Liquid mirrors for space telescopes?
Robert Clark
Telescopes on the cheap?
[9 April 1999] Telescopes are becoming increasingly large and expensive.
Reducing the cost has therefore become a priority for some astronomers. One
example of this is the suggestion by a group of astronomers from Laval
University in Quebec, Canada, that mirrors with a diameter of 8 metres could be
made of metallic liquids rather than glass. The method works by placing the
liquid mirror on a spinning platter. As the liquid spins, gravitational forces
cause the liquid to deform into the shape of a perfect parabolic mirror
(Astrophysical Journal 516 May 10 1999).
http://physicsweb.org/archive/news/1999/04?show=stands
Ultra-Thin Floating Mirrors
http://xxx.lanl.gov/abs/astro-ph/9901255
The authors describe their research in producing liquid mirrors. They note
that the primary problem is that of the mirror deviating from its parabolic
shape when tilted. If the mirror were used in an orbiting telescope wouldn't
this problem be eliminated?
The parabolic shape the mirror obtains is described as being due to
'gravitational' forces. But centrifugal forces alone would cause the liquid
surface to assume a curved shape even without gravity. Would it also assume a
parabolic shape in this case? Perhaps we could choose the right liquid so that
it would assume this shape.
________________________________
"In science, everything is significant."
-- Bob Clark
________________________________
Paul Lutus
eliminated? >>
Yep, sure would -- along with the parabolic shape. The parabolic shape is
created by a balance between gravitation and centrifugal force. No gravity,
no parabola.
<< But centrifugal forces alone would cause the liquid surface to assume a
curved shape even without gravity. >>
No, it wouldn't. Absent gravity, the liquid metal would quickly run to the
sides of the container and assume the shape of a cylinder (if confined).
<< Perhaps we could choose the right liquid so that it would assume this
shape. >>
Good idea -- how about glass, which is technically a liquid?
--
Paul Lutus
www.arachnoid.com
Robert Clark wrote in message <01be87d0$dc912680$31113018@default>...
<snip>
Gerrit Schügerl
substituting
the gravitatinoal force. I see two possibilities to do that:
1) Applying an aditional centrifugal force: Let the whole satelite rotate around
the x-Axes and the mirror around y.
Paul Lutus schrieb:
ydu...@my-dejanews.com
Gerrit =?iso-8859-1?Q?Sch=FCgerl?= <e922...@fbma.tuwien.ac.at> wrote:
> It might be possible to build a liquid mirror on an orbiting telescope by
> substituting
> the gravitatinoal force. I see two possibilities to do that:
> 1) Applying an aditional centrifugal force: Let the whole satelite rotate
around
> the x-Axes and the mirror around y.
It is not that simple. In such situation you will have serious Coriolis
force which will distord your mirror.
Borra thought about a solar sail to produce the gravitationnal force.
Unfortunatly when you put the number togheter, you find than the smallest
meteorid will produce waves meters high on the surface. Not very good for
the optic. Also the mercury will boil off rapidly in space.
Yvan Dutil
-----------== Posted via Deja News, The Discussion Network ==----------
http://www.dejanews.com/ Search, Read, Discuss, or Start Your Own
Nathan Urban
> It might be possible to build a liquid mirror on an orbiting telescope by
> substituting the gravitatinoal force. I see two possibilities to do that:
> 1) Applying an aditional centrifugal force: Let the whole satelite rotate
> around the x-Axes and the mirror around y.
That's not going to be very useful in a telescope -- it will prevent
the mirror from being pointed in one fixed direction.
What's possibility 2?
Magnus Nyborg
Robert Clark wrote in message <01be87d0$dc912680$31113018@default>...
>
..snip...
>
> The authors describe their research in producing liquid mirrors. They note
>that the primary problem is that of the mirror deviating from its parabolic
>shape when tilted. If the mirror were used in an orbiting telescope
wouldn't
>this problem be eliminated?
> The parabolic shape the mirror obtains is described as being due to
>'gravitational' forces. But centrifugal forces alone would cause the
liquid
>surface to assume a curved shape even without gravity. Would it also assume
a
>parabolic shape in this case? Perhaps we could choose the right liquid so
that
>it would assume this shape.
>
>________________________________
>
> "In science, everything is significant."
> -- Bob Clark
>________________________________
A centrifuge alone would make the mirror spherical, but if combined with
'rotation' I am not sure what it would be. A conic section, maybe, but I
doubt soo... And even more unlikely a parabolic shape.
But consider this, how do you point a telescope that is, and needs to be,
constantly rotating with respect to the observing direction?
Clear Skies,
Magnus
Robert Clark
Paul Lutus <nos...@nosite.com> wrote in article
<wiCR2.1829$L66.1...@news1.giganews.com>...
| << If the mirror were used in an orbiting telescope wouldn't this problem be
| eliminated? >>
|
| Yep, sure would -- along with the parabolic shape. The parabolic shape is
| created by a balance between gravitation and centrifugal force. No gravity,
| no parabola.
|
| << But centrifugal forces alone would cause the liquid surface to assume a
| curved shape even without gravity. >>
|
| No, it wouldn't. Absent gravity, the liquid metal would quickly run to the
| sides of the container and assume the shape of a cylinder (if confined).
|
| << Perhaps we could choose the right liquid so that it would assume this
| shape. >>
|
| Good idea -- how about glass, which is technically a liquid?
| --
|
| Paul Lutus
| www.arachnoid.com
|
| Robert Clark wrote in message <01be87d0$dc912680$31113018@default>...
|
| <snip>
|
|
I don't think it would arbitrarily assume a cylindrical shape on physical
grounds. If the material were of very high viscosity such as glass for example
then it would have to be spun at a very high rate to get it to move at all. And
in the case of mercury which the researchers are using, if spun at a very slow
speed it would change only minimally from its flat shape. So it appears the
degree of curvature would be determined both by the fluid and the rate of spin
even in a non-gravity environment. The question is would this curvature be that
of a parabola.
Given the number of experiments that have been conducted in space testing
space-based manufacturing, this question probably has been investigated before.
Perhaps, in testing crystal formation or drug production in space where a
centrifuge is used.
Charles W. Shults III
>
> Telescopes on the cheap?
> [9 April 1999] Telescopes are becoming increasingly large and expensive.
> Reducing the cost has therefore become a priority for some astronomers. One
> example of this is the suggestion by a group of astronomers from Laval
> University in Quebec, Canada, that mirrors with a diameter of 8 metres could be
> made of metallic liquids rather than glass. The method works by placing the
> liquid mirror on a spinning platter. As the liquid spins, gravitational forces
> cause the liquid to deform into the shape of a perfect parabolic mirror
> (Astrophysical Journal 516 May 10 1999).
> http://physicsweb.org/archive/news/1999/04?show=stands
>
> Ultra-Thin Floating Mirrors
> http://xxx.lanl.gov/abs/astro-ph/9901255
>
> that the primary problem is that of the mirror deviating from its parabolic
> this problem be eliminated?
> The parabolic shape the mirror obtains is described as being due to
> surface to assume a curved shape even without gravity. Would it also assume a
> it would assume this shape.
>
First, as the others have pointed out- no gravity,
no parabolic shape. As I understood it, the parabola
would form only at specific rates of spin. Too fast,
and you get a spherical, to slow, and you get a catenary.
I could be wrong here, but you get the idea.
Next, if you are in low gravity or microgravity, why
not -use-a-solid-mirror-? Gravity will no longer be
a problem, distorting it. I know this isn't the cheap
solution you were lookinh for, but liquid metals for this
sort of thing introduce a -lot- of problems. What sort of
window will you look through to keep the mercury from
boiling away? It could cost as much as polishing a
real glass mirror...
Cheers!
Chip Shults
Paul Lutus
used regularly to shape (1) molten glass mirror blanks that are then allowed
to cool very slowly to retain the parabola permanently, and (2) mercury
mirrors that are used as telescope objectives as they spin. The first
proposal of this idea took place about the turn of the century. The first
example is presently being used to shape the largest current mirrors (8
meter class) for the largest new telescopes. Some finishing work and
polishing is performed after the spinning phase is complete (about a year
for cooling), but the majority of the shaping is conducted by simply
rotating the molten glass.
<< So it appears the degree of curvature would be determined both by the
fluid and the rate of spin even in a non-gravity environment. >>
In an orbital situation with just rotation applied, the mercury would
quickly assume a cylindrical shape. No acceleration, no curvature.
--
Paul Lutus
www.arachnoid.com
Robert Clark wrote in message <01be8817$3e3468c0$37113018@default>...
<snip>
Paul Lutus
substituting the gravitatinoal force. I see two possibilities to do that:
1) Applying an aditional centrifugal force: Let the whole satelite rotate
around
the x-Axes and the mirror around y. >>
This would simply change the axis around which the cylinder of liquid metal
would rotate. Think. Any combination of rotations would integrate (if you
will) into a single axis of rotation, which would produce the original
effect I describe.
The parabola requires, as one of two components, a linear acceleration such
as is provided by gravity.
--
Paul Lutus
www.arachnoid.com
Gerrit Schügerl wrote in message <3716FFAD...@fbma.tuwien.ac.at>...
<snip>
Joe Fischer
: Paul Lutus <nos...@nosite.com> wrote in article
: | Yep, sure would -- along with the parabolic shape. The parabolic shape is
: | created by a balance between gravitation and centrifugal force.
: :
: : No gravity, no parabola.
: | << But centrifugal forces alone would cause the liquid surface to assume
: | curved shape even without gravity. >>
: |
: | No, it wouldn't. Absent gravity, the liquid metal would quickly run to the
: | sides of the container and assume the shape of a cylinder (if confined).
: I don't think it would arbitrarily assume a cylindrical shape on physical
: grounds. If the material were of very high viscosity such as glass for
: example then it would have to be spun at a very high rate to get it to
: move at all.
It wouldn't "arbitrarily" assume a cylindrical shape,
it would assume the shape of the walls perpendicular to
the spin axis.
All large telescope mirror blanks are formed by spinning,
but gravity controls the shape.
Spinning could possibly hold a flexible mirror
straight, but not a liquid one, it would probably
be plastic with a vapor deposited aluminum surface.
Joe Fischer
Robert Clark
Joe Fischer <joe...@iglou.com> wrote in article <37175...@news.iglou.com>...
Huh? I'm asking if it would assume a cylindrical shape (in a cylindrical
container) regardless of the fluid used and the rate of spin. It's difficult to
believe that it would on physical grounds. It may indeed be the case that the
rate of spin for mercury that could be used in a weightless environment is so
low that it wouldn't produce a useful focal length.
But your post raises an interesting possibility. Perhaps the parabolic shape
could be induced by using an appropriately shaped container.
Paul Lutus
container) regardless of the fluid used and the rate of spin. It's difficult
to believe that it would on physical grounds. >>
No, it is easy, and geometrically inescapable.
If you shaped the container in a peculiar way, let's say a torus with the
liquid inside, and rotated the torus with the axis of rotation running
through the center, the liquid would flow so that its inside surface
resembled a cylinder. This is inescapable and rather trivial.
All you need to do is think about this -- use your visualization skills. The
forces on the liquid are uniform along the axis of rotation. The liquid
would therefore do what one would expect a small lake to do if it were
included in the design of a spinning space habitat -- it would assume a
cylindrical segment shape, with a size governed by its enclosure.
--
Paul Lutus
www.arachnoid.com
Robert Clark wrote in message <01be8829$8b28dbe0$37113018@default>...
Paul Lutus
This is not correct. The rate of rotation governs the focal length of the
mirror, but it is most certainly a parabola, not a spherical cross-section.
Think about the balance between centrifugal and gravitational vectors
created by the rotation at various positions along the mirror's radius to
see why.
It is always a parabola. Spin it fast, you get a deep parabola (short focal
length). Spin it slow, you get a long focal length.
<< But consider this, how do you point a telescope that is, and needs to be,
constantly rotating with respect to the observing direction? >>
Earthbound telescopes of this kind have this tiny, little, itty bitty
drawback -- they can only be pointed straight up :) You can image some
things near the zenith by using the periphery of the proper focal point, but
this is a compromise. This is why these devices are more a laboratory
curiosity than general astronomical instruments.
This remark, by the way, also applies to the Arecibo dish in Puerto Rico --
it also is most useful when the target is straight up. But the main Arecibo
dish is spherical, and a very clever plastic "secondary" lens is used to
achieve more of a parabolic overall curvature, and also to make off-axis
observations possible.
--
Paul Lutus
www.arachnoid.com
Magnus Nyborg wrote in message <7f7cvd$jur$1...@vega.lejonet.se>...
<snip>
Robert Clark
| << Huh? I'm asking if it would assume a cylindrical shape (in a cylindrical
| container) regardless of the fluid used and the rate of spin. It's difficult
| to believe that it would on physical grounds. >>
|
|
| If you shaped the container in a peculiar way, let's say a torus with the
| liquid inside, and rotated the torus with the axis of rotation running
| through the center, the liquid would flow so that its inside surface
| resembled a cylinder. This is inescapable and rather trivial.
|
| All you need to do is think about this -- use your visualization skills. The
| forces on the liquid are uniform along the axis of rotation. The liquid
| would therefore do what one would expect a small lake to do if it were
| included in the design of a spinning space habitat -- it would assume a
| cylindrical segment shape, with a size governed by its enclosure.
|
| --
|
| Paul Lutus
| www.arachnoid.com
I presume you are responding to this specific question of my post. Then may I
assume your opinion is that glass would also flow into a cylindrical shape even
if spun say at a rate of 1 revolution per year?
________________________________
Paul Lutus
spin. Too fast, and you get a spherical, to slow, and you get a catenary.
>>
No. It is always a parabola. You may simply change the rate of spin to
acquire different focal lengths, but they are all parabolas.
--
Paul Lutus
www.arachnoid.com
Charles W. Shults III wrote in message <371772...@gdi.net>...
<snip>
Paul Lutus
may I
assume your opinion is that glass would also flow into a cylindrical shape
even
if spun say at a rate of 1 revolution per year? >>
Only if the glass were heated to a molten state. If this were true, and the
experiment were conducted in space (no gravitational field), and enough time
were permitted, then yes.
Glass, although technically a liquid, has a very, very high viscosity at
room temperature. It won't flow over time at room temperature, various urban
myths notwithstanding. You must heat it to a molten state to produce a flow.
--
Paul Lutus
www.arachnoid.com
<snip>
James Hunter Heinlen
dangerous fits of clarity and wrote:
> Yep, sure would -- along with the parabolic shape. The parabolic shape is
> created by a balance between gravitation and centrifugal force. No gravity,
> no parabola.
If you are intent on using a liquid metal (such as mecury) in a zero gee
environment, why don't you use one that is affected by magnetic fields, and use
magnets (or electro-magnets, if you want to fine tune the field, or save on
weight) to replace the gravity force?
--
-------------------------------------------------------------------------------
dracus __ __ ____ ___ ___ ____
dra...@primenet.com /__)/__) / / / / /_ /\ / /_ /
/ / \ / / / / /__ / \/ /___ /
-------------------------------------------------------------------------------
Robert Clark
| Earthbound telescopes of this kind have this tiny, little, itty bitty
| drawback -- they can only be pointed straight up :) You can image some
| things near the zenith by using the periphery of the proper focal point, but
| this is a compromise. This is why these devices are more a laboratory
| curiosity than general astronomical instruments.
|
| This remark, by the way, also applies to the Arecibo dish in Puerto Rico --
| it also is most useful when the target is straight up. But the main Arecibo
| dish is spherical, and a very clever plastic "secondary" lens is used to
| achieve more of a parabolic overall curvature, and also to make off-axis
| observations possible.
|
| --
|
| Paul Lutus
| www.arachnoid.com
|
In the news report and in the paper, the researchers claim an angle of tilt of
10 degrees and expect to be able to reach 20 degrees.
Telescopes on the cheap?
http://physicsweb.org/users/article/news/03/4/8 [requires registration]
Ultra-Thin Floating Mirrors
http://xxx.lanl.gov/abs/astro-ph/9901255
The researchers also have a web page devoted to their work:
Optical Correctors for Liquid Mirror Telescopes:
http://wood.phy.ulaval.ca/lmt/home.html
Paul Lutus
tilt of 10 degrees and expect to be able to reach 20 degrees >>
it's a question of acceptable amounts of distortion. For a small mirror, 20
degrees of tilt may not produce unacceptable results, especially if the
telescope is being used for spectroscopy, where great resolution is not
required. The larger the mirror, and the greater the importance of
resolution, the smaller the acceptable tilt.
--
Paul Lutus
www.arachnoid.com
Robert Clark wrote in message <01be8831$c872e2e0$37113018@default>...
<snip>
Hiram Berry
> << As I understood it, the parabola would form only at specific rates of
> spin. Too fast, and you get a spherical, to slow, and you get a catenary.
> >>
>
> No. It is always a parabola. You may simply change the rate of spin to
> acquire different focal lengths, but they are all parabolas.
To a first order approximation it's always a parabola, anyway. Surface
tension effects on a curved surface will perturb the paraboloid a little bit.
I think your main problem will be mechanically isolating the liquid mirror so
that vibrations and rotation-speed variations don't propagate from the motor
into the liquid. Thermal convection cells in the liquid which could vary the
smoothness of the surface might also be a problem.
I tried letting a pan of polyester resin set up while on a turntable to test
this idea out-- the surface formed is definitely close to parabolic, but isn't
optically smooth (probably due to vibrations from the motor).
-- Hiram Berry
Paul Lutus
that, assuming control over these factors, you get a very good parabola.
Finishing work is all that is required.
--
Paul Lutus
www.arachnoid.com
Hiram Berry wrote in message <371792AB...@burningbridges.com>...
<snip>
Chris Matthaei
>To a first order approximation it's always a parabola, anyway. Surface
>tension effects on a curved surface will perturb the paraboloid a little bit.
>I think your main problem will be mechanically isolating the liquid mirror so
>that vibrations and rotation-speed variations don't propagate from the motor
>into the liquid. Thermal convection cells in the liquid which could vary the
>smoothness of the surface might also be a problem.
>
>I tried letting a pan of polyester resin set up while on a turntable to test
>this idea out-- the surface formed is definitely close to parabolic, but isn't
>optically smooth (probably due to vibrations from the motor).
I'm glad I'm not the only one who tried this! My dad wasn't too happy when
the vapors from the resin fogged the plastic turntable lid.
Chris
Joe Fischer
: Earthbound telescopes of this kind have this tiny, little, itty bitty
: drawback -- they can only be pointed straight up :) You can image some
: things near the zenith by using the periphery of the proper focal point, but
: this is a compromise. This is why these devices are more a laboratory
: curiosity than general astronomical instruments.
Mercury telescope mirrors were used for observing,
but a long time ago.
Joe Fischer
John Rehling
> Glass, although technically a liquid, has a very, very high
> viscosity at room temperature. It won't flow over time at room
> temperature, various urban myths notwithstanding. You must heat it
> to a molten state to produce a flow.
I have seen an exhibit where a horizontal glass rod about an
inch thick supported a heavy metal weight on one end. It was the
better part of a century old and had bent maybe 30 degrees. So you can
get it to do tricks, but not very fast.
-JAR
--
Knowledge is power if you know it about the right person.
-Ethel Watts Mumford
Paul Lutus
other than glass. A great deal has been written on this subject. An example
can be found at http://www.earthsky.com/1997/es971014.html
--
Paul Lutus
www.arachnoid.com
John Rehling wrote in message <7f86f5$fj5$1...@jetsam.uits.indiana.edu>...
Regnirps
>I tried letting a pan of polyester resin set up while on a turntable to test
>this idea out-- the surface formed is definitely close to parabolic, but
>isn't
>optically smooth (probably due to vibrations from the motor).
>
>-- Hiram Berry
The Navy had a spinning mercury mirror for transits at one time. Nearly all the
big mirrors are being made this way now with turntables in ovens that run for
months or a year for a very slow cool down.
I made plaster molds on potter wheels in the '70s and tried some glass in
electric kilns but my faculty advisors thought I was wasting my time. Now a guy
in Arizona (?) has made the idea into a huge mirror fabrication fascility and
laboratory. Never listen to your advisors!
BTW, you can make perfect (nearly) off axis parabolas this way or rings or
segments. The plaster and fibreglass stuff I made was in many different
variations including Fresnel mirrors with the spin rate slightly different for
each ring to get a common focal point/interference filter for sound.
I missed the beginning of this thread, how could this work in a space
telescope?
Charlie SPringer
Paul Lutus
>>
Any chance you might do a little research before posting? This idea is less
than a century old.
Among others, there is such a telescope currently in operation, at Malcolm
Knapp Research Forest, British Columbia. Not the greatest location for
observing, but on the other hand, one must be concerned about mercury vapors
that are more likely at elevated temperatures.
The project's description, and references to other such telescopes, may be
found at http://www.astro.ubc.ca/LMT/lzt.html
--
Paul Lutus
www.arachnoid.com
Joe Fischer wrote in message <37179...@news.iglou.com>...
Bob May
to assume the parabolic shape because the radius of the rotation would never
be big enough. The Paraboloid would be astigmatic in the direction of the
rotation.
Have Fun and Keep Looking UP!
Bob May
Joe Fischer
: Huh? I'm asking if it would assume a cylindrical shape (in a cylindrical
: container) regardless of the fluid used and the rate of spin.
: It's difficult to believe that it would on physical grounds.
That's ok, NASA originally thought they could
stop a satellite spinning by using one man to grab hold
of it, but I spent about $30 explaining why that didn't
work, and why they needed more than one man.
: It may indeed be the case that the
: rate of spin for mercury that could be used in a weightless environment
: is so low that it wouldn't produce a useful focal length.
: But your post raises an interesting possibility. Perhaps the parabolic
: shape could be induced by using an appropriately shaped container.
No, a liquid cannot be used in space at all,
if any published article suggested this, it must have
been April First. (Actually, theoretically, a telescope
could be built this way, but it would need to have two
or more mirrors mounted on the "walls" of a spinning
cylinder, and the mirrors would have to spin on axes
perpendicular to the spin axis of the main cylinder, but,
this would probably be unacceptable complicated).
They have contemplated using large _thin_ blanks
made by spinning, and using computer control to maintain
the correct shape, but the only spinning would be on the
ground to make the grinding work easier (and to reduce
the problem of relieved stresses in the glass which
would cause problems over time.
Actually, spinning any mirror in space would
probably be a bad idea, and spinning a liquid mirror
is totally out of the question.
Joe Fischer
Uncle Al
John Rehling wrote:
>
> Paul writes:
>
> > Glass, although technically a liquid, has a very, very high
> > viscosity at room temperature. It won't flow over time at room
> > temperature, various urban myths notwithstanding. You must heat it
> > to a molten state to produce a flow.
>
> I have seen an exhibit where a horizontal glass rod about an
> inch thick supported a heavy metal weight on one end. It was the
> better part of a century old and had bent maybe 30 degrees. So you can
> get it to do tricks, but not very fast.
It's worse that that. If you took the deformed rod, laid it
horizontally in an annealing furnace, and heated it to *below* its
strain point temp it would straighten over a day or three interval.
Deforming glass at room temp leaves strain and an ionic memory in the
disordered lattice. Net material flow is something entirely
different.
You can see the internal strain between crossed polarizers. It
doesn't dissipate.
--
Uncle Al Schwartz
http://uncleal.within.net/
http://www.mazepath.com/uncleal/
http://www.ultra.net.au/~wisby/uncleal/
http://www.guyy.demon.co.uk/uncleal/
(Toxic URLs! Unsafe for children and most mammals)
"Quis custodiet ipsos custodes?" The Net!
Richard A. Schumacher
>> > Glass, although technically a liquid, has a very, very high
>> > viscosity at room temperature. It won't flow over time at room
>> > temperature, various urban myths notwithstanding. You must heat it
>> > to a molten state to produce a flow.
>>
>> I have seen an exhibit where a horizontal glass rod about an
>> inch thick supported a heavy metal weight on one end. It was the
>> better part of a century old and had bent maybe 30 degrees. So you can
>> get it to do tricks, but not very fast.
How do you know that it had flowed, instead of merely being bent
under the weight? Did you have the opportunity to remove the
weight to see whether the glass rod rebounded? Betcha a buck
that this was a demonstration of glass' flexibility and not
of it's viscosity.
Hiram Berry
BTW, you can make perfect (nearly) off axis parabolas this way or rings or
> segments. The plaster and fibreglass stuff I made was in many different
> variations including Fresnel mirrors with the spin rate slightly different for
> each ring to get a common focal point/interference filter for sound.
Great idea! And I'd think it would work for other waves than sound, too...
> I missed the beginning of this thread, how could this work in a space
> telescope?
It probably can't, at least not in free space; the original poster didn't seem to
quite grasp the physics of why the liquid had a paraboloid surface (the normal g
force being essential), and noone has been able to think of a reasonable
replacement for gravity. But as an inexpensive (in space = low mass) way to erect a
large mirror on an asteroid, the spinning liquid mirror approach should be really
useful. The layer of mercury doesn't need to be very thick.
As to the inability to target anywhere but straight up, why not use a pivoting flat
mirror before the focusing mirror in the light path? I realize there could be
problems with maintaining a planar surface, but active flexure-compensating
transducers ought to be able to take care of this. Just a thought,
-- Hiram Berry
Robert Clark
Hiram Berry <burn...@burningbridges.com> wrote in article
<3717C215...@burningbridges.com>...
| > I missed the beginning of this thread, how could this work in a space
| > telescope?
|
| It probably can't, at least not in free space; the original poster didn't
seem to
| quite grasp the physics of why the liquid had a paraboloid surface (the
normal g
| force being essential), and noone has been able to think of a reasonable
| replacement for gravity. But as an inexpensive (in space = low mass) way to
erect a
| large mirror on an asteroid, the spinning liquid mirror approach should be
really
| useful. The layer of mercury doesn't need to be very thick.
|
| As to the inability to target anywhere but straight up, why not use a
pivoting flat
| mirror before the focusing mirror in the light path? I realize there could be
| problems with maintaining a planar surface, but active flexure-compensating
| transducers ought to be able to take care of this. Just a thought,
|
| -- Hiram Berry
|
Well, I can believe it wouldn't assume a PARABOLIC shape, if this has been
proven mathematically or demonstrated in actual experiments in space in
cylindrical containers. What I was disputing was the assertion that regardless
of the fluid or rate of spin the mirror would deform to a cylindrical shape
about the sides of cylindrical container with a bottom.
So if the surface can assume a curved shape, could we choose the shape of the
container and the fluid so the shape is parabolic?
If this doesn't work the most obvious way to induce false gravity is to have
the scope be continually accelerated. Using rockets for this would work but
would be prohibitively expensive since they would have to be fired continually
during the period the scope is observing. Note that the problem with the grav.
field in regards to deforming the shape of the tilted mirror is that the grav.
field is directed in a direction other than the one the scope is pointing. If
the scope were accelerated however, we could always insure it was in the
direction the scope was pointing.
Another possibility for achieving this acceleration that would require no use
of fuel is by the use of space tethers:
"Space Tethers"
http://www.sciam.com/1999/0299issue/0299beardsleybox3.html
"Tethers in Space"
http://infinity.msfc.nasa.gov/Public/ps01/ps02/space.html
"Tethers Unlimited, Inc."
http://www.tethers.com/
By the way, the authors of the paper do argue that in general it is better
to use small mirror correctors to view stars from the vertical rather than
tilting the entire scope:
"CORRECTORS FOR FIXED TELESCOPES"
http://wood.phy.ulaval.ca/lmt/CorrAct.html
me...@cars3.uchicago.edu
>
> Well, I can believe it wouldn't assume a PARABOLIC shape, if this has been
>proven mathematically or demonstrated in actual experiments in space in
>cylindrical containers. What I was disputing was the assertion that regardless
>of the fluid or rate of spin the mirror would deform to a cylindrical shape
>about the sides of cylindrical container with a bottom.
There is nothing to dispute here. The shape that the fluid will
assume will be such that the normal to the surface at any location is
in the direction of the force acting on the liquid. Draw the
conclusions yourself.
Mati Meron | "When you argue with a fool,
me...@cars.uchicago.edu | chances are he is doing just the same"
Robert Clark
me...@cars3.uchicago.edu wrote in article <FABAy...@midway.uchicago.edu>...
The problem with this argument is that it would work for any material, even
for a solid. So we would conclude that solid steel when spun, regardless of the
spin rate, would deform into a cylindrical shape in a cylindrical container.
The problem is that the complicated viscosity, frictional, intermolecular,
surface tension, and convection forces aren't being considered.
In any case, the problem of inducing an acceleration has been considered in a
paper presented at the recent "Ultra Lightweight Space Optics Challenge
Workshop", http://origins.jpl.nasa.gov/meetings/ulsoc/presentations.html
In the paper "Space Based Liquid Ring Mirror",
http://origins.jpl.nasa.gov/meetings/ulsoc/papers/magnuson.pdf, the authors
suggest using an ion propulsion system which need only provide 10^-3 g's thrust
for the system they envisage.
me...@cars3.uchicago.edu
>
>me...@cars3.uchicago.edu wrote in article <FABAy...@midway.uchicago.edu>...
>| In article <01be8866$b9205c20$25113018@default>, "Robert Clark"
><rcl...@op.net> writes:
>| >
>| > Well, I can believe it wouldn't assume a PARABOLIC shape, if this has been
>| >proven mathematically or demonstrated in actual experiments in space in
>| >cylindrical containers. What I was disputing was the assertion that
>regardless
>| >of the fluid or rate of spin the mirror would deform to a cylindrical shape
>| >about the sides of cylindrical container with a bottom.
>|
>| There is nothing to dispute here. The shape that the fluid will
>| assume will be such that the normal to the surface at any location is
>| in the direction of the force acting on the liquid. Draw the
>| conclusions yourself.
>|
>
>for a solid.
No. It would work for material which cannot sustain internal strains.
Which happens to be the definition of a liquid. You may want to
consult a decent book on mechanics of fluids and deformable bodies.
So we would conclude that solid steel when spun, regardless of the
>spin rate, would deform into a cylindrical shape in a cylindrical container.
>The problem is that the complicated viscosity, frictional, intermolecular,
>surface tension, and convection forces aren't being considered.
They are considered. Lets go in order:
1+2) Viscosity and frictional: All it does is to introduce a time
scale. You didn't think that the liquid obtains the minimum energy
shape in zero time, did you.
3) Intermolecular: If there are sufficient intermolecular forces to
enable the material to sustain stress, then it isn't liquid. If there
aren't tnhey don't matter.
4) Surface tension: Introduces extra energy term, proportional to
the surface area. Of significance only for very small volumes.
5) Convection forces: There ain't no such thing. There is
convection which is a different matter.
Anything else?
Regnirps
> Well, I can believe it wouldn't assume a PARABOLIC shape, if this has
>been
>proven mathematically or demonstrated in actual experiments in space in
>cylindrical containers. What I was disputing was the assertion that regardless
>of the fluid or rate of spin the mirror would deform to a cylindrical shape
>about the sides of cylindrical container with a bottom.
> So if the surface can assume a curved shape, could we choose the shape
>of the
>container and the fluid so the shape is parabolic?
> If this doesn't work the most obvious way to induce false gravity is to
>have
>the scope be continually accelerated. Using rockets for this would work
>but
>would be prohibitively expensive since they would have to be fired continually
>during the period the scope is observing. Note that the problem with the
>grav.
>field in regards to deforming the shape of the tilted mirror is that the
>grav.
>field is directed in a direction other than the one the scope is pointing.
>If
>the scope were accelerated however, we could always insure it was in the
>direction the scope was pointing.
I see now you are posing a far from trivial problem. Take a cylindrical
container (I'm pretty sure as long as there is radila sysmetry, the shape
doesn't matter, it could be a cone for example) closed at one end and nearly
full of fluid. Spin it very slowly. Question: Is there a combination of
materials of container and fluid such that surface tension and adhesion will
force the fluid into an optically useful surface? In other words, it won't be
forced out the end or wet the entire container inside and out, etc, and become
parabolic or spherical or something easily corrected.
How about an incremental approach? If you can describe the shape for the
non-spinning situation, then add just a little spin, etc. Start with some
difference equations and it can be easily simulated.
Its a tough one. The folks with the quick no for an answer should ponder a
little longer.
Charlie Springer
Wolfram Schmied
wrote:
>Among others, there is such a telescope currently in operation, at Malcolm
>Knapp Research Forest, British Columbia.
The Canucks have a "Research Forest"?
Now I've seen everything
Wolfram "Hill Bill" 8-)#
Asimov
Re: Liquid mirrors for space telescopes? (16 Apr 99 14:47:29)
dr> On Fri, 16 Apr 1999 01:13:15 -0700, Paul Lutus (nos...@nosite.com)
dr> suffered dangerous fits of clarity and wrote:
> Yep, sure would -- along with the parabolic shape. The parabolic shape is
> created by a balance between gravitation and centrifugal force. No gravity,
> no parabola.
dr> If you are intent on using a liquid metal (such as mecury) in a zero
dr> gee environment, why don't you use one that is affected by magnetic
dr> fields, and use magnets (or electro-magnets, if you want to fine tune
dr> the field, or save on weight) to replace the gravity force?
Or even a static electric field, which consumes no power, pulling into
shape a silvered electret plastic membrane (mylar?) as the mirror. Vary
the field strength to change focal length. No need to spin it either.
... "Give me a place to stand, and I will move the world." -- Archimedes
--
| Return Address: mike...@juxta.mnet.pubnix.ten
| Standard disclaimer: The views of this user are strictly her/his own.
| From addresses mangled solely to block spamming.
| Apologies to those wishing to respond, correct suffix with .net
| Signature auto-added at gateway.
Gregory L. Hansen
Robert Clark <rcl...@op.net> wrote:
> The authors describe their research in producing liquid mirrors. They note
>that the primary problem is that of the mirror deviating from its parabolic
>shape when tilted. If the mirror were used in an orbiting telescope wouldn't
>this problem be eliminated?
> The parabolic shape the mirror obtains is described as being due to
>'gravitational' forces. But centrifugal forces alone would cause the liquid
>surface to assume a curved shape even without gravity. Would it also assume a
>parabolic shape in this case? Perhaps we could choose the right liquid so that
>it would assume this shape.
A few things come to mind, I'm not sure if any are practical.
Instead of tilting the mirror, how about moving the eyepeice (camera,
whatever)? I'd have to pull out pencil and paper to check, but I think
you still get focused imaged from different directions if you put your
eyepeice in different locations. I think that's how they do it at
Aricebo.
Don't use a lens with a parabolic curve. Maybe a lens can be made that
will correct for the distortion of the mirror if the distorted image is
being projected on a different part of the lens when the mirror tilts. Or
perhaps make no effort at all to correct the image with the lens, but
instead make a lens that produces an image most suited for being corrected
by a computer.
Maybe you could keep the shape in space if you can find a shiny
ferromagnetic fluid and put a magnet on the bottom. It would be hard to
keep the field uniform. But if either of the aforementioned points can be
used, then maybe you don't have to. Make a lens that's specially designed
to focus an image from a mirror that's shaped in part by a dipole-like
field.
--
The above opinions are licensed from Microsoft Corporation.
Gregory L. Hansen
>In article <01be8866$b9205c20$25113018@default>, "Robert Clark"
><rcl...@op.net> writes:
>>
>>proven mathematically or demonstrated in actual experiments in space in
>>cylindrical containers. What I was disputing was the assertion that regardless
>>of the fluid or rate of spin the mirror would deform to a cylindrical shape
>>about the sides of cylindrical container with a bottom.
>
>assume will be such that the normal to the surface at any location is
>in the direction of the force acting on the liquid. Draw the
>conclusions yourself.
Well, maybe not if you use a mucous instead of a liquid...
William Hamblen
wrote:
> Well, I can believe it wouldn't assume a PARABOLIC shape, if this has been
>proven mathematically or demonstrated in actual experiments in space in
>cylindrical containers. What I was disputing was the assertion that regardless
>of the fluid or rate of spin the mirror would deform to a cylindrical shape
>about the sides of cylindrical container with a bottom.
It isn't all that difficult to show in the case of a rotating
container in free fall the surface of the liquid would form a hollow
cylinder with the axis of the cylinder coinciding with the axis of
rotation. It is the same reason the surface of a liquid in a
container resting on the surface of the earth is level regardless of
the shape of the container. Think about it.
William Hamblen
wrote:
> The problem with this argument is that it would work for any material, even
>for a solid. So we would conclude that solid steel when spun, regardless of the
>spin rate, would deform into a cylindrical shape in a cylindrical container.
>The problem is that the complicated viscosity, frictional, intermolecular,
>surface tension, and convection forces aren't being considered.
The problem with this is that while liquids have little resistance to
shear and tension forces, solids do. It's one of the reasons you are
not a layer of goo spreading across the floor.
Mark Hagerman
> Telescopes on the cheap?
> [9 April 1999] Telescopes are becoming increasingly large and expensive.
> Reducing the cost has therefore become a priority for some astronomers. One
> example of this is the suggestion by a group of astronomers from Laval
> University in Quebec, Canada, that mirrors with a diameter of 8 metres could be
> made of metallic liquids rather than glass. The method works by placing the
> liquid mirror on a spinning platter. As the liquid spins, gravitational forces
> cause the liquid to deform into the shape of a perfect parabolic mirror
> (Astrophysical Journal 516 May 10 1999).
> http://physicsweb.org/archive/news/1999/04?show=stands
>
> Ultra-Thin Floating Mirrors
> http://xxx.lanl.gov/abs/astro-ph/9901255
>
> The authors describe their research in producing liquid mirrors. They note
> that the primary problem is that of the mirror deviating from its parabolic
> shape when tilted. If the mirror were used in an orbiting telescope wouldn't
> this problem be eliminated?
> The parabolic shape the mirror obtains is described as being due to
> 'gravitational' forces. But centrifugal forces alone would cause the liquid
> surface to assume a curved shape even without gravity. Would it also assume a
> parabolic shape in this case? Perhaps we could choose the right liquid so that
> it would assume this shape.
>
> ________________________________
>
> "In science, everything is significant."
> -- Bob Clark
> ________________________________
Briefly, the parabolic shape of the surface is a consequence of the constant
gravitational acceleration and the fact that the centripetal acceleration from
rotation is proportional to the radius. No other combination of forces would
induce that shape. The shape of the container has no effect on the shape of the
inner surface.
In zero G, a spinning fluid WILL assume a shape that conforms to its container,
with the inner (unconstrained) surface being a portion of a cylindrical locus. If
the material is too viscous to do this, then you won't get an optically useable
surface at all.
Using orbital tethers to provide pseudo-gravity might work, but remember that the
orbital complex is already rotating synchronously with its orbital period; this
will complicate matters. In any case, you'd want to cast your mirror under
rotation, then freeze it solid, as is done with ground-based mirrors.
Mark Hagerman
Mark Hagerman
> In zero G, a spinning fluid WILL assume a shape that conforms to its container,
> with the inner (unconstrained) surface being a portion of a cylindrical locus. If
> the material is too viscous to do this, then you won't get an optically useable
> surface at all.
One exception: if the rate of rotation is low enough, the surface tension of the
liquid will hold the mass together in a distorted spherical shape (oblate
spheroid?).
Mark
Magnus Nyborg
then read your own answer.
Paul Lutus wrote in message <2nKR2.1407$kG6....@news2.giganews.com>...
><< A centrifuge alone would make the mirror spherical, >>
A centrifuge in space can be made to create a gravitational field.
>
>This is not correct. The rate of rotation governs the focal length of the
>mirror, but it is most certainly a parabola, not a spherical cross-section.
>Think about the balance between centrifugal and gravitational vectors
>created by the rotation at various positions along the mirror's radius to
>see why.
A centrifuge would make the surface spherical, since the sphere is the
'surface', as will gravity make a surface spherical, but with much longer
radius (6400 km). Not flat, spherical, but with so little curvature that it
for all purposes is flat.
>
>It is always a parabola. Spin it fast, you get a deep parabola (short focal
>length). Spin it slow, you get a long focal length.
When rotating, yes, but not when centrifuging. Different axis of rotation.
>
><< But consider this, how do you point a telescope that is, and needs to
be,
>constantly rotating with respect to the observing direction? >>
>
>Earthbound telescopes of this kind have this tiny, little, itty bitty
>drawback -- they can only be pointed straight up :) You can image some
>things near the zenith by using the periphery of the proper focal point,
but
>this is a compromise. This is why these devices are more a laboratory
>curiosity than general astronomical instruments.
But this was supposed to be a space bound telescope. Does this still apply?
...snip...
>
>Paul Lutus
>www.arachnoid.com
>
Clear Skies,
Magnus
Paul Lutus
This is not true. It can be used to create a centrifugal force, something
else entirely.
<< A centrifuge would make the surface spherical, >>
This is not true on Earth or in space. In space, a rotating platform has an
axis of rotation -- the liquid would move away from this axis in a linear
way, and form a cylinder if confined. On Earth, the combination of
gravitation and a centrifugal force creates a parabola.
<< It is always a parabola. Spin it fast, you get a deep parabola (short
focal length). Spin it slow, you get a long focal length.
When rotating, yes, but not when centrifuging. Different axis of rotation.
>>
How do you distinguish rotating from "centrifuging"? They are the same thing
with two different names. In an Earthbound scenario, you have two factors --
a gravitational field and a centrifugal force. This produces a parabola.
<< But this was supposed to be a space bound telescope. Does this still
apply? >>
My point is this cannot be done in space. You need two factors, one linear
acceleration such as gravity, another provided by rotation.
--
Paul Lutus
www.arachnoid.com
Magnus Nyborg wrote in message <7feivk$1hb$1...@vega.lejonet.se>...
<snip>
Robert Clark
>On Sat, 17 Apr 1999 01:07:29 GMT, "Robert Clark" <rcl...@op.net>
>wrote:
>
Recall the meniscus. It's due to surface tension. Therefore, it is also
dependent on the fluid used:
Meniscus - An Example of Surface Tension
http://www.wl.k12.in.us/depts/science/earth_science/frameworks/ch11/meniscus.
html
Surface Tension
http://hyperphysics.phy-astr.gsu.edu/hbase/surten2.html
See also the NASA web pages on the effects of surface tension in space:
Surface Tension-Driven Convection Experiments
http://zeta.lerc.nasa.gov/6712/Hall/ostrach/stdce/science.htm
___________________________________________
"In science, everything is significant."
-- Bob Clark
___________________________________________
Robert Clark
This one exception is big enough to drive a space telescope through:
Surface Tension-Driven Convection Experiments
http://zeta.lerc.nasa.gov/6712/Hall/ostrach/stdce/science.htm
In other words, the properties of the particular fluid are important, as is
the spin rate.
___________________________________________
"In science, everything is significant."
-- Bob Clark
___________________________________________
-----------== Posted via Deja News, The Discussion Network ==----------
http://www.dejanews.com/ Search, Read, Discuss, or Start Your Own
Regnirps
>My point is this cannot be done in space. You need two factors, one linear
>acceleration such as gravity, another provided by rotation.
I think everyone who has rejected this idea is suffering from over
simplification. No one has addressed the mater of the container being closed at
one end, nearly full, and reasonable surface tension. What surface is formed if
this container is slowly spun (without spillage)?
How about rubbery or gooey stuff that isn't really a liquid, but deforms
easily?
Charlie Springer
mem...@yahoo.com
wrote:
><< A centrifuge in space can be made to create a gravitational field. >>
>
>This is not true. It can be used to create a centrifugal force, something
>else entirely.
>
><< A centrifuge would make the surface spherical, >>
>
>This is not true on Earth or in space. In space, a rotating platform has an
>axis of rotation -- the liquid would move away from this axis in a linear
>way, and form a cylinder if confined. On Earth, the combination of
>gravitation and a centrifugal force creates a parabola.
>
><< It is always a parabola. Spin it fast, you get a deep parabola (short
>focal length). Spin it slow, you get a long focal length.
>
>When rotating, yes, but not when centrifuging. Different axis of rotation.
>>>
>
>How do you distinguish rotating from "centrifuging"? They are the same thing
>with two different names. In an Earthbound scenario, you have two factors --
>a gravitational field and a centrifugal force. This produces a parabola.
I think that he is talking about 2 axis rotation. One is the axis
of the 'disk' of the mirror, another intersects the first axis at
right angles and is displaced from face of the disk by some distance.
>
><< But this was supposed to be a space bound telescope. Does this still
>apply? >>
>
>My point is this cannot be done in space. You need two factors, one linear
>acceleration such as gravity, another provided by rotation.
In an earlier part of this thread, I started to wonder about the
possibilities of using surface tension, and/or pressure to de/form
the mirror.
For the pressure idea, assume a closed cylinder where one of the
ends is the mirror and the other is a window. Pressurize the
interior of the cylinder. Light shines into the window, bounces
off the mirror to a prime focus or secondary that can be either
inside the cylinder or outside.
Paul Lutus
of the 'disk' of the mirror, another intersects the first axis at
right angles and is displaced from face of the disk by some distance. >>
This doesn't solve any problems. The two axes of rotation are trivially
additive -- for a normal rigid object, you get a new, single, third axis of
rotation that is the vector sum of the original two. For this not to be
true, the rotated object would have to map into at least two
three-dimensional spaces at once.
<< For the pressure idea, assume a closed cylinder where one of the ends is
the mirror and the other is a window. Pressurize the interior of the
cylinder. >>
Interesting idea, but for a flexible membrane, the curvature would be
spherical. This is not fatal -- a secondary mirror could in principle
correct the curvature, or the front glass, as in the classic Cassegrain
design (which also uses a spherical primary).
--
Paul Lutus
www.arachnoid.com
mem...@yahoo.com wrote in message <371c9f91....@news.onramp.net>...
<snip>
MARK F FOLSOM
>In article <371A0379...@worldnet.att.net>,
> Mark Hagerman <mhag...@worldnet.att.net> wrote:
>> Mark Hagerman wrote:
>>
>> > In zero G, a spinning fluid WILL assume a shape that conforms to its
container,
>> > with the inner (unconstrained) surface being a portion of a cylindrical
locus. If
>> > the material is too viscous to do this, then you won't get an optically
useable
>> > surface at all.
>>
>> One exception: if the rate of rotation is low enough, the surface tension
of the liquid will hold the mass together in a distorted spherical shape
(oblate
>> spheroid?).
>>
>> Mark
>
> This one exception is big enough to drive a space telescope through:
>
Try forming an image with an oblate ellipsoid.
> Surface Tension-Driven Convection Experiments
> http://zeta.lerc.nasa.gov/6712/Hall/ostrach/stdce/science.htm
>
> In other words, the properties of the particular fluid are important, as
is
>the spin rate.
Only to you.
>
Mark Folsom
Hiram Berry
> >"Paul Lutus" <nos...@nosite.com> wrote:
>
> >My point is this cannot be done in space. You need two factors, one linear
> >acceleration such as gravity, another provided by rotation.
>
> I think everyone who has rejected this idea is suffering from over
> simplification.
Charlie, I agree with you that we ought to look at alternative approaches, and
consider as many different ones as possible. However, Paul's basic criteria above
(a uniform parallel acceleration acting intensively on the fluid in conjunction
with a force perpindicular to that one which varies proportionally to the radius)
are the only ones that obviously create a paraboloid surface, so in lieu of
knowledge of a similarly acting pair of forces, we have to accept gravity vs.
rotation as a necessary mechanism.
Also, the forces mustn't vary over the time scale of an observation-- this pretty
much rules out kinetic solutions like rocket acceleration to provide the linear
force. You're not going to get the needed spatial uniformity from artificially
generated electromagnetic fields either, at least not from any apparatus I know
of. There are a couple of gravity/rotation solutions applicable to a space
telescope: (1) location on a body with gravity, such as a large asteroid, and (2)
position the telescope on one of two masses, each in a different (by several
kilometers) radius circular orbit which are connected by a long tether: each end
experiences a net acceleration; if located at a mean of geosynchronous distance,
the field won't rotate any more quickly than for a similar instrument on the
ground.
> No one has addressed the mater of the container being closed at
> one end, nearly full, and reasonable surface tension. What surface is formed if
> this container is slowly spun (without spillage)?
For a focusing mirror you need the surface to be concave, and to get a good focus
it ought to be as close to a paraboloid as possible; the case you propose would
necessarily be a convex surface if vacuum were above the mirror liquid (there's no
axial component to the centrifugal force, so the surface has to balance the axial
component of the internal pressure by bulging outward if stasis is to be
maintained). If you put a pressurized gas in there, well... we know the pressure in
the liquid Pl varies with its rate of increase being in proportion to the radius,
in the radial direction, without regard to anything going on on the surface. At a
point on the surface, that pressure must be offset by a curved surface with surface
tension, by the relation
Pn = gamma * (1/Rt + 1/Rr); Rt and Rr are radii of curvature tangentially and
radially, Pn is the pressure difference across the interface induced by surface
tension; + Pn means a concave surface, directed gasward.
Rt is zero everywhere. Pl = P0 + k*r^2. The pressure Pg in the gas for our
purposes is a constant. Pg = Pn + Pl, for stasis to occur on the surface, so I get
a relation for the radially directed radius of curvature on the surface of the
liquid of:
Rr = gamma/(Pg-P0-k*r^2);
and radius of curvature can be related to derivatives of axial height (y) respect
to r by:
Rr = (1 + (y')^2)^3/2 / y'' = gamma/(Pg-P0-k*r^2);
Invert and integrate both sides to get an expression for the slope of the liquid,
y':
y' = sqrt( Q / (1-Q)), where Q = [(Pg-P0)/gamma] * r - [k/(3*gamma)] * r^3;
I don't know if you can analytically integrate that to get an explicit formula for
y as a function of r, but you can tell a couple of things from this form: (1) at
some point with large enough r it will flatten out, and (2) in areas near the
central axis, Q is approximately proportional to r, thus y' will be approximately
proportional to sqrt(r), ie. y will be approximately proportional to r^(1.5). That
shape doesn't look like it would be very useful to me, but maybe I'm wrong-- is
there some strange optical arrangement that could use it?
>
>
> How about rubbery or gooey stuff that isn't really a liquid, but deforms
> easily?
Well, increasing viscosity to damp out transient variations is certainly a good
idea. So instead of pure Hg for the mirror we might use an amalgam, for instance?
-- Hiram Berry
Paul Lutus
value for the second derivative of position. Levitation has a first
derivative of zero in the general case (but not a requirement).
On a space platform, making things float around isn't quite the carnival
stunt that it is here. And it wouldn't create the right conditions to
parabolize a liquid mirror.
--
Paul Lutus
www.arachnoid.com
Robert Lynn wrote in message <371EB173...@peterlynnkites.co.nz>...
>Another idea for producing the slight acceleration required to create
>the parabolic shape: Use a large superconducting solenoid around the
>whole mirror and utilise either the paramagnetic or diamagnetic
>qualities of the metal (depends on material properties) in much the same
>way that frogs are levitated in high-field superconducting solenoids.
>
>Robert Lynn
Robert Lynn
James Hunter Heinlen
> Use a large superconducting solenoid around the
> whole mirror and utilise either the paramagnetic or diamagnetic
> qualities of the metal (depends on material properties) in much the same
> way that frogs are levitated in high-field superconducting solenoids.
Please refer to my previous post in this topic. Also, we are only wanting to
apply constant force to the liquid metal, not levitate it, so we do not need
to have nearly as complex a design as you imply.
--
-------------------------------------------------------------------------------
dracus __ __ ____ ___ ___ ____
dra...@primenet.com /__)/__) / / / / /_ /\ / /_ /
/ / \ / / / / /__ / \/ /___ /
-------------------------------------------------------------------------------
Jim Carr
} whole mirror and utilise either the paramagnetic or diamagnetic
} qualities of the metal (depends on material properties) in much the same
} way that frogs are levitated in high-field superconducting solenoids.
Gee, I think "sweet" ... and then I see this response ...
In article <sKyT2.15494$L66.1...@news1.giganews.com>
"Paul Lutus" <nos...@nosite.com> writes:
>
>Um, levitation is not acceleration.
Levitation results _on_earth_ because diamagnetism results in a
repulsive force in the presence of a magnetic field. Orient the
field vertically and, with sufficient strength, you can create
an upward force equal to that of gravity. Net force = 0 means
levitation, and it is stable because of the structure of the
magnetic field. Absent gravity, or if the field is not pointed
up, and this results in acceleration.
>On a space platform, making things float around isn't quite the carnival
>stunt that it is here.
Just more expensive. Whether it is a carnival stunt to study
water droplets on earth and not in space is a matter of opinion.
However, the problem that was being solved was how to apply a
force to the mirror material when it is in free fall (orbit)
rather than on earth. The force produced on a diamagnetic
material by a magnet would not levitate an object in space, it
would accelerate it -- just as it accelerates an object on earth
if the field is not vertical.
>And it wouldn't create the right conditions to
>parabolize a liquid mirror.
That would be a technical problem, because you would require
extreme uniformity of the magnetic field over the volume of
the mirror. Otherwise, I don't see why it can't be done while
in orbit.
--
James A. Carr <j...@scri.fsu.edu> | Commercial e-mail is _NOT_
http://www.scri.fsu.edu/~jac/ | desired to this or any address
Supercomputer Computations Res. Inst. | that resolves to my account
Florida State, Tallahassee FL 32306 | for any reason at any time.
Peter R Newman
> Otherwise, I don't see why it can't be done while
> in orbit.
I'm just butting in here for no particular reason.
One problem nobody seems to have mentioned in those parts of this thread
that I've read: how would re-orientation of the liquid-in-space mirror
be achieved? Having spent all that time and money to get a stable
paraboloid pointing in one direction, surely re-pointing the mirror
would make it slop around, put you back at square one...
Christopher B Specker
: I'm just butting in here for no particular reason.
: One problem nobody seems to have mentioned in those parts of this thread
: that I've read: how would re-orientation of the liquid-in-space mirror
: be achieved? Having spent all that time and money to get a stable
: paraboloid pointing in one direction, surely re-pointing the mirror
: would make it slop around, put you back at square one...
What I'm wondering is, what if you use a slow spin rate? Sure an
ideal liquid would form a cylinder, but an ideal liquid has no surface
tension. Or would surface tension make a perfect paraboloid impossible
even with gravity?
Robert Lynn
Paul Lutus wrote:
>
> Um, levitation is not acceleration. Acceleration is represented by a nonzero
> value for the second derivative of position. Levitation has a first
> derivative of zero in the general case (but not a requirement).
>
> On a space platform, making things float around isn't quite the carnival
> stunt that it is here. And it wouldn't create the right conditions to
> parabolize a liquid mirror.
The earth's surface is an _accelerating_ frame of reference, hence
levitation requires the application of a force. If you constrain the
motion of the liquid metal with the underlying rotating plattern then
the applied magnetic force distributed through the liquid should produce
the same effect as an acceleration would.
> --
>
> Paul Lutus
> www.arachnoid.com
>
> Robert Lynn wrote in message <371EB173...@peterlynnkites.co.nz>...
> >Another idea for producing the slight acceleration required to create
> >the parabolic shape: Use a large superconducting solenoid around the
> >whole mirror and utilise either the paramagnetic or diamagnetic
> >qualities of the metal (depends on material properties) in much the same
> >way that frogs are levitated in high-field superconducting solenoids.
Robert Lynn
Robert Lynn
James Hunter Heinlen wrote:
>
> On Thu, 22 Apr 1999 17:19:47 +1200, Robert Lynn (rob...@peterlynnkites.co.nz)
> suffered dangerous fits of clarity and wrote:
>
> > Use a large superconducting solenoid around the
> > whole mirror and utilise either the paramagnetic or diamagnetic
> > qualities of the metal (depends on material properties) in much the same
> > way that frogs are levitated in high-field superconducting solenoids.
>
> Please refer to my previous post in this topic. Also, we are only wanting to
> apply constant force to the liquid metal, not levitate it, so we do not need
> to have nearly as complex a design as you imply.
I concur with the application of a distributed bulk force, hence the
application of a large stationary magnetic field that produces a bulk
force using either diamagnetism or paramagnetism with the liquid
effectively being presses against the rotating plattern. I used the
levitation example only to demonstrate the scale of forces possible. I
imagine that it is very important to get a homogenous perpendicular
field to limit eddy currents and movements that would be caused by
same. (sorry couldn't find your previous post)
Robert Lynn
Robert Lynn
Robert Lynn wrote:
>
> Another idea for producing the slight acceleration required to create
I should apologise for having used the word acceleration rather than
force here, sorry for the confusion.
> the parabolic shape: Use a large superconducting solenoid around the
> whole mirror and utilise either the paramagnetic or diamagnetic
> qualities of the metal (depends on material properties) in much the same
> way that frogs are levitated in high-field superconducting solenoids.
>
> Robert Lynn
Robert Lynn
Peter R Newman wrote:
>
> Jim Carr wrote:
> > Otherwise, I don't see why it can't be done while
> > in orbit.
>
> I'm just butting in here for no particular reason.
>
> One problem nobody seems to have mentioned in those parts of this thread
> that I've read: how would re-orientation of the liquid-in-space mirror
> be achieved? Having spent all that time and money to get a stable
> paraboloid pointing in one direction, surely re-pointing the mirror
> would make it slop around, put you back at square one...
>
> Pete
> --
> http://sa1.star.uclan.ac.uk/~prn
Yes this is a problem, it would be helped by using a very thin layer of
liquid metal to aid viscous damping, you might be able to use some form
of electromagnetic damping as well (just waving my hands around here).
Robert Lynn
Paul G. White
>
> Jim Carr wrote:
> > Otherwise, I don't see why it can't be done while
> > in orbit.
>
> I'm just butting in here for no particular reason.
>
> One problem nobody seems to have mentioned in those parts of this thread
> that I've read: how would re-orientation of the liquid-in-space mirror
> be achieved? Having spent all that time and money to get a stable
> paraboloid pointing in one direction, surely re-pointing the mirror
> would make it slop around, put you back at square one...
>
> Pete
> --
> http://sa1.star.uclan.ac.uk/~prn
Let's see. If you can control the magnetic field accurately enough to
get the paraboloid, why not make the field stronger on one side, causing
the mirror to precess in the direction you want? Not easy but an 8-meter
mirror in space isn't easy either.
On another point, if you are limited in the diameter of solid mirror you
can lift to orbit by the dimensions of the launch vehicle, why not an
elliptical mirror? Then, in order to get an image which takes full use
of the major axis of the mirror, you could rotate it about the line of
sight.
PGWHITE
Charles W. Shults III
At this point, I must inject a very serious note
about this plan. In Earth orbit, unless you are
-very far out-, there will be very measurable tidal
forces. You cannot have this idea work in Earth
orbit very well. At shuttle height (low but as a
point of demonstration) the tidal forces can be as
much as 0.15 microgees per meter in a plane tangent
to the Earth and 0.3 microgees per meter vertically
from the Earth.
Sound trivial but it will definitely distort the
mirror. I still think that liquid mirrors in free
space are a bad idea, and a solid mirror can far
outperform it with minimal support hardware.
On the Moon, a liquid mirror might be okay, but
for space, let's think of a foamed metal with a
very low coeffecient of expansion. You can aim it
anywhere without a lot of planning!
Cheers!
Chip Shults
Mark Folsom
Christopher B Specker <spe...@email.uah.edu> wrote in message
news:7fo4ik$ljb$2...@info2.uah.edu...> : I'm just butting in here for no particular reason.
>
> : One problem nobody seems to have mentioned in those parts of this thread
> : that I've read: how would re-orientation of the liquid-in-space mirror
> : be achieved? Having spent all that time and money to get a stable
> : paraboloid pointing in one direction, surely re-pointing the mirror
> : would make it slop around, put you back at square one...
>
> ideal liquid would form a cylinder, but an ideal liquid has no surface
> tension. Or would surface tension make a perfect paraboloid impossible
> even with gravity?
The shape for a liquid with surface tension dominant is a sphere, which
would become oblate when rotated. That's pretty useless for a telescope.
--
Mark Folsom, P.E.
Consulting Mechanical Engineer
http://www.redshift.com/~folsom
Mark Folsom
Peter R Newman <s...@my.web.page> wrote in message
news:371F46...@my.web.page...
> Jim Carr wrote:
> > Otherwise, I don't see why it can't be done while
> > in orbit.
>
>
> One problem nobody seems to have mentioned in those parts of this thread
> that I've read: how would re-orientation of the liquid-in-space mirror
> be achieved? Having spent all that time and money to get a stable
> paraboloid pointing in one direction, surely re-pointing the mirror
> would make it slop around, put you back at square one...
>
Viscosity in the fluid would damp out the sloshing eventually.
Gregory L. Hansen
Mark Folsom <fols...@redshift.com> wrote:
>
>Christopher B Specker <spe...@email.uah.edu> wrote in message
>news:7fo4ik$ljb$2...@info2.uah.edu...
>> Peter R Newman (s...@my.web.page) wrote:
>>
>> : One problem nobody seems to have mentioned in those parts of this thread
>> : that I've read: how would re-orientation of the liquid-in-space mirror
>> : be achieved? Having spent all that time and money to get a stable
>> : paraboloid pointing in one direction, surely re-pointing the mirror
>> : would make it slop around, put you back at square one...
>>
>> ideal liquid would form a cylinder, but an ideal liquid has no surface
>> tension. Or would surface tension make a perfect paraboloid impossible
>> even with gravity?
>
>The shape for a liquid with surface tension dominant is a sphere, which
>would become oblate when rotated. That's pretty useless for a telescope.
Don't use a parabolic lens for the eyepeice.
--
"And don't skimp on the mayonnaise!"
Andrew Plotkin
> The shape for a liquid with surface tension dominant is a sphere, which
> would become oblate when rotated. That's pretty useless for a telescope.
Useless? If the liquid is transparent, rather than reflective, it's a
*lens*.
Okay, there are still practical problems.
--Z
--
"And Aholibamah bare Jeush, and Jaalam, and Korah: these were the
borogoves..."
Paul Lutus
magnetic field accurately enough to
get the paraboloid, why not make the field stronger on one side, causing the
mirror to precess in the direction you want? Not easy but an 8-meter mirror
in space isn't easy either. >>
I assume you meant this in fun. It is the space equivalent of blowing on
your own sail.
--
Paul Lutus
www.arachnoid.com
Paul G. White wrote in message <37205C...@ids.net>...
<snip>
me...@cars3.uchicago.edu
>Mark Folsom (fols...@redshift.com) wrote:
>
>> The shape for a liquid with surface tension dominant is a sphere, which
>> would become oblate when rotated. That's pretty useless for a telescope.
>
>Useless? If the liquid is transparent, rather than reflective, it's a
>*lens*.
>
you know.
Mati Meron | "When you argue with a fool,
me...@cars.uchicago.edu | chances are he is doing just the same"
Christopher B Specker
: At this point, I must inject a very serious note
: about this plan. In Earth orbit, unless you are
: -very far out-, there will be very measurable tidal
: forces. You cannot have this idea work in Earth
: orbit very well. At shuttle height (low but as a
: point of demonstration) the tidal forces can be as
: much as 0.15 microgees per meter in a plane tangent
: to the Earth and 0.3 microgees per meter vertically
: from the Earth.
: Sound trivial but it will definitely distort the
: mirror. I still think that liquid mirrors in free
: space are a bad idea, and a solid mirror can far
: outperform it with minimal support hardware.
: On the Moon, a liquid mirror might be okay, but
: for space, let's think of a foamed metal with a
: very low coeffecient of expansion. You can aim it
: anywhere without a lot of planning!
OK, how about a mylar balloon (low internal pressure of course)
with one side metallized to reflect light? A little creative
engineering could then be used to make sure the reflective
surface is parabolic. Granted, it wouldn't work for IR or
UV light, but should be OK for visible light if I'm not
mistaken. The mass might be negligible compared with the
pressure holding the surface in place. Granted, you'd have to
replace it now and then, but again the weight factor would make
this more convenient as well.
Paul Lutus
with one side metallized to reflect light? A little creative
engineering could then be used to make sure the reflective
surface is parabolic. >>
Cute idea. These thoughts:
1. No need to make it parabolic. You could use a corrector plate, such as is
used in the classic Cassegrain design, or you could even shape the secondary
mirror to compensate.
2. The optical-window problem. The light would have to pass through the far
side of the balloon, causing a loss of some wavelengths. Apart from
Cassegrains, reflecting telescopes tend not to have anything in the light
path.
--
Paul Lutus
www.arachnoid.com
Christopher B Specker wrote in message <7fqq2j$b1r$1...@info2.uah.edu>...
<snip>
Gregory L. Hansen
>In article <erkyrathF...@netcom.com>, erky...@netcom.com (Andrew
>Plotkin) writes:
>>Mark Folsom (fols...@redshift.com) wrote:
>>
>>> The shape for a liquid with surface tension dominant is a sphere, which
>>> would become oblate when rotated. That's pretty useless for a telescope.
>>
>>Useless? If the liquid is transparent, rather than reflective, it's a
>>*lens*.
>>
>Which is prety useless. There are reasons why telescopes use mirrors,
>you know.
Uh... isn't that because a five meter diameter lens is inordinately heavy
and expensive?
There's also various abberations, but I'd think those can be corrected by
smaller lenses.
Gregory L. Hansen
Christopher B Specker <spe...@email.uah.edu> wrote:
>OK, how about a mylar balloon (low internal pressure of course)
>with one side metallized to reflect light? A little creative
>engineering could then be used to make sure the reflective
>surface is parabolic. Granted, it wouldn't work for IR or
>UV light, but should be OK for visible light if I'm not
>mistaken. The mass might be negligible compared with the
>pressure holding the surface in place. Granted, you'd have to
>replace it now and then, but again the weight factor would make
>this more convenient as well.
Hey, I kind of like that. Put a vacuum on the backside to pull the lens
into a spheroidal shape, or slightly pressurize the inside of a space
telescope. I'm pretty sure spherical abberation is routinely corrected.
Paul Lutus
and expensive? >>
That is certainly not the only reason, although it is one. Mirrors have no
chromatic aberration, and they accept a much wider wavelength range.
<< There's also various abberations, but I'd think those can be corrected by
smaller lenses. >>
The point is to avoid lenses and windows of all kinds. Modern telescope
design tries to have as much reflection, and as little refraction, as
possible.
--
Paul Lutus
www.arachnoid.com
Gregory L. Hansen wrote in message <7fsfbj$r0u$5...@jetsam.uits.indiana.edu>...
<snip>
Christopher B Specker
: Hey, I kind of like that. Put a vacuum on the backside to pull the lens
: into a spheroidal shape, or slightly pressurize the inside of a space
: telescope. I'm pretty sure spherical abberation is routinely corrected.
I was just thinking about using the vacuum of space behind the balloon.
Of course, there would be shielding to protect it from small particles
to prevent leaks. That's nothing too difficult.
As for the front surface of the balloon being opaque to some wavelengths,
the liquid mirror won't work either.
1) It must be heated so it will not freeze.
2) It must be enclosed so it does not evaporate.
Enclosing the liquid would impose a barrier to some wavelengths of light,
and the balloon might not require heating if it is filled with helium.
Wait a second... any astronomers out there object to the absorbance of
atomic helium spectral lines? This is the main flaw I can see.
Like I say, the balloon can be vacuum-formed during manufacture to produce
a paraboloid shape. (If you doubt this, where have you been? Go to a
large store's florist section and see what they have to offer.)
me...@cars3.uchicago.edu
>In article <FAntM...@midway.uchicago.edu>, <me...@cars3.uchicago.edu> wrote:
>>In article <erkyrathF...@netcom.com>, erky...@netcom.com (Andrew
>>Plotkin) writes:
>>>Mark Folsom (fols...@redshift.com) wrote:
>>>
>>>> The shape for a liquid with surface tension dominant is a sphere, which
>>>> would become oblate when rotated. That's pretty useless for a telescope.
>>>
>>>Useless? If the liquid is transparent, rather than reflective, it's a
>>>*lens*.
>>>
>>Which is prety useless. There are reasons why telescopes use mirrors,
>>you know.
>
>and expensive?
>
>smaller lenses.
>
complexity. And I would expect an experimentalist to know that this
is not the way to go, unless you've no choice.
George Herbert
>: Hey, I kind of like that. Put a vacuum on the backside to pull the lens
>: into a spheroidal shape, or slightly pressurize the inside of a space
>: telescope. I'm pretty sure spherical abberation is routinely corrected.
>
>I was just thinking about using the vacuum of space behind the balloon.
>Of course, there would be shielding to protect it from small particles
>to prevent leaks. That's nothing too difficult.
Given that mirror telescopes are preferable to lens telescopes
to avoid ever having to worry about chromatic abberation (as I
understand this situation; I don't do telescope design professionally,
though one of my uncles by marriage does...), the logical expansion
of this sort of concept would be to use a gas-pressure formed
pseudospherical thin mirror formed out of aluminized mylar or
a thin layer of aluminum metal something similar.
The only problem with that, and all the other proposals here,
is that the tolerances required to be held appear to me to be
sufficiently tight that minor material variations in the
membranes for any of these proposals, lens or mirror,
will introduce wider than allowable variations from the
desired mirror shape over the surface, ruining its use.
Glass is used for a reason. You can machine it to where
you need the surface to be, and it stays there once you've
machined it, to within optical tolerances...
-george william herbert
Retro Aerospace
gher...@retro.com gher...@crl.com
Allen Thomson
>
>Given that mirror telescopes are preferable to lens telescopes
>to avoid ever having to worry about chromatic abberation (as I
>understand this situation; I don't do telescope design professionally,
>though one of my uncles by marriage does...), the logical expansion
>of this sort of concept would be to use a gas-pressure formed
>pseudospherical thin mirror formed out of aluminized mylar or
>a thin layer of aluminum metal something similar.
>
>The only problem with that, and all the other proposals here,
>is that the tolerances required to be held appear to me to be
>sufficiently tight that minor material variations in the
>membranes for any of these proposals, lens or mirror,
>will introduce wider than allowable variations from the
>desired mirror shape over the surface, ruining its use.
Several years ago, the JASONS did a study of future verification
technologies, part of which was a study of just this sort of
inflatable optics for use in space. Unfortunately, my
copy is in storage, but I'll try to dig it out and summarize
it in the next couple of weeks. IIRC, they thought it was doable,
but I don't recall the details.
Regnirps
It could be huge. Think of it, tectonically stable, much slower rotation so
derformation from precession is nil, no wind loading or convection current from
a heated mirror. It could be focused on a detector of the shifting pixel type
to match the (slow) movement of the sky or an area detector could be moved
across the focus for time exposure.
In the long run make it out of hot glass and let it cool the usual way. But
liquid would be nice because you can change focal length.
Charlie Springer
Robert Clark
ydu...@my-dejanews.com wrote:
>...
> Borra thought about a solar sail to produce the gravitational force.
> Unfortunately when you put the numbers together, you find that the smallest
> meteoroid will produce waves meters high on the surface. Not very good for
> the optics. Also the mercury will boil off rapidly in space.
>
> Yvan Dutil
>
> -----------== Posted via Deja News, The Discussion Network ==----------
> http://www.dejanews.com/ Search, Read, Discuss, or Start Your Own
>
On NASA's Astrophysics Data System (ADS) I found some of Borra's papers:
http://adswww.harvard.edu/abstract_service.html.
He discusses the possibility of using solar sails to produce the g-force in
space in "The Case for Liquid Mirrors in Orbiting Telescopes,"
1992ApJ...392..375B. He mentions the possibility of meteorite hits but only
in respect to their puncturing the container. He suggests that a liquid
mirror system should be self-sealing for small size meteorites, larger size
ones being less common. The recent article about micron size meteor damage
on HST does make this a likely problem for future space scopes. Does anyone
know if Hubble's mirror was damaged? One advantage of the liquid mirror as
opposed to a solid glass one is that after the hit, the liquid would subside
back to its original shape, while a solid mirror would retain the damage,
possibly requiring replacement. I didn't see anything about meter high waves
being formed. Do you mean on the solar sail or on the mirror itself? There
are other possible ways of producing the required acceleration of course.
Such as for example the ion engine proposal made in the paper: "Space Based
Liquid Ring Mirror",
http://origins.jpl.nasa.gov/meetings/ulsoc/papers/magnuson.pdf. In this
proposal the ion propulsion system would only need to provide 10^-3 g's
thrust. The recent successful testing of the Deep Space 1 Probe, being able
to provide continuous thrust over long periods, shows that this is indeed a
legitimate possibility. It doesn't seem likely that there would be formed
meters high waves on the mirror since in the systems Borra's team have
researched the liquids are only millimeters thick. Borra does discuss the
problem of mercury evaporation in space and suggests gallium alloys as better
possibilities. Another interesting paper of Borra is: "The Case for a Liquid
Mirror in a Lunar-Based Telescope," 1991ApJ...373..317B. In this paper Borra
describes the weight advantage of transporting a liquid mirror to the moon
(and by extension to space in general.) He gives the estimated weight of a 4
meter mercury mirror, including container, motor, and mount as only 188kg,
and for a gallium one as only 137kg. This is compared to the Hubble's 2.4
meter mirror's weight of 900kg. Note also that a liquid mirror eliminates the
problem of requiring a large size payload bay to hold the large mirror, such
as the 8 meter proposed for the Next Generation Space Telescope (NGST).
To get the Borra papers, enter Borra in the author field at the ADS site,
http://adswww.harvard.edu/abstract_service.html. Then click on the paper to
bring up the abstract. Click on the Print button to download a postscript
version of the paper.
________________________________________
"In science, everything is significant"
-- Bob Clark
________________________________________
-----------== Posted via Deja News, The Discussion Network ==----------
http://www.dejanews.com/ Search, Read, Discuss, or Start Your Own
Joe Fischer
: It seems to me the far side of the Moon is the ideal place for a
Glass requires both grinding and extensive polishing,
so that is probably not a viable option.
And chances are even a mercury mirror might not be
suitable for deep sky photography, because the surface
must be within a fraction of a wavelength of light.
When large mercury mirrors were used, photography
wasn't an option, and galaxies were not recognized as
anything but fuzzy nebulae.
But I do think all SETI efforts should have
antennae on the other side of the moon, to eliminate
most of the Earth source emissions.
Joe Fischer
Jimmy Dean
> The authors describe their research in producing liquid mirrors. They note
> that the primary problem is that of the mirror deviating from its parabolic
> shape when tilted.
What about the Aricebo (sp?) approach. Make a humongous mirror and just
point it vertically.
jd
ydu...@my-dejanews.com
You got it, body! Starring the zenith is to only practical way to use it.
Any other scheme make to cost skyrocketing! Dont dream about tiltable
liquid mirrors or space based one. These scheme are not working and
they are not going to work from basic physical ans economical reason.
Hiram Berry
> Regnirps (regn...@aol.com) wrote:
[... some good reasons for a moon-based liquid mirror, which unfortunately did
not show up on my news server ...]
> Glass requires both grinding and extensive polishing,
> so that is probably not a viable option.
Not much grinding is required if you've used the spin technique to create a
nearly paraboloid surface to begin with. And the moon is full of silica and
alkaline earth oxides for the glass, as well as alumina for the finishing
abrasive. Modern mirror manufacture and the personal experience Charlie Springer
related earlier in this thread seem to show that getting very close to a
paraboloid isn't too hard.
>
> And chances are even a mercury mirror might not be
> suitable for deep sky photography, because the surface
> must be within a fraction of a wavelength of light.
> When large mercury mirrors were used, photography
> wasn't an option, and galaxies were not recognized as
> anything but fuzzy nebulae.
It's an interesting technical problem; from the ideas put forward here it looks
like a liquid mirror in free-fall might be theoretically possible, but either
practically impossible or requiring so much support equipment that it wouldn't be
competitive with solid mirrors. On the moon, though, I think it's a very
different situation. Sure, you want the paraboloid surface to be accurate to
within a quarter wavelength or so, but on the moon it's not a case of actually
creating such a surface, but rather of doing some minor perturbations to overcome
the slight distortion due to surface tension. It's been pointed out that mercury
is diamagnetic (bismuth is also, and has a fairly low melting point, so it
probably ought to be considered too) -- minimal correction of the surface by
solenoid current shouldn't be hard to do, and I don't think superconducting coils
are required here, either.
> But I do think all SETI efforts should have
> antennae on the other side of the moon, to eliminate
> most of the Earth source emissions.
I certainly agree with you on that.
-- Hiram Berry
ro...@mauve.demon.co.uk
: Christopher B Specker <spe...@email.uah.edu> wrote:
:>Gregory L. Hansen (glha...@steel.ucs.indiana.edu) wrote:
:>: Hey, I kind of like that. Put a vacuum on the backside to pull the lens
:>: into a spheroidal shape, or slightly pressurize the inside of a space
:>: telescope. I'm pretty sure spherical abberation is routinely corrected.
:>
:>I was just thinking about using the vacuum of space behind the balloon.
:>Of course, there would be shielding to protect it from small particles
:>to prevent leaks. That's nothing too difficult.
<snip>
: the logical expansion
: of this sort of concept would be to use a gas-pressure formed
: pseudospherical thin mirror formed out of aluminized mylar or
: a thin layer of aluminum metal something similar.
: The only problem with that, and all the other proposals here,
: is that the tolerances required to be held appear to me to be
: sufficiently tight that minor material variations in the
: membranes for any of these proposals, lens or mirror,
: will introduce wider than allowable variations from the
: desired mirror shape over the surface, ruining its use.
You could do active corrections, by heating (patterend IR) of the
backside of the mirror.
stardot
near zenith?
--
Randy J. Rogers
Dallas, TX
To reply, remove "nospam" from my email address.
Robert Clark wrote in message <01be87d0$dc912680$31113018@default>...
>
>Telescopes on the cheap?
>[9 April 1999] Telescopes are becoming increasingly large and expensive.
>Reducing the cost has therefore become a priority for some astronomers. One
>example of this is the suggestion by a group of astronomers from Laval
>University in Quebec, Canada, that mirrors with a diameter of 8 metres could be
>made of metallic liquids rather than glass. The method works by placing the
>liquid mirror on a spinning platter. As the liquid spins, gravitational forces
>cause the liquid to deform into the shape of a perfect parabolic mirror
>(Astrophysical Journal 516 May 10 1999).
>http://physicsweb.org/archive/news/1999/04?show=stands
>
>Ultra-Thin Floating Mirrors
>http://xxx.lanl.gov/abs/astro-ph/9901255
>
> The authors describe their research in producing liquid mirrors. They note
>that the primary problem is that of the mirror deviating from its parabolic
>shape when tilted. If the mirror were used in an orbiting telescope wouldn't
>this problem be eliminated?
> The parabolic shape the mirror obtains is described as being due to
>'gravitational' forces. But centrifugal forces alone would cause the liquid
>surface to assume a curved shape even without gravity. Would it also assume a
>parabolic shape in this case? Perhaps we could choose the right liquid so that
>it would assume this shape.
>
>________________________________
>
> "In science, everything is significant."
> -- Bob Clark
>________________________________
P. Edward Murray
I do seem to recall an article in a recent S&T about a liquid mirror
'scope...the mirror was made out of liquid mercury and could only point
towards the zenith!
Might be useful to some extent, but mercury is a poison and no doubt
that would make this type of telescope very difficult to build.
Ed Murray
BMAA
douglas dwyer
<edwa...@erols.com> writes
>Gee Doug, a holographic mirror is a new one to me!:)
reciprocal device so if it should be possible to produce a flat mirror
that concentrates parallel light to a point source by reversing the
photographic process, now if an imperfect concentrator was in the
picture forming path it may be possible to correct the imperfection .
Quite possible that inefficiecies in the holoic mirrld offset any gain.
from the big mirror.
--
douglas dwyer
Gregory L. Hansen
Hmm... holograms have (or can have) better resolution than the human eye.
I wonder if hologram technology can be made to produce optical-quality
fresnel lenses or flat parabolic mirrors?
--
And remember, when packing up biohazardous waste, don't "burp" the bag!
Charles W. Shults III
No need for the Fresnel route if you use a
hologram- just make a plain old lens hologram.
Still, they are lossy and you need them to be
very flat for accuracy, so you're right back
where you started.
As for large, flat parabolic mirror holograms,
the depth of the image becomes a limiting factor.
And, that pesky optical loss...
Cheers!
Chip Shults
Gregory L. Hansen
Charles W. Shults III <aic...@gdi.net> wrote:
>Gregory L. Hansen wrote:
>> Hmm... holograms have (or can have) better resolution than the human eye.
>> I wonder if hologram technology can be made to produce optical-quality
>> fresnel lenses or flat parabolic mirrors?
>
> No need for the Fresnel route if you use a
>hologram- just make a plain old lens hologram.
>Still, they are lossy and you need them to be
>very flat for accuracy, so you're right back
>where you started.
> As for large, flat parabolic mirror holograms,
>the depth of the image becomes a limiting factor.
>And, that pesky optical loss...
Well, I thought it might be easier to make large, flat surfaces than
large, parabolic surfaces. Or it might be easier to make a six foot
diameter window pane than a six foot diameter lens.
Still, losses and flatness aside, the mere fact that you can make a lens
hologram is pretty darn cool.
Robert Clark
>...
> For a focusing mirror you need the surface to be concave, and to get a good
focus
> it ought to be as close to a paraboloid as possible; the case you propose
would
> necessarily be a convex surface if vacuum were above the mirror liquid
(there's no
> axial component to the centrifugal force, so the surface has to balance the
axial
> component of the internal pressure by bulging outward if stasis is to be
> maintained). If you put a pressurized gas in there, well... we know the
pressure in
> the liquid Pl varies with its rate of increase being in proportion to the
radius,
> in the radial direction, without regard to anything going on on the surface.
>...
> -- Hiram Berry
>
>
I don't think it would remain convex for sufficiently high rotation rates.
For if spun fast enough the fluid would begin to centrifuge to the sides of
the container. So at some point the surface would become nearly flat. Then at
increasingly higher speeds it would become more and more concave. The web
site devoted to experiments in the temporary zero-g created by aircraft
undergoing parabolic trajectories, shows that in general in zero-g without
rotation the surface will be a portion of a sphere: "Physical Principles",
http://polar.space.swri.edu/hudd/Phys_Prin.html. As indicated on the web site
the radius of the sphere and the angular size of the spherical segment
depends on both the fluid and the material of the container. On this web
page, the examples do contain some gas above the surface, however, I think
the result would be the same in vacuum. For we could choose the fluid and
container material so that the contact angle was nearly flat, close to zero,
forcing the surface to assume a concave shape. The authors of this site
suggest the reference: Antar, Basil and Vappu; Fundamentals of Low Gravity
Fluid Dynamics and Heat Transfer ; Boca Raton : CRC Press Inc.; 1993. In this
book it is shown that there are an infinite range of *kinds* of shapes for
the surface in zero-g when rotation is allowed, not just segments of spheres.
The possible shapes include convex ones, concave ones, and surprisingly even
convexo-concave ones. I'm checking the equations to see if parabolic ones
are possible.
________________________________________
"In science, everything is significant"
-- Bob Clark
________________________________________
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