet of white paper, it should work.
Seems to me that the resulting hologram would do what you wanted, but only
for the wavelengths of light that were coherent laser light when you
made the hologram. For any other colors of incident light, you'd get
blurring.
But there's one application where this may work out nicely: viewing the
Sun's chromosphere at the Hydrogen-alpha wavelength. This could do
several jobs at once: filtering out all but a narrow bandwidth of
desired wavelengths, focusing that desired light, and sending all the
rest elsewhere without becoming hot and causing tube currents. It may
work better in reflection, as a flat virtually-parabolic mirror, than
by transmission, as a virtual lens. If this works at all, it may also
be practical to go with some unobstructed design without becoming more
expensive to reproduce.
Is it possible to get coherent laser light at the Hydrogen-alpha
wavelength? If not, the holographic pattern would have to be computer
generated, rather than made optically.
This reminds me of an article in Scientific American about 3 years ago,
on a holographic-like trick I think was called "binary optics". It's a
way of using silicon-chip-imprinting methods to make holographic
mirrors that do what you want. The word "binary" is odd; it came from
the tiny hills and ridges on the chip surface being shaped into
stairstep patterns rather than smooth slopes. The chip-etching process
only allows a few discrete levels in the design. But if the steps are
sufficiently small compared to the wavelength of light being imaged, it
apparently works nearly as well as a conventional holograph with
continuous gradations between min and max.
Andrew Stephenson
properly machined mirrors, what hope is there for a cheap "active
mirror", possibly using a mess of circuitry, a sheet of pressed
metal foil, integrated-circuit position sensors and printed-coil-
plus-permanent-magnet micro-actuators?
--
Andrew Stephenson (who finds back-of-envelope discussions fun)
Greg Granville
yio...@ix.netcom.com(............................... ) writes:
>Tuesday I will have a 30" piece of Aluminum machined as close to a
>mirror surface as is possible with my money and the technology that is
>available to the shop. Then I will have it polished.
>I will keep you posted
>Best regards
>Yiorgos
You're gonna end up with the world's largest shaving mirror!
Just because you saw a reflection of the moon in your bumper
doesn't mean that you can stamp or machine a quality telescope
mirror from a hunk of aluminum.
Lemme ask you something... What shape are you going to tell your
friendly machinist that you want on this metal disk? A parabola,
right? What is your machinist friend gonna say when you tell him that
the shape you need only deviates from a sphere by 1/50,000 of an
inch, and that you need a surface accuracy of 1/500,000 of an inch?
... He's gonna laugh his *ss off!
Anyway, you say you are gonna have this surface machined and then you're
just gonna polish it. How are you gonna polish it? With a buffing wheel
and some metal polish? I hardly think so. Try this and you'll end up
with the large shaving mirror I was talking about... You might see
larger craters on the moon with it, and you might even get real lucky and
get a fuzzy view of M42. However, I wouldn't exactly call that a
telescope. The surface of this mirror will likely have peaks and valleys
that are hundreds of waves in height... not even close to the 1/4 wave
criterion that all "decent" scopes should meet or exceed.
The surface figure of a mirror is attained during *carefully* controlled
polishing, not the cutting or grinding stages. If you want this metal
mirror to have any optical quality at all, I strongly suggest that you
use a pitch based polishing lap and follow traditional mirror polishing
and figuring techniques. While other approaches may be technically
*possible* - nothing else is economically feasible - at least not yet.
If there was a cheaper/better way - someone would be doing it.
---
Greg Granville - Applied Research Lab @ PSU
ga...@psu.edu (work) - gr...@penn.com (home)
http://users.aol.com/greggran15/
Doug Bock
spend on the machine shop to accomplish this 30" feat? Or are you assuming
everyone has access to one for free?
Doug
Gregory Edwards
............................... <yio...@ix.netcom.com> wrote:
>In <4c7ceo$g...@crl6.crl.com> edw...@crl.com (Gregory Edwards) writes:
>>
>>Ever price Al?
>>Ever buy a bag sand?
>>They both need to be melted and slowly cooled.
>>They both need grinding (even with spin casting), figuring and final
>polish.
>>They both need a good tube and a better mount.
>>
>>And as for Pentium computers costing $1500, telescopes will be much
>>cheaper when 40,000,000+ a year are sold.
>>
>>I'd also like to point out that the good Pentium computers with much
>>RAM, disk, tape, 20" monitor, +software cost a little more than $1500.
>>
>>Also, 50% clarity may work for light buckets, but not at 1000x.
>>
>
>
>
>Are you then trying to tell me that for less than $2000 I CANNOT make a
>30" metal mirror telescope that could show me a quality image?
>
>Just curious
>
>
>Best regards
>Yiorgos
>
I am saying that you can not make a 30" metal telescope any cheaper
than a 30" glass telescope (both delivering high quality images at
some reasonable, say 20x per inch power).
I am also saying that you can not build any sort of decent clock driven
computer controlled mount (or even not computer controlled) with
that $2000 if it is a one of a kind telescope (rather than 100,000s)
even if you only value your time at $5/hour. In fact, unless you have
built a number of telescopes before, I doubt that you can build a 30"
scope mount that is easy and safe to use, fast damping, easy tracking
(even with alt/az rathen than eq), easy to view through (I think you
mentioned an 84" focal length, you will need some fancy corrective
optics or eyepieces to have a sharp, coma-free image at around f2.9).
BTW, is this 30" to be portable or will it have it's own dome? If
portable, how long do you consider acceptable for setup and tear down?
If fixed, does that $2000 cover the cost of a dome?
Look, if you can build this for $2k and it works great, you
then turn around and sell them for $6k, and you will make more $$ than
you could dream of. If you fail, well, you are out $2k and you have
learned something. But if you yell at people who have built telescopes
when you have gotten nothing to work yet (if it is working, please
post your images) you are a fool.
Greg Edwards
(who has built more than one scope...and most of the early ones with many
unusual ideas had many problems)
>
William Sommerwerck
> Given today's cheap computer electronics and the cost of properly
> machined [flat?] mirrors, what hope is there for a cheap "active
> mirror", possibly using a mess of circuitry, a sheet of pressed
> metal foil, integrated-circuit position sensors and printed-coil-
> plus-permanent-magnet micro-actuators?
The same idea has crossed my mind. However, I don't think it's
practical, for a number of reasons that have nothing to do with the
basic validity of the idea.
There are two "modes" an "active" mirror could be used in -- static
and active.
In the static mode, the submirror positions are adjusted to closely
simulate a paraboloidal surface (or whatever surface was wanted). It
seems to me that you could not divide even a 12" surface into enough
small sections to do (for a reasonable price) a better job than
actually grinding a single mirror. (This is speculation. A few
calculations might show what the actual tradeoffs of such a design
are.)
In the active mode, the submirror positions are adjusted to compensate
for atmospheric disturbances. For this to work, the telescope needs a
reference -- an "artificial star." An artificial star is usually
created by beaming a dangerously high-powered laser into the lower
atmosphere. For what the laser costs, you could buy or build a big,
highly corrected telescope.
The basic idea is valid. However, it probably isn't practical for
consumer telescopes.
Wolff Bloss
>
> David Jacob <dja...@toto.csustan.edu> writes:
>
> > I tried making a refractor telescope using a hologram
> >for a lense. I figured it this way: What if you took a hologram of a
> >lense, and then used the hologram as the lense itself?
>
> you just be taking a 3D picture of the lens? How would your hologram
> have the ability to bend light? If you took a hologram of a
> mirror, you would just be taking a 3D picture of the mirror, i.e.
> a picture of reflections on the face of the mirror. Why would you
> expect a hologram of a lens to have the optical properties of the
> lens?
>
> As a matter of fact, what does happen when you take a hologram of a
> transparent object? Does that work?
I have seen a clever hologram of a telescope and when I looked into the
"eyepiece" I could see an image of a woman. I don't know how it was
made. All of the holograms that I have made were of opaque objects.
Wolff
Bill Nelson
: In <4c6jrj$a...@cnn.Princeton.EDU> t...@franck.Princeton.EDU (Tim
: Hollebeek) writes:
: >(2) what you know is actually worth getting excited about, and (3)
: >you actually have thought of something noone else has thought of
: before.
: I think I know what I am talking about. For less than two thousand
: dollars I could have a 30" metal mirror telescope. And that is much
: bigger than anything I could make of Glass.
And, a number of people have pointed out why you do not know what you
are talking about.
It appears that you will have to follow through with the experiment, to
be convinced. So be it. There are many who insist on repeating previous
failures, simply because they refuse to listen to those who have already
made those mistakes.
: >People do; mercury mirrors are becoming quite popular lately. The
: >fact that you think the idea of not using glass is a new idea only
: >shows how ignorant you are. In fact, polished metal mirrors were used
: >long before glass ones; people don't use them any more because glass
: ones work better ...
: Ok Great ! Hey like I said it was a new Idea to me, When I saw the
: Moon On my car bumber thats all. I am excited about it. Havent you ever
: had a great idea?
Sure, I have had many "great" ideas. Fortunately, with a bit of research,
I have found that most of them were actually worthless.
: I dont think anyone talked about metal mirrors on this echo though.
We don't need to - as it would be about as useful as discussing the
construction of a a nuclear power plant in the basement.
: >>After all that is
: >>what the INFORMATION SUPERHIGHWAY is all about sharing information,
: >>accelerating research etc.
: >The internet was around long before people started babbling about the
: >information superhighway in all caps.
: it sounds to me like you are a little bit.... irritated?
No, just that it is a far better use of the internet to discuss practical
matters, and not spend a lot of time on those that have been shown to be
failures.
: Anyway thank you for your support. and wish me luck. I am on my way to
: making a 30" mirror.
Good luck.
Have you ever figured a mirror? Do you know how to do so? Do you realize
how much work it would be - even if you obtained a practical pregenerated
curve?
Bill
Bill Nelson
David Jacob (dja...@toto.csustan.edu) wrote:
: Once, I had the same type of thought. But my tack on the solution was
: slightly different. I tried making a refractor telescope using a hologram
: for a lense. I figured it this way: What if you took a hologram of a
: lense, and then used the hologram as the lense itself? My physics teacher
: at the time said if I took the hologram with the lense in front of a
: sheet of white paper, it should work. I was all ready to begin conducting
Your physics teacher did not know much about holography, or the making of
holograms.
: experiments, but the lousy astronomers were *very* hostile to my idea and
: would not let me use their half meter lense. They refused to even let me
: touch their $35,000 'baby', even though all I wanted to de was take a
: picture of it. I realize now thay really did nt like the idea of using
: cheap holograms instead of real glass. It would let too many
: 'undesirables' into the field.
I think they were quite reasonable in their attitude. Simply taking a
picture of the lens would give you nothing but a picture of the lens.
You would have to dismount the lens, and mount it in a very stable optical
shop. You would then have to set up a very expensive laser to make the
hologram. I doubt if you had either available.
: Anyway, the idea is still valid, and seems better than actually making a
: metal lense.
You could try it with a small lens. Try taking the objective out of one
side of a pair of binoculars, making a hologram, then placing the hologram
where the lens used to be.
I think you will be sadly disappointed with the results. There are a
number of reasons why this will be the case - one of the most obvious
being the resolution of the film.
Bill
Bill Nelson
: It isn't *exactly* what the original poster was talking about but there
: are astronomers at the university of british columbia who use a metal
: mirror. I'm a little sketchy on the details but apparently what they
: do is use a continuously rotating pool of mercury as the primary and
: are able to make a quite large primary this way (i.e. no rigid mirror
: is ever generated - the mercury assumes the desired shape during
: observing).
That is not what is being discussed here, and it has one major problem.
It can only be used to view directly overhead.
Bill
Bill Tschumy
>
> I do not share your beliefs.
> I ask you this, which requires more science,engineering,tooling,and
> precision ? A Pentium PC , a $15000 new car, or a 16" LX 200.
>
> If you say a LX200, you are a either a big fool or just very very
> naive.
> Mass production has given us quality computers for about $1000 dollars
> and very good automobiles for $15000.
>
> The reasons telescope prices are high are several
>
> 1 there is an eleitist mentality among amature astronomers
>
> 2 many of us are not willing to try anything new.(apathy)
>
> 3 I dont think it has occured to enough small companies to start making
> high quality low priced telescopes to compete with the monopolies.
>
> 4 Not enough people have expressed demand because they think a good
> telescope is unnatainable.
5. I think you are forgetting the most important: Economy of Scale. Vastly more computers and
cars are made than amateur telescopes. That large volume is what is pushing prices down.
Prices are high for any niche product where economy of scale doesn't come into play. There is
no conspiracy going on to inflate prices.
>
> Besides go to your local library and see if you find a book that could
> tell you how to make a pentium PC from beach sand, or a car from a
> chunk of Iron.
>
> But telescopes, Everyone can write a book on how to make a telescope.
> So where is the "HIGH TECH"? The drive perhaps? Ok how much is an
> LX-200 without a drive?
>
> Sorry I just do not agree. The status quo must go.
>
> Best regards
> Yiorgos
>
--
Bill Tschumy
Otherwise
bi...@otherwise.com
Bill Tschumy
Hey guys, I think old David here was making a joke -- at least I hope he was. Don't
forget to laugh.
_ _ _ _ _ MARK W. LUND _ _ _ _ _
>>> I will try my hardest to make an optical metal mirror no
>>>matter what the cost. Research always cost more anyway.
>
>>>Yiorgos
>>>
>> An aluminum
>>mirror sounds like a fine idea.
>>
>>Jerry
>
> Aluminum mirrors are such a cool idea they were tried many years ago, by
> no less than Harold Johnson, who developed the UBV photometry system still in
> widespread use in professional astronomy. Why haven't you heard about them?
> Because they didn't work for sour apples, that's why. After being temperature
> cycled in observatory domes for a year or two, the metal crept enough so the
> optical surface was destroyed. The last of them (there were many) was retired
> from service [read: replaced with a decent glass-based mirror] just a few years
> ago.
>
> A lot of people lost a lot of money and a lot of time on this idea for cheap
> mirrors. "Those who do not read history are condemned to repeat it."
>
> ed
I don't know how long ago this was, but the diamond turning people have
solved this problem by carefully annealing the metal before, during,
and after machining. The trick is to eliminate all residual stresses
so the mirror has no reason to deform over time.
best regards
mark
>
>
>
--
Mark W. Lund, PhD
Director
MOXTEK, Inc.
Orem UT 84057
801-225-0930
FAX 801-221-1121
lu...@xray.byu.edu
Mel Bartels
> properly machined mirrors, what hope is there for a cheap "active
> mirror", possibly using a mess of circuitry, a sheet of pressed
> metal foil, integrated-circuit position sensors and printed-coil-
> plus-permanent-magnet micro-actuators?
I've looked into this briefly - the stumbling block that I hit was the $
of the piezo actuators, though a friend told me recently that they were
dropping in price. Right now, you are looking at thou $ just for the
actuators...
--
Regards, Mel Bartels programmer/technical analyst
mailto:mbar...@efn.org
homepage http://www.efn.org/~mbartels/
How To Computerize a Dobsonian Telescope:
http://zebu.uoregon.edu/~mbartels/altaz/altaz.html
Mike Gore
This can be done because the light source can be made of ONE color. The
problem you face with replacing a mirror is that there are many wavelenghts
to worry about. The holograms are created by computer...
Tom Lum
: I think I know what I am talking about. For less than two thousand
: dollars I could have a 30" metal mirror telescope. And that is much
: bigger than anything I could make of Glass.
Whose $200K+ CNC machine are you going to use? Are you going to pay the
real costs for the machine time and the machinist's time? Besides, have
you priced raw aluminum stock lately? And don't forget the very special
annealing the blank will need before final machining and before the nickle
electroless coating and the final machining after coating? Oh, don't
forget the cost of the nickle coating too. And I'm not sure if you're
planning to polish the final figuring into the surface. If you do, how
much is your or the optician's time worth. Finally, have you priced out
the plain aluminizing costs for a 30" (I have, it's more than $2000 I'm
afraid). The nickle coating won't reflective enough by itself to justify
the trouble. Have you done the economics komrad?
BTW, I bought a new 30" Pyrex (tm) slumped glass blank a few years ago for
$1400 (after shipping). It's still probably less than $2000 for the same
piece of glass.
Someone mentioned the spook stuff machining capabilities. It's highly
probable that visual wavelength optics can be machined, but the cost of
those CNC machines (assuming they can even be bought by non-NSA
organizations) is probably beyond even Meade which probably has the
deepest pockets of anyone in the amateur telescope field. Last time I
looked, Meade was in it for a profit or at least breaking even.
: Anyway thank you for your support. and wish me luck. I am on my way to
: making a 30" mirror.
I'm not going to wish you luck as I have a feeling I'm going to pay as a
real taxpayer (ie. I work for a real commercial company which is not a
military contractor (or the leeches around them) or research lab) for
==>YOUR <== personal 30" mirror. I don't wish to sound like some ultra
right wing capitalist nut, but If you're not paying the REAL costs, then
you are really stealing from someone else. There is no free lunch, if you
don't pay, you took some else's lunch from them.
Bill Nelson
: > I tried making a refractor telescope using a hologram
: >for a lense. I figured it this way: What if you took a hologram of a
: >lense, and then used the hologram as the lense itself?
: How could this possibly work? If you took a hologram of a lens, wouldn't
: you just be taking a 3D picture of the lens? How would your hologram
: have the ability to bend light? If you took a hologram of a
: mirror, you would just be taking a 3D picture of the mirror, i.e.
: a picture of reflections on the face of the mirror. Why would you
: expect a hologram of a lens to have the optical properties of the
: lens?
If you just took it of the lens, that would be true. If there was something
behind the lens, which was also "holographed", then the "lens" would work
on that object.
A hologram works by refracting light - just like a lens.
: As a matter of fact, what does happen when you take a hologram of a
: transparent object? Does that work?
Yes and no. See above.
Unfortunately, even if it worked - the hologram would not have anywhere
close to the resolution of a glass or plastic lens. Even 1:1 images tend
to be grainy, due to the grains in the holographic film. If you enlarge
the image, the grain becomes terrible.
Bill
Martin Cohen
Jacob <dja...@toto.csustan.edu> writes:
>
>Once, I had the same type of thought. But my tack on the solution was
hologram
>for a lense. I figured it this way: What if you took a hologram of a
teacher
>at the time said if I took the hologram with the lense in front of a
>sheet of white paper, it should work. I was all ready to begin
conducting
and
>would not let me use their half meter lense. They refused to even let
me
>touch their $35,000 'baby', even though all I wanted to de was take a
>picture of it. I realize now thay really did nt like the idea of using
>cheap holograms instead of real glass. It would let too many
>'undesirables' into the field.
>
>Anyway, the idea is still valid, and seems better than actually making
a
>metal lense.
If a hologram of a lens could be used as a lens,
then a hologram of a car could be driven.
The people who refused to let you try
your nonsensical idea on their valuable equipment
are being quite reasonable, IMO.
I can see why this was posted to alt.sci.physics.plutonium.
Doug McDonald
d.s...@ix.netcom.com (Duane Sand) wrote:
>
> I think there is an unrelated trick using holographic film patterns that
> does have the desired effect: modelling and effecting a lens-like
> behavior where light rays of arbitrary new scenes are bent inward or
> outward by the desired amount, depending on distance from the "optical
> axis". This trick is not being used much today, because it requires use
> of monochromatic light, and maybe because it is grainy.
>
> Today I was puzzled about how a flat hologram could imitate the
> light-retarding effects of lens glass or bowl-shaped mirrors. Real
> lenses and mirrors work by adjusting the travel time (speed X path
> length) of light beams so that they all arrive at the same focus point
> at the same instant. But each spot on the holographic plate can only
> respond instantaneously when it is activated by the incoming light; it
> can't wait awhile before forwarding the revised light, nor can it send
> off the revised light before being activated. The speed and path of the
> light rays is the same as with an ordinary flat sheet of white paper.
It works in monochromatic light because the travel distance from each fringe
on the hologram to the focus is different from the adjacent fringe by exactly
one wavelength of light. Thus you get a slightly modified Airy pattern. And yes,
the arrival times are wrong, so in fact as the light pulse gets short (shorter
than about a nanosecond for a 10 inch lens) the resolution gets worse, as only
smaller areas of the lens reach focus at the same time. I've never seen this
effect with a holographic lens, but I have actually seen it with a grating
spectrograph, where it is very dramatic ... I can change the length of my
laser pulses from 0.3 to 20 trillionths of a seconds, and it's easy to see
the diffraction limited spot size change.
Doug McDonald
mcdo...@aries.scs.uiuc.edu
Mark VandeWettering
>polish it that could be done by another method and it is easier than
>polishing glass.
Please back this statement up. Please. All us idiots who
have been pushing glass for years would like to know why
we have been mislead for so long.
>If a machine can give an accuracy of .0001 then polishing/finishing the
>mirror would be easier than it would be grinding a glass blank. it
>would be unrealistic to expect a mirror finish from a milling machine.
It might surprise you, but it is really trivial to rough grind
a mirror to that accuracy. Note my words _TRIVIAL_. It just
takes time. For amateurs, we equate time == free, and we
grind. We would probably all like to have diamond generators,
but we don't. They aren't justified if you only make a couple of
mirrors a year. It is more economical to pay somebody who has
the equipment $100 to rough out your 20" blank than to try to
reproduce that equipment yourself.
Note that when you are done, you still have to fine grind, polish
and notably FIGURE the mirror. There is absolutely nothing trivial
about figuring a 20" mirror. There is as yet no cost-effective
procedure for manufacturing large precision aspheric optics. NONE.
Work is progressing in this, but when done will likely still require
machinery costing millions of dollars, at least initially. I'll
stick with my grinding stand thank you.
Have you made even one mirror? Somehow, I think not.
>Yiorgos > > > > >
--
Mark T. VandeWettering Telescope Information (and more)
Email: <ma...@pixar.com> http://webspace.com/markv/
<ma...@webspace.com> Clear Skies!
Mark VandeWettering
Mike Zorn <oz...@kaiwan009.kaiwan.com> wrote:
> Interesting thought: why not metal? Early mirrors (1700s & 1800s)
>were made of metal (speculum metal [I can't remember the
>complosition]).
Speculum metal contains 68.2% copper and 31.8% tin, consisting of small
crystals of Cu4Sn. (From Proceedings in Experimental Physics by John
Strong, an excellent book, available from Lindsay Publications). It lists
Lord Ross' famous 60" mirror to have been 70 Cu and 30 Tin. Other specula
alloys have contained lead and arsenic.
They made some pretty big ones: Lord Rosse's [can't
>remember the size]. But glass has the delightful property of being
>able to be polished to 1/10 or 1/20 of a wavelength of light
>(conversion to inches or cm. left as an exercize to the reader) across
>its entire surface. And this by an average dedicated workman, in his
>own garage.
> Glass mirrors, silvered [this is the process of putting a thin film
>of aluminum on the glass (they used to use silver - you can still find
>the formulas in Ingalls), will reflect way over 90% of the light
>falling on them. The best metal mirrors ran about 40-60%. They gave
>up there because glass came along.
> One thing amateurs don't seem to be able to make easily is the
>eyepiece. [I'd be glad to hear from anyone who has, though.] A Really
>Good eyepiece can run about $200 or so.
And would be difficult to reproduce by the amateur. Unfortunatly, simpler
designs leave something to be desired in the way of performance when used
with the fast telescopes of today. You just can't get by with a Ramsden
today, whereas in the f/20 refractors of old, they were entirely adequate.
I have considered making a small lens grinding machine as my first lathe
project.
> As long as we're talking about metal, don't forget mercury: a
>rotating pool assumes a nice paraboloid shape (though the axis has to
>be vertical). And when the night's over, you just pour it back in the
>bottle.
I have a link to some information on liquid mirror telescopes on my
web page (see below for the addresss).
Mark VandeWettering
>This whole thing occured to one evening when I was triyng to find
>Saturn on my small refractor. I thought: gee I wish I had a 20"
>reflector. And then there it was the answer was right there on the back
>of my car. I looked at the bumper of all things, and I could see almost
>every detail of the Moon. The craters were there the dark areas the
>lights areas. It was all there.
>A bumber is not even made to be an optical surface. It is chromed steel
>for Christs' sake. If a stamped, dusty bumber can show that much detail
>of the moon then certainly a mirror could be made of steel say 20,30,40
>inches in diameter, and cromed and we can then even make it ractangular
>or even octagonal anything goes. And as for weight a fraction of the
>weight of a thick glass mirror. I think its time we explode this 500
>year old tradition of glass grinding for something better.
Well, some real suggestions would be useful, instead of just idle speculation.
>Yiorgos
Mark
Mark VandeWettering
............................... <yio...@ix.netcom.com> wrote:
>Yes it does work I finally finished polishing a 99cent stainless steel
>SALAD bowl 9" in diameter. I polished it using jewlery polish left over
>from classes I had taken. It as close to a mirror finish as I get.
>Although I cannot be sure if this thing is paraboloid or spherical when
>I held it to the moon I saw an image of the moon magnified at the focal
>point.It has a flat bottom however.I suggest everyone get polishing and
>we soon will have huge mirrors to look up to the stars with. And get
>this the thing weighs only several ounces. I am extatic. YES, YES, YES
>!!!!!!
Great, you have a shiny salad bowl. Next time, try to build a telescope.
>Yiorgos The Greek
Mark VandeWettering
The advances that make adaptive optics can also be used to make a deformable
corrector that can correct aberrations for large fields (1/2 degree) more than
45 degrees from zenith. Check out
http://www.geop.ubc.ca/~cabanac/lmt.html
for more information.
Mark VandeWettering
............................... <yio...@ix.netcom.com> wrote:
>FOR THE RECORD
>
>Are you then trying to tell me that for less than $2000 I CANNOT make a
>30" metal mirror telescope that could show me a quality image?
>
If no one else will say that, then YES I WILL SAY THAT.
YOU CANNOT DO IT.
YOU CANNOT BUILD A 30" METAL MIRROR TELESCOPE THAT WILL PASS ANY KIND
OF OPTICAL TEST.
Prove me wrong, and report back.
>Best regards
>Yiorgos
Richard A. Schumacher
with a friend, consultant or reference librarian and looking foolish
in front of two or three people, one can make a fool of one's self
in front of tens or thousands of strangers all over the world!
Duane Sand
>>> What if you took a hologram of a lens, and then used
>>> the hologram as the lense itself?
>>
>> If you took a hologram of a lens, wouldn't
>> you just be taking a 3D picture of the lens? How would your
hologram
>> mirror, you would just be taking a 3D picture of the mirror, i.e.
>> a picture of reflections on the face of the mirror. Why would you
>> expect a hologram of a lens to have the optical properties of the
>> lens?
>>
>> transparent object? Does that work?
>
the
>"eyepiece" I could see an image of a woman. I don't know how it was
>made. All of the holograms that I have made were of opaque objects.
>Wolff
The holograms like this that I've seen, featured what looked like an
old-fashioned microscope, and and old-fashioned eyeglass. When you
look at the hologram from a distance, what you see is the microscope
and its surroundings, at normal size scale. When you get closer and
position your head just right, you can peer down the barrel of the
eyepiece of the microscope or eyeglass, and see the magnified image of
whatever the thing was focused on. Maybe some of these holograms were
made by some trickery, like combining two separate exposures of two
different scenes. But I seem to recall that one such hologram featured
a plain magnifying glass, and the inner and outer views matched
perfectly, and behaved just like a real magnifying glass when you look
both thru it and around it.
The main problem of this virtual magnifying glass is that it could only
be used for viewing the stuff it was posed with, not things of your
choosing. The hologram did not capture a model of how the glass
refracts any light passing thru, for all possible object images.
Instead, it captured the complete wavefront info of the posed scene,
which happened to include the bent light rays coming thru the
magnifying lens at that time. (By the way, holograms work via
diffraction and interference effects only, not by refraction as someone
else claimed.)
That's a good trick, but it's of no use for telescopes looking at some
new scene.
I think there is an unrelated trick using holographic film patterns that
does have the desired effect: modelling and effecting a lens-like
behavior where light rays of arbitrary new scenes are bent inward or
outward by the desired amount, depending on distance from the "optical
axis". This trick is not being used much today, because it requires use
of monochromatic light, and maybe because it is grainy.
Today I was puzzled about how a flat hologram could imitate the
light-retarding effects of lens glass or bowl-shaped mirrors. Real
lenses and mirrors work by adjusting the travel time (speed X path
length) of light beams so that they all arrive at the same focus point
at the same instant. But each spot on the holographic plate can only
respond instantaneously when it is activated by the incoming light; it
can't wait awhile before forwarding the revised light, nor can it send
off the revised light before being activated. The speed and path of the
light rays is the same as with an ordinary flat sheet of white paper.
If lens-like holograms do exist, I suspect they work only for steady
streams of coherent light. Then the hologram doesn't have to mess with
the travel time of individual photons; it merely needs to mess with the
phase of the wave trains. This is again of no use for telescopes, since
starlight is not coherent.
Kurt Hillig
In article <4c3k4g$r...@texas.nwlink.com>, William Sommerwerck <will...@nwlink.com> writes:
|>
|>The whole point of an S-C is that the main mirror is spherical ^, with
|>the corrector plate fixing the errors. Unfortunately for Yiorgis's
|>suggestion, spherical mirrors are relatively easy to grind and finish.
|>The expensive part of an S-C is the corrector plate and its sub-mirror
|>-- not the main mirror. And you can't make a corrector from metal.
|>
|>^ If this is wrong, would someone please correct me? What would be the
|>point of a compound telescope with a paraboloidal mirror?
While a simple paraboloid eliminates spherical abberation - as do the
Schmidt and Maksutov correctors - it still suffers from coma and field
curvature.
Most refractive optics to correct these are mounted after the primary,
rather than in front of it as in the S-C and Mak scopes - e.g. the
commercial coma correctors, the focal reducer/field flatteners for SCT's
etc. But some other designs - e.g. the Wright - use refractive elements
in front of a parabolic primary. There are also Mak-Newtonian hybrids
and such...
--
Dr. Kurt Hillig
Dept. of Chemistry I always tell the phone (313)747-2867
University of Michigan absolute truth X.500 khi...@umich.edu
Ann Arbor, MI 48109-1055 as I see it. khi...@chem.lsa.umich.edu
Computers were invented to help people waste more time faster
Oz
wonder if anyone has a technique for the following small
problem. Apologies if anyone is seriously offended by the
off thread posting.
Due to advanced old age (hits most people after mid forties)
I have to wear glasses. Trouble is that you invariably put
them lenses down and you get little scratches (more a
foggyness) right in front of both pupils, which is tedious.
As a matter of principle, rather than buy a new pair, it
should be possible to polish off the scratches with a
suitable abrasive. Think of the ecological benefits this
would have world-wide.
So: anybody know the most suitable grade and type of
abrasive?
-------------------------------
'Oz "When I knew little, all was certain. The more I learnt,
the less sure I was. Is this the uncertainty principle?"
James E. Klein
>In <4cdibr$r...@topcat.uk.gdscorp.com> ste...@uk.gdscorp.com (Steve Gilham)
>writes:
>>
>>I've been quite surprised while following this thread with the
>>attitude towards metallic mirrors as either "can't be done" or
>>"future/high tech".
>>
>>Check out references in the history of astronomy to "speculum metal" -
>>this alloy was used by Newton for the main mirror of his original
>>reflector telescope.
>How About Herchels 49" in mirror in 1789. Or Rosse reflector 72" in
>1845.
>I am convinced now that I,ve started reading (ever since my vision on
>my car bumper) some more on this that it can be done. Lord Rosse with
>his 72" mirror discovered spiral galaxies. And we all know it's not the
>easiest thing to separate out the arms of Galaxies.
>I think the negativism that follows this thread is simply resistance to
>Change. I think the problem is simply this. No one has seriously tried
>to manufacture metal mirrors since those early years. It's an art thats
>been forgotten. I am trying right now to at least make myself one
>mirror from aluminum(although not the best possible material).
>Best Regards
>Yiorgos
>
The choice of glass, pyrex or other non-metallic materials for
telescope mirrors is partially driven by the lower coefficient of
thermal expansion as compared to most metals.
Find a metal which is cheap, easy to work, has a low thermal expansion
coefficient, can be easily coated and is lighter than the non-metalics
now used and you won't get too much resistance.
Mark VandeWettering
E3 thn akra Aseas <yio...@ix.netcom.com> wrote:
>> Please back this statement up. Please. All us idiots who
>> have been pushing glass for years would like to know why
>> we have been mislead for so long.
>I will give you a good example lately I have been looking at ordinary
>plastics under a 1200x microscope and I am amazed at how clear their
>surfaces are of defects. Things like CD cases, Plastic lenses, Any kind
>of clear plastic. Not knowing anything about molding plastics I would
>imagine that the mold is propably metal, If the metal had any
>imperfections in it should they not be transfered to the Plastic?
If this is a good example, I would hate to see what a bad example is.
A couple of _small nits_. The problem is that it is not sufficient to
remove pitting from a piece of whatever to make it an effective
objective. IT HAS TO HAVE THE PROPER SHAPE TO VERY TIGHT TOLERANCES!
All that glitters isn't gold, and all that is shiny ain't a telescope.
You can even have a telescope mirror that works quite well that has
many very obvious pits and scratches, so pitfree isn't even necessary
(although it is desirable). Try taking your CD case and using it as
an optical flat or window. It won't work. It will never work. You can't
make silk purse out of a sow's ear.
>Plastics that are made at a dime a dozen like flashlight parabolic
>reflectors. They Show an amazing reflectivity even at 600x under a
>microscope. These are the things I am looking at now and I am amazed
>that the plastics companies Have not gotten into making telescope
>mirrors.They make every other thing out of plastic except mirrors.
>I am convinced that there is a quick and easy way to make a large high
>quality mirror very cheap out of different materials.
You are wrong. PERIOD. YOU ARE WRONG. WRONG WRONG WRONG WRONG WRONG!
>I will devote lots of effort to this end.
s/effort/research/ and you cannot help but realize just how absolutely
ludicrous this is.
Judy Cheek
create optical elements which reshape the wave front of light entering the
system.
The elements are created by computer and consist of wavelength specific
diffraction patterns.
Steve Gilham
attitude towards metallic mirrors as either "can't be done" or
"future/high tech".
Check out references in the history of astronomy to "speculum metal" -
this alloy was used by Newton for the main mirror of his original
reflector telescope.
-- st...@windsong.demon.co.uk (home) ---- PGP keys available on keyservers --
Steve Gilham |GDS Ltd.,Wellington Ho. |Lives of great men all remind us
Software Specialist|East Road, Cambridge |We may make our lives sublime
steveg@ |CB1 1BH, UK |And departing, leave behind us
uk.gdscorp.com |Tel:(44)1223-300111x2904|Footprints in the sands of time.
Key fingerprint = 08 8A 67 70 6E 86 09 B4 38 0A BD C4 53 1C 88 99
Rob Robinson
: Due to advanced old age (hits most people after mid forties)
: I have to wear glasses. Trouble is that you invariably put
: them lenses down and you get little scratches (more a
: foggyness) right in front of both pupils, which is tedious.
: As a matter of principle, rather than buy a new pair, it
: should be possible to polish off the scratches with a
: suitable abrasive. Think of the ecological benefits this
: would have world-wide.
: So: anybody know the most suitable grade and type of
: abrasive?
there is none. I have already been down this road before. In fact I have
friend that owns an optical shop, and he claims that once the
polycarbonate lenses become scratched there is no way to polish the
scratches out.
Rob Robinson
Bonner Springs KS, USA
http://www.sky.net/~robinson
Stephen Waelder
First of all, you do not take a hologram of a lens and use the hologram in
place
of the lens. What you need to create is a hologram which will reshape the wave
front as a lens would. This is refered to as diffractive optics. The best way
to do this is with a computer generated hologram.
Another problem that you will run into is that these optical elements are
wavelength specific. Since the optical element consists of a diffraction
pattern, the effects are the same but the extent of the wavefront modification
will vary with wavelength. If you construct a telescope employing diffractive
optics, you will have to compensate for, or eliminate, unwanted wavelengths.
Andrew Cooke
>Change. I think the problem is simply this. No one has seriously tried
>to manufacture metal mirrors since those early years. It's an art thats
>been forgotten. I am trying right now to at least make myself one
>mirror from aluminum(although not the best possible material).
i went down to london a couple of months ago to pick up a
telescope with a metal mirror - it had a diameter of about a
metre and had been flown under a balloon (infra-red). so they
certainly do exist.
other materials are used when they are more suited to the
engineering problem - making the mirrors for the next generation
of telescopes (the 8 to 10m diameter things like keck, eso's vlt,
gemini, etc) is one of the most expensive and difficult parts
of the whole engineering process. other materials were considered
early in the design - since glass was chosen the engineers
involved must have decided that its advantages (low expansion
coefficient being the most obvious) outweigh any problems.
andrew
--
work phone/fax: 0131 668 8356, office: 0131 668 8357
institute for astronomy, royal observatory, blackford hill, edinburgh
http://www.roe.ac.uk/ajcwww
Byron1646
recall the date but it was in the NY Times Science section (Tuesdays)
sometime over the past 2 months.
Joseph Roland Kraus
> mirror from aluminum(although not the best possible material).
>
> Yiorgos
Aluminum has a much, much higher coefficient of expansion compared to
pyrex or quartz, etc.
With changes in temperature, the metal will deform and ruin the figure
much more quickly.
Joe
Allen Gilchrist
been used before. In fact Herschel's large 40 foot (focal length) scope used
such an objective. They worked, but they were abandoned in favor of glass.
Metal mirrors tarnish or corode or rust, and it was not possible to either
prevent the deterioration or clean it off the mirrors without destroying the
figure of the mirror. Imaging having to refigure a 48" objective every year
or so. You wouldn't get much observing done.
The price is a telescope is dictated just like anything else, supply and demand.
The demand for telescopes just isn't what it is for automobiles or PC's. As an
illustration, consider the prices of computer hard disks. You can get a Gig now
for less than we used to pay for an 80 Meg. Certainly, the technology has
improved, but the vastly increased damand has provided even larger economies of
scale. The technology required to make a fine telescope mirror is neither new
nor complex, but it is still very hard to do because the level of accuracy and
precision requires is so great a 1/4 wave optic must conform to the specified
shape to within about 150 nm, thats 150x10^(-9) meters. Sure the technology to
do this is well known, but not everyone can do it. I know how to hit a major
league curve ball, I've seen it done thousands of times, but that doesn't meen
I could take a bat, go out to the stadium and do it.
Allen Gilchrist
Mark VandeWettering
>1845.
Ummm. Check your calendar. This is 1996. ~150 years separate these
achievements. Overwhelmingly the move has been from using metal to
glass. You know why? Because making large metal mirrors was _more_
difficult than making large glass mirrors. Try reading the treatment
of metal mirrors in Amateur Telescope Making (volume 1 I believe)
by Ingalls.
>I am convinced now that I,ve started reading (ever since my vision on
>my car bumper) some more on this that it can be done. Lord Rosse with
>his 72" mirror discovered spiral galaxies. And we all know it's not the
>easiest thing to separate out the arms of Galaxies.
Bah, I have seen spiral arms through a simple 10" and I imagine that
they can be seen in much smaller scopes. As for your hopes of building
a 72" telescope, let me remind you that Lord Rosse was a _Lord_. He had
beacoup $$$. From the book _Unusual Telscopes_ (books, useful things,
perhaps you have heard of them?)
William Parsons, the Third Earl of Rosse, tried several exotic
techniques in casting large metal disks over 0.5m (19in) accross.
He experimented in the late 1800s with soldering speculum metal
plates onto a solid brass backing fixture then grinding the whole
assembly tto the proper curve. Because of the diffraction problems
associated with the joins in the plates, he dropped the idea and
developed methods for casting large specula up to about 2m (6ft) in
diameter.
It was a research problem back in the late 1800s. Have we solved this problems
today? No, of course not. While material science has advanced, and relatively
homogenous chunks of aluminum can be had cheaply, one still needs to figure
out effective ways to ensure dimensional stability (annealing), figure, polish,
and above all mount your optics. These are not cheap.
I admit to a basic philosophical difference to some people. I would never
seriously consider making a telescope mirror out of some random porthole that
somebody found. The reason is simple: that "cheap" glass requires many more
hours of work to turn into a telescope. While I count my time as nearly free,
beating my head against a wall isn't fun. I like figuring, but I like it best
when I can make relatively predicatable changes toward the final figure.
>
>I think the negativism that follows this thread is simply resistance to
>Change. I think the problem is simply this. No one has seriously tried
>to manufacture metal mirrors since those early years. It's an art thats
>been forgotten. I am trying right now to at least make myself one
>mirror from aluminum(although not the best possible material).
>
>Best Regards
>Yiorgos
>
>
>
Mark VandeWettering
Joseph Roland Kraus <joek...@wam.umd.edu> wrote:
>> been forgotten. I am trying right now to at least make myself one
>> mirror from aluminum(although not the best possible material).
>>
>> Best Regards
>> Yiorgos
>
>pyrex or quartz, etc.
>
>With changes in temperature, the metal will deform and ruin the figure
>much more quickly.
While this view is conventional, there seems to be a lack of agreement.
While the coefficient of expansion is certainly higher, metals conduct heat
relatively quickly, and therefore should come to thermal equilibrium faster
than similar glass mirrors. can somebody point to definitive research about
this?
>Joe
byw...@tiac.net
regularly use to take impressions of mouths. He spritzed it with
mercury and resolved great stuff in deep space. He did not think
this incredible and does not want to go on with it. I, on the
other hand, was amazed.
Richard A. Schumacher
>I am convinced now that I,ve started reading (ever since my vision on
>my car bumper) some more on this that it can be done. Lord Rosse with
>his 72" mirror discovered spiral galaxies. And we all know it's not the
>easiest thing to separate out the arms of Galaxies.
Rosse was a millionaire Lord with nothing but free time and nothing
else to spend his money on. Why do you suppose no one has built a
metal mirror in the 150 years since his 72"? Some conspiracy by the
Glasspushers Union? No. It's a conspiracy of nature. Glass is the
easiest material to figure accurately and precisely, and hand-polishing
is the only cheap technique for figuring it. When you talk about
machining metal to within 100 nanometers across a width of 1 meter
or stamping out optical grade mirrors like hubcaps you're talking
through your hat. You just don't realize it yet :->
Go do a little reading and find out _why_ Lord Rosse's monster was
the last of the metal scopes. When you can demonstrate answers to
those physical objections, then you'll have something. I for one
will not hold my breath waiting.
Phil Fischer
E3 thn akra Aseas <yio...@ix.netcom.com> wrote:
>I will devote lots of effort to this end.
>
Seems hard to believe, since based on the number of postings I've seen from
you, you must spend most of your waking hours posting stuff to sci.astro.
Phil
Richard Klappal
1845.
my car bumper) some more on this that it can be done. Lord Rosse with
his 72" mirror discovered spiral galaxies. And we all know it's not the
easiest thing to separate out the arms of Galaxies. <<<
... except that Rosse needed a 72" mirror because it only reflected
40% of the light striking it, so his 72" mirror gave him the brightness
of a 45" mirror.
Oh, Rosse also had 3 mirrors for the scope-- 2 were in perpetual refinishing
while he used the third, then play musical mirrors-- take out the tarnished
one, put in the finished re-finished one, with one yet in-process.
Foucault improved the effectiveness by 50% when he learned to put silver
on glass. No one has improved on the metal-film on glass concept for
optical wavelength work. -- surface accuracy, polish, substrate stability,
tarnish, $, hazardous materials--something else has always been wrong.
Rich
Richard Klappal
Yea... spin a bowl of mercury at the right speed and it'll assume the right
shape. Breathe the vapors that come off it, and you'll go "madder than a
hatter!" Look it up! 18th century hat makers went mad from heavy metal
poisoning they got from working with mercury.
Plus, if you spill it, the EPA will be all over you.
Plus if you get it on your wedding ring, the ring dissolves and you wife
kills you.
MERCURY IS NOT PLEASANT STUFF.
Mark VandeWettering
>we are able to order 20,30,40 inch epoxy, metal mirrorrs or algae
>mirrors ready to go for $100 or $200.
>
Anything can be made into a shaving mirror. But a shaving mirror cannot
be made into a telescope (except for Kevin Medlocks 18" shaving mirror :).
As for not being able to wait, I am afraid you are going to be waiting a
very long time.
Here is another little factoid about the dynamics of telescope pricing:
Go to the library and find an issue of Sky and Telescope from
about 20 years ago. Look up the price for a simple 6" f/8
Newtonian. Mark down the price and the date. Now, get a recent
issue and do the same thing. Now, figure out what the total
inflation rate between those two dates were. Report back what
you find.
Frankly, I find it amazing that telescopes can be manufactured as cheaply
as they are.
Mark
Bratislav Curcic
|>
|> Most refractive optics to correct these are mounted after the primary,
|> rather than in front of it as in the S-C and Mak scopes - e.g. the
|> commercial coma correctors, the focal reducer/field flatteners for SCT's
|> etc. But some other designs - e.g. the Wright - use refractive elements
|> in front of a parabolic primary.
The Wright does not use parabolic primary. It is exactly OPPOSITE from
a parabola, having a focus of the edge area shorter than mirror's centre
(it's called oblate spheroid, or prolate ellipse, whichever suits you).
I should know, I made one ... :-)
Lawrence D. Lopez
I do not use polycarbonate lenses.
Glass lenses last much longer as
eyeglass lenses.
Phil Ward
>Yea... spin a bowl of mercury at the right speed and it'll assume the right
>shape.
>
>MERCURY IS NOT PLEASANT STUFF.
>
Nonetheless, this is still being done, in Canada, I think. See a recent
(1995) issue of New Scientist. I have also heard that you can place
a vacuum behind a disc of flexible material which comprises one end of
a cylinder, and that this assumes a parabolic shape.
See Ashby "Materials Selection in Mechanical Design" for criteria for
materials to be used in telescope mirrors. According to his work, steel
concrete and Al-alloys are all less subject to distortion than glass, so
ease of achieving a good surface finish must be important.
P Ward (University of Sheffield, UK)
Bill Wyatt
: >>much more quickly.
: >While this view is conventional, there seems to be a lack of agreement.
: >While the coefficient of expansion is certainly higher, metals conduct heat
: >relatively quickly, and therefore should come to thermal equilibrium faster
: >than similar glass mirrors. can somebody point to definitive research about
: >this?
One other point is that it's expensive to get pure enough metal that
the figure won't corrupt due to inhomogeneous expansion. Pure glass is cheaper
than pure metal, and the lower coef. of expansion helps, tooo.
--
Bill Wyatt (wy...@cfa.harvard.edu)
Smithsonian Astrophysical Observatory (Cambridge, MA, USA)
24 White Doves
thn akra Aseas ) wrote:
> In <30EAD7...@tiac.net> byw...@tiac.net writes:
> Well there ya go anything could be made into a mirror I can't wait till
> we are able to order 20,30,40 inch epoxy, metal mirrorrs or algae
> mirrors ready to go for $100 or $200.
>
> And Glass mirrors will go the way of glass pop bottles.
Hey, they will when my chrome-plated, balsa wood mirrors hit the market!
--
[24]
Dr. Donald G. Bruns
ThermoTrex Corporation, are working on just such a mirror for the 6.5
meter MMT near Tucson. We are using a 2 mm thin glass shell for an
adaptive secondary which is 64 cm across. This shell is supported by
about 300 voice coil actuators and referenced by capacitor position
sensors. In addition to correcting the mirror shape due to figuring
errors aor gravity, the actuators will also correct for atmospheric
turbulence distortions. Som eof the problems in building such a mirror
are removing the heat from all of the electronics and the actuators, and
sensing the wavefront fast enough to allow good corrections. We will be
using laser guide stars, as well as state of the art CCDs and computers.
See Steward Observatories home page for more details about the adaptive
optics programs there!
E. J. Moler
at synchrotron radiation facilities for x-ray optics. X-ray quality
mirrors have extremely tight tolerances for finish, slope error, etc.
because of a) the short wavelengths of the x-rays, ~ 1 nm scale, and
b) they are used at grazing incidence.
The primary reason for using metal is that the high heat load requires
good thermal conductivity, and sometimes water cooling, to maintain
the proper shape and figure. Usually a copper alloy is used like
Glidcop. These mirrors are manufactured by Photon Sciences and
Zeiss, among others. I have seen a 1 meter x 0.3 meter
cylindrical mirror that is among the smoothest pieces of metal ever
manufactured.
So, it is possible to make excellent mirrors from metal and it is
done. I have never compared the specifications for telescope optics
to x-ray optics, but I would guess the x-ray optics are more demanding.
You can even buy these mirrors, made to order. All you need is money
that only nobility or governments could afford. I'm talking in the
$10,000-50,000 range. Thats not including the mechanical mount design
to hold a 150 lb piece of metal without letting it bend or sag.... not
trivial.
Incidentally, if the heat load requirement is not so great people
prefer to buy/make glass mirrors. Usually ULE is used.
Eddie Moler
EJM...@LBL.GOV
Rob Robinson
: n article <4c6ho2$s...@cloner3.netcom.com>
: yio...@ix.netcom.com(............................... ) writes:
: >Tuesday I will have a 30" piece of Aluminum machined as close to a
: >mirror surface as is possible with my money and the technology that is
: >available to the shop. Then I will have it polished.
: >I will keep you posted
: >Best regards
: >Yiorgos
Okay, now I have determined what is going on here....
First it was a TWENTY INCH, then I saw it was a TWENTY THREE inch, now I
see it is THIRTY INCH.....
A 30" square piece of aluminum at least 3" thick (in order for it to
structurally hold a flat shape) is about 2700 cu in of aluminum which would
weigh approximately 256 lbs. Calling calling my local steel/aluminum
supplier I got a price of $2.14 per pound or $548 + a $35 shear charge
making the total price of material equal to $583. If I wanted a circular
piece laser cut it was going to add another $300 plus shipping charge
from Chicago, or another $375 to the price (or I could have spent the
time sawing it out myself - remember we are talking THREE INCHES THICK).
So if I do it myself it is $583 for square chunk, and $958 if I have them
laser cut the disk.
Since we do not have a maching center large enough to handle 30" blanks,
I called a shop we farm work out to. They said they could hold a 50
millionths tolerance on the machine. They quoted the job (setup,
programming, and machine time) at $3700. They showed me a piece of
aluminum that had been machined with 50 millionths tolerance, and there
were still fine line tool marks. Their mold department could polish the
blank, to remove the fine tool marks, at an additional $750. We are now
at $5033, if *I* cut the material.
Now there are the hundreds of hours to figure the blank. Your time and
money.
Plating was another story....
Only one place around here could manange a piece that size, and they
wanted $825 to chrome. Thickness of chrome was .005, but they could not
guarantee a uniform thickness anywhere close to .0005. So now you have to
go back a refigure the chrome plating....
To make a story short....
Our club bought a replace 30" mirror for the observatory for $5000 DONE,
ready to go.
The discussion of this topic is stupid......
I have an EXCELLENT 8" f7 Newtonian I paid $900 for. I have observed all
the Messiers and about 300 of the Herschels with it....
In the past 30 years, I have observed 1,000's of objects ranging from
double stars, variables, clusters, galaxies (and yes even the 8" shows
spiral arms), planets, comets and asteroids....
If someone wants to spend their precious time, trying to re-event the
wheel, to prove a point....let them waste their time...
I am going observing.
Twit filter on.....
Tom Picciani
yio...@ix.netcom.com(E3 thn akra Aseas ) writes:
>In <4cbvck$7...@pixar.com> ma...@pixar.com (Mark VandeWettering) writes:
>
>>
>>>polish it that could be done by another method and it is easier than
>>>polishing glass.
>>
>> Please back this statement up. Please. All us idiots who
>> have been pushing glass for years would like to know why
>> we have been mislead for so long.
>
>I will give you a good example lately I have been looking at ordinary
>plastics under a 1200x microscope and I am amazed at how clear their
>surfaces are of defects. Things like CD cases, Plastic lenses, Any kind
>of clear plastic. Not knowing anything about molding plastics I would
>imagine that the mold is propably metal, If the metal had any
>imperfections in it should they not be transfered to the Plastic?
>
>reflectors. They Show an amazing reflectivity even at 600x under a
>microscope. These are the things I am looking at now and I am amazed
>that the plastics companies Have not gotten into making telescope
>mirrors.They make every other thing out of plastic except mirrors.
>I am convinced that there is a quick and easy way to make a large high
>quality mirror very cheap out of different materials.
>
>
>>--
>>Mark T. VandeWettering Telescope Information (and
>more)
>>Email: <ma...@pixar.com> http://webspace.com/markv/
>> <ma...@webspace.com> Clear Skies!
>
have horrible expansion coefficients from what I hear (and that could be wrong)
They are also too soft to polish. Just because the surface appears smooth
does not mean it has no error. And most importantly you are looking at
the project as a mechanical exercise rather than one measured by a much
more stringent wavelength of light measurement. Remember that you have to
be within 5 millionth of an inch in accuracy. You can't measure that in
a mechanical manner. You have to use foucalt testing, or laser testing.
Tom P.