SCT, 1/4 wave & 33% obstruction
Bruce Fleming
According to Suiter or someone who wrote the article in
Sky&Telescope. An SCT with 33% obstruction has a maximum
of 1/4 wave optics. But how come I was able to look at
someone's 8" SCT and it is 1/8 wave?? I used the Suiter
chart and the scope has the same look at either side
of focus to 1/8th wave in the chart. How the hell can
this be possible when it is stated that a 33% obstruction
can produce 1/4th wave wavefront error at its best (assuming
the optics of the mirror itself is 0 wavefront error. Or
less than 1/4th wave if the mirror and corrector has more
error). Anyone can please enlighten on this mystery? Thanks.
Bruce
--== Sent via Deja.com http://www.deja.com/ ==--
---Share what you know. Learn what you don't.---
Armando Rio
If you look at page 50 Fig. 3-6 of the Suiter's book, you can see that a
perfect scope with a 33% obstruction has a MTF curve similar to that one of
a scope with 1/4 wavelenght sperical aberration with no obstruction.
So a perfect SCT will have a MTF plot as in fig. 3-6. If the scope has other
aberrations, let's say a spherical aberration, than the MTF curve will be
reduced as you can see on page 176 Fig. 10-5.
If you see that the SCT has the same look at either side of focus to 1/8th
wave as on page 184, this means the scope has a spherical aberration of
1/8th of wave.
The MTF plot will be slightly degraded, as each aberration contributes to
deteriorate the MTF curve. If you return on page 176 Fig 10-5 you can see
that a spherical aberration of 1/8 wavelength reduces only slightly the
contrast, but you can see what happens if the spherical aberration increases
and don't forget at the end we have also to consider the degradation for the
obstruction.
Armando
Bruce Fleming
"Armando Rio" <rioar...@EUnet.at> wrote:
> Hi Bruce,
> If you look at page 50 Fig. 3-6 of the Suiter's book, you can see that
>a perfect scope with a 33% obstruction has a MTF curve similar to that
>one of a scope with 1/4 wavelenght sperical aberration with no
>obstruction.
>1/8th wave as on page 184, this means the scope has a spherical
>aberration of 1/8th of wave.
So the above means that if there are two scopes and there are both
1/8th wave. If the MTF of one is higher from the other. Then the
performance of both 1/8th wave is NOT similar?? I thought wavefront
error is the ultimate characteristic to look to see how good an optics
is. So it's the MTF. I wonder if bad coatings or chromatic
aberration can also further decrease the MTF? I think a scope has
to give MTF rating rather than wavefront error. I am still looking
for a good big SCT because my 1/10th wave apo refractor is
only mere 60mm. :( Not enough to see lunar or deep sky. But it is
great terrestrially. I guess I have to be near the birds rather
than looking at the barcony a mile away. Anyway. Thanks for the good
info.
AndersonRM
writes:
>According to Suiter or someone who wrote the article in
>Sky&Telescope. An SCT with 33% obstruction has a maximum
>of 1/4 wave optics. But how come I was able to look at
>someone's 8" SCT and it is 1/8 wave??
Take Suiter's claims lightly.
In the book he claims this, and we all know SCT's have
more residual optical errors than small refractors,
plus if the mirrors aren't exactly the right distance
apart, you get more wavefront error. Add it all up
and you have about 1/2 wave cumulative, according to
Suiter? That's a guess. But, anyone who has
seen through a good SCT knows there is more to the
story than blandly taking that at face value and you
can see more in a good SCT than a Newtonian confirmed as 1/2
wave accurate.
-Rich
"GO SEE STAR WARS P.M.;
PUT ANOTHER NAIL INTO THE
COFFIN OF QUALITY SCIENCE FICTION"
William Sommerwerck
> How the hell can this be possible when it is stated that a 33% obstruction can
> produce 1/4th wave wavefront error at its best (assuming the optics of the
> mirror itself is 0 wavefront error? Or less than 1/4th wave if the mirror and
> corrector has more error)? Anyone can please enlighten on this mystery?
> Thanks.
Because wavefront error and percentage of obstruction have absolutely nothing
whatsoever to do with each other.
William Sommerwerck
whatsoever to do with each other. They aren't related -- mathematically or
"practically" -- in any way.
Del Johnson
forget that it is aperture that matters most. A 1/4 wave larger telescope
will still outperform a 1/100 wave smaller telescope. This is because the
wavefronts errors are not absolute, but a function of aperture.
Del Johnson
Bruce Fleming wrote in message <7hmbmr$jhk$1...@nnrp1.deja.com>...
David W. Knisely
> According to Suiter or someone who wrote the article in
> Sky&Telescope. An SCT with 33% obstruction has a maximum
> of 1/4 wave optics. But how come I was able to look at
> someone's 8" SCT and it is 1/8 wave??
Well, it isn't quite the whole story. What the central obstruction does
is move energy from the diffraction disk and put it into the diffraction
rings (mostly the first ring out from the diffraction disk for common
central obstructions). Wave front errors caused by an inaccurate mirror
will cause a scattering of light and a reduction in image quality, but
not in quite the same predictable manner as a central obstruction does.
The two effects (central obstruction and mirror wavefront error), cause
similar problems, but are not really "equivalent", except under certain
well-defined conditions. What Suiter clearly shows on his table on page
198 is that a 33% central obstruction causes a reduction of energy in
the diffraction disk which is nearly equivalent to the loss caused by
1/4 wave of "simple" first-order spherical aberration due to a
misfigured but unobstructed primary mirror. Some SCT's I have looked
through have had very fine images, so I suspect that their mirrors were
better than 1/4 wave wavefront (sometimes significantly better). An 8
inch Newtonian with an 1/8th wave primary mirror and a 20 percent
obstruction would outperform an 8 inch SCT with 1/8th wave mirrors in
terms of high power image contrast, but often, the difference is less
than many on this group make it out to be. However, and 8 inch
Newtonian with 20% obstruction and a 1/2 wave wavefront error mirror
would be significantly inferior to the above SCT. The single most
important thing to worry about when talking about telescopes is optical
quality, rather than the exact size of the central obstruction. Clear
skies to you.
--
David Knisely KA0CZC dk8...@navix.net
Prairie Astronomy Club, Inc. http://www.4w.com/pac
Hyde Memorial Observatory, http://www.blackstarpress.com/arin/hyde
************************************************
* Attend the 6th Annual NEBRASKA STAR PARTY *
* August 7-14th, 1999 http://www.4w.com/nsp *
************************************************
KB6UDA
AndersonRM
(KB6UDA) writes:
>Is this why people freq report that a 4" refractor does better than an 8"
>SCT?
Most people who know those scopes don't. What you read is from
people who simply aren't comparing them under the proper conditions.
If you want to compare an 8" SCT to a 4" refractor you must:
-Make sure the seeing is excellent
-Make sure the 8" SCT has good optics
-Make sure the collimation in the SCT is adjusted right.
-Make sure the SCT has reached thermal equilibrium
If you fail to meet any of those four requirements then the 4" scope
might be better and you will get a false idea of the true performance
level of the SCT. I suspect most people who post that the refractor
is superior were subject to one of those four problems.
PI3P1415
lunar observing. Even when a SCT is well collimated and cooled down it just
does not produce the fine image a refractor does. There is a washed out look.
Of course in a side by side comparison a 8" SCT will beat a 4 inch any day of
the week. It just looks a lot nicer in a for inch florite scope.
tomd...@my-dejanews.com
I do understand your point, Rich, but with all these caveats, it seems
clear to me that a typical true 4" APO, which is what most people
compare to (as opposed to a generic 4" refractor), performs better off
the shelf and in the field than a typical 8" SCT.
--
Tom DeMary
tomd...@my-dejanews.com
Harry Pulley
The question you must ask yourself is whether you want more detail in a
less pleasing image or less detail in a more aesthetically pleasing
image? If you want to see smaller lunar rilles and craters, more
planetary detail, resolved clusters instead of fuzzballs, close split
doubles or wide split doubles, then an 8" SCT is a better choice. The
big secondary does indeed give it a somewhat washed out or fuzzed out
appearance when compared to a smaller refractor. Plus even cooled down
an 8" scope will always show more seeing effects than a 4" scope. I
will take the detailed view in a less steady and sharp image every time.
Some people prefer the apparently steadier, more contrasty, sharper
image in the smaller refractor, but not me.
--
<:-{} Harry C. Pulley, IV
\ member RASC Hamilton/KW centres, ALPO, IOTA, TPS
mailto:hpu...@home.com http://members.home.com/hpulley
Guelph, Ontario, Canada, 43.51N, 80.28W
Ratboy99
>If you fail to meet any of those four requirements then the 4" scope
>might be better and you will get a false idea of the true performance
>level of the SCT.
That would seem to indicate that it is easier for the 4" to outperform the SCT
than vice versa. There are a lot of things to make sure of with the SCT, but
none with the 4" refractor.
rat
~( );>
AndersonRM
(PI3P1415) writes:
>There is a washed out look.
>Of course in a side by side comparison a 8" SCT will beat a 4 inch any day of
>the week. It just looks a lot nicer in a for inch florite scope.
That's the crux. The image "looks" better (maybe) in the 4" but
it certainly isn't delivering more information.
Fcathell
refractor. Let's stop kidding ourselves.
Frank
Chris1011
If properly collimated most 8" SCTs today will absolutely outperform a 4"
refractor. Let's stop kidding ourselves.
>>
I have a properly collimated 8"SCT with good optics and it does not absolutely
outperform my good 4" refractor most of the time on the planets. The SCT has an
advantage only on deep sky objects but even there the difference is less than
you would think. In fact, my 4" refractor can almost always show 6 Trapezium
stars at 100x, yet the SCT struggles to see 5 most of the time. Only in
pristine seeing on the seashore in the Florida Keys did the 8"SCT clearly
outperform the 4" on elusive unequal brightness doubles.
Roland Christen
Shedir
>I have a properly collimated 8"SCT with good optics and it does not
>absolutely
>outperform my good 4" refractor most of the time on the planets. The SCT has
>an
>advantage only on deep sky objects but even there the difference is less than
>you would think. In fact, my 4" refractor can almost always show 6 Trapezium
>stars at 100x, yet the SCT struggles to see 5 most of the time. Only in
>pristine seeing on the seashore in the Florida Keys did the 8"SCT clearly
>outperform the 4" on elusive unequal brightness doubles.
>
>Roland Christen
I guess Rats claim to his 70mm Pronto outperforming his 8'' SCT and
my claim to my 3.5 Questar outperforming my 8'' SCT on planetary and lunar
detail doesn't sound so far fetched anymore does it? :) I'm like Rat. I tell
it like I see it!
Shedir
PI3P1415
after collimation and cooling just does not look very nice.
Chris1011
I guess Rats claim to his 70mm Pronto outperforming his 8'' SCT and
my claim to my 3.5 Questar outperforming my 8'' SCT on planetary and lunar
detail doesn't sound so far fetched anymore does it? :) I'm like Rat. I tell
it like I see it! >>
I know it sounds far fetched that a scope of half the size and 1/4 the light
grasp could even come close to the bigger Cat. Consider this: The Airy disc
brightness in a perfect 35% obstructed telescope is 64% of the total. The first
diffraction ring's brightness is 23%. Compare this to an unobstructed aperture
of 84% and 7%. If the obstructed optic has 1/4 wave error, the Airy disc drops
to about 50% and the first diffraction ring rises to 32% and the 2nd
diffraction ring rises to a prominent 17%. Now add atmospheric turbulence of
1/3 to 1/4 wave. What happens is that the very bright first diffraction ring
merges with the Airy disc. Voila - you have a diffraction pattern resembling
that of a 60mm refractor where the second diffraction ring has the brightness
and appearance of a pseudo 1st diffraction ring. That 8" aperture suddenly
performs like a much smaller telescope. Of course turbulence tends not to be
constant, so you will get tantalizing glimpses now and then of better detail,
but you will also be chasing the focus back and forth trying to eke out any
kind of detail most of the time. With the smaller unobstructed aperture,
especially one with very accurate and smooth optical correction, the image will
literally sit there at focus showing sharp detail under the very same lousy
seeing.
How about brightness. The measured throughput of my SCT is as follows: .92
primary reflectivity, .92 secondary, .93 corrector transmission, .89 secondary
shading. Total throughput = 70%. This was when the mirrors were brand new.
With aged mirrors the reflectivity drops to .88 for a total of 64%. Even at
64%, the total light throughput of an 8" SCT is higher than a 4" perfect
aperture. However, the brightness of the background sky (assuming some light
pollution) is considerably lower in a well baffled refractor vs. a typical SCT.
Even though faint fuzzies are intrinsically fainter in the 4", their contrast
against the blacker background makes them appear brighter to a dark adapted
eye. Even so, the 8" still shows them brighter, but the difference is not 4
times. The one place where this argument falls apart is in extremely dark
locations where the background sky is black even in unbaffled scopes. There the
8" SCT clearly wins on deep sky.
Roland Christen
Pablo
absolutely
>outperform my good 4" refractor most of the time on the planets. The SCT
has an
>advantage only on deep sky objects but even there the difference is less
than
>you would think. In fact, my 4" refractor can almost always show 6
Trapezium
>stars at 100x, yet the SCT struggles to see 5 most of the time. Only in
>pristine seeing on the seashore in the Florida Keys did the 8"SCT clearly
>outperform the 4" on elusive unequal brightness doubles.
>
>Roland Christen
If what you're reporting is true, then you MUST have a problem with your
SCT. Most likely
its optics. Take care..
jschibelli
well, Rich doesn't sell anything.
If you are going to pick on Rich, pick a topic that doesn't make you look
off base.
Jim
Michael Edelman wrote in message <374189C6...@michdeletethis.com>...
>
>
>AndersonRM wrote:
>
>> In article <19990517151537...@ng-cc1.aol.com>,
chri...@aol.com
>> (Chris1011) writes:
>>
>> >I have a properly collimated 8"SCT with good optics and it does not
>> >absolutely
>> >outperform my good 4" refractor most of the time on the planets.
>>
>> So you contend your 4" is outperforming an 8" SCT "most" of
>> the time? That's what I'd call bad seeing conditions on average....
>
>Let's see: We have Roland, who makes some of the best scopes on the planet
on one
>side of the coin, and opposing him, we have Rich, who has apparantly never
had an
>unexpressed thought on *any* topic, whether he has any ideas what he's
talking
>about or not..
>
>Who to listen to? Wow, tough call.
>
Del Johnson
embarassment for supporting his opinions.
Del Johnson
Fcathell
turbulance. Ihave a C-5 and a C-8 and lately the C-5 has been consistently
outperforming the C-8 because of bad seeing despite excellent transparency.
However, when the seeing is good the 8 "clobbers" the 5.
This also brings up another effect I call the "lazy" effect. After over 20
years of observing thru large and small scopes I have found that because it is
easier to set up the small scope, I have done more observing thru them. I also
know many amateurs who have eventually "dumped" their ligh buckets for smaller
scopes for this very reason. More observing gets done with a small scope. I
think the last few years has seen a resurgence of interest in small, optically
high quality scopes. I personally am very interested in seeing some of the
upcoming 4" APO vs Meade ETX-127 "shootout' comparisons.
Frank
Chris1011
If what you're reporting is true, then you MUST have a problem with your
SCT. Most likely
its optics. Take care..
>>
No, the SCT has been thouroughly tested by interferometry. It is well
collimated. There is no problem except seeing and cooldown problems.
Roland Christen
AndersonRM
>I do understand your point, Rich, but with all these caveats, it seems
>clear to me that a typical true 4" APO, which is what most people
>compare to (as opposed to a generic 4" refractor), performs better off
>the shelf and in the field than a typical 8" SCT.
I still disagree. A 4" won't begin to show intricate detail on
planets and on deepsky, there is no contest. Besides, straining
to see detail that isn't there in a good 4" isn't that much fun.
The compact ones make nice portable scopes for those wishing
to go beyond the ETX, Shortube setups.
AndersonRM
(Ratboy99) writes:
>That would seem to indicate that it is easier for the 4" to outperform the
>SCT than vice versa. There are a lot of things to make sure of with the SCT,
but
>none with the 4" refractor.
It comes down to this; I personally don't want to waste time
looking at planets during bouts of really bad seeing. Seeing
that might allow a 4" to work to it's limits a few seconds out of
every hour and will never allow an 8" SCT to meet it's seeing
ability. During times like that, I'm either not observing or
looking at deepsky objects that are less susceptable to atomospheric
turbulence.
Were sort of getting back to that question of: "Is there anytime when
a 4" exceeds the detail shown by an 8" SCT and how often does that
happen." Rarely, if ever in my experience.
AndersonRM
>I have a properly collimated 8"SCT with good optics and it does not
>absolutely
>outperform my good 4" refractor most of the time on the planets.
So you contend your 4" is outperforming an 8" SCT "most" of
the time? That's what I'd call bad seeing conditions on average.
Looking at the Moon in a good apo 4" always impresses people,
but after looking carefully side by side with an 8" SCT, most
notice there is more detail in the SCT, despite the fact it doesn't
quite look as "good." There's contrast, then there is resolution.
>In fact, my 4" refractor can almost always show 6 Trapezium
>stars at 100x, yet the SCT struggles to see 5 most of the time.
This I understand and more or less agree. This situation is kind
of unique in that seeing conditions that will "boil away" the 5th and
6th components of the Trapezium in an 8" SCT will often allow
a good 4" refractor to show them. I'd attribute it to a particular
contrast effect inherent in both scopes. The 4" has high contrast
and presents the 4 main stars as not as bright as in the 8" SCT.
That, coupled with less than perfect seeing tends to keep the
E and F components more visible in 4" than the 8." But, sometimes
if you reduce the light coming through the 8" with an ND filter, it will allow
you to see the E and F components because the other 4 main stars
are dominating the scene. There may be other examples of this,
but I haven't come across them.
>Only in
>pristine seeing on the seashore in the Florida Keys did the 8"SCT clearly
>outperform the 4" on elusive unequal brightness doubles.
That's kind of odd considering the brightness of the diffraction rings
in the highly obstructed 8." Generally, it makes some unequal doubles
harder to spot than in an unobstructed refractor of smaller size.
However, 80% of the time I can split sub-1 arc sec doubles with
an 8" SCT in Toronto, something that can't be done too well with
a 4" apo owing to it's having only half the resolving power of the
8" SCT.
AndersonRM
(Shedir) writes:
> I guess Rats claim to his 70mm Pronto outperforming his 8'' SCT and
>my claim to my 3.5 Questar outperforming my 8'' SCT on planetary and lunar
>detail doesn't sound so far fetched anymore does it? :)
Where were you guys when Questar tried to "pull the wool over people's
eyes" in the late 1970s by claiming this? They could have used the
support. :)
AndersonRM
(PI3P1415) writes:
>I wont argrue a 8" sct will out perform a 4 inch refractor but the image even
>after collimation and cooling just does not look very nice.
The best SCT's do produce crisp, refractor-like images at very low
power. But the moment you raise the power over 100x, the super-bright
diffraction rings make stars look "bloated." Plus, it's a rare night when
a diffraction pattern in an 8" SCT will be dead-still and solid which is
what is required to see things in a refractor-like (read: small scope)
view. If we all had 8" refractors, we'd have the same image problems
due to seeing that we get in an 8" SCT. However, the stars would be
tighter looking due to the greatly diminished diffraction ring
illumination.
Ratboy99
>really kicks butt?
>
>rat
>~( );>
Hey! My new scope is the same size as my old scope's central obstruction, and
it's almost as good! The obstruction in the 2.75" SCT would be almost 1 inch!
rat
~( );>
Ratboy99
>a diffraction pattern in an 8" SCT will be dead-still and solid which is
>what is required to see things in a refractor-like (read: small scope)
>view.
Rare indeed. Took the words right out of my mouth. My SCT only produced star
'blobs' at high power 99 percent of the time. I know that it was ok because on
ONE occasion I saw it produce perfect Airy disks, meaning tiny central spot
with diffraction rings. It could usually split E Lyrae, almost always but
recently, I even had difficulty getting it to split Alpha Gemini. The images
would not hold steady enough. The Ranger yields an airy disk albeit larger,
every single time I go out. I am not imagining it's good performance. I
honestly believe that more aperture would be better, but I don't think it takes
that much more to get some great views. In fact I have observed 18 NGC and 44
Messier objects since April 7, found by star hopping and not observed by myself
before, in a 2.75" scope. I'm just pointing out that for 2.75" it is a very
impressive little scope. I think that has at least something to do with it's
being a refractor. If it was an SCT what would I be saying? Man, my 2.75" SCT
AndersonRM
(Chris1011) writes:
>How about brightness. The measured throughput of my SCT is as follows: .92
>primary reflectivity, .92 secondary, .93 corrector transmission, .89
>secondary
>shading. Total throughput = 70%.
Your scope must have an uncoated corrector because even old
magfluoride coatings on both surfaces gives a throughput of
97%, even after allowing for a small (less than a percent)
bit of absorbtion. A modern SCT will have 94% mirrors and
98-99% at the corrector surfaces. So a new SCT will give
76% or better. That gives it the throughput of a 7" refractor.
On deepsky objects, that's how it will behave, minus some
contrast due to the secondary.
AndersonRM
(Ratboy99) writes:
> It could usually split E Lyrae, almost always but
>recently, I even had difficulty getting it to split Alpha Gemini. The images
>would not hold steady enough. The Ranger yields an airy disk albeit larger,
>every single time I go out.
Watching Mars on a video screen with my 9.25" you could see
actual planet features, in their entirety "move" around the planet
due to poor seeing. In addition, the whole planet migrated by
it's own diameter quite a bit. There is no doubt that as you go up
in scope size, your chances of utilizing that aperture to it's limit
drop. But, I consider my seeing to be average for a city and
I can use an 8" 80% of the time to it's limit on fuzzy deep
sky objects, 50% of the time on resolving globulars and fine deepsky
detail, 15-20% of the time on double stars to their 0.56-.053" limit
(equal doubles!) and maybe 10% of the time on planets to
the limit of it's capabilities. That's not bad and it justifies using
the 8". But, for the Sun, maybe 0.000000000001% of the time!
AndersonRM
(Ratboy99) writes:
> I'm just pointing out that for 2.75" it is a very
>impressive little scope. I think that has at least something to do with it's
>being a refractor. If it was an SCT what would I be saying? Man, my 2.75"
>SCT
>really kicks butt?
No, if refractors were the same price as SCT's, inch per inch and
had F5 focal ratios, no one would own an SCT. I can imagine
a colour-corrected, 45" long 8" refractor being quite popular!
But, small refractors are affordable, and refractors are still the
best scopes, inch per inch.
Chris1011
Your scope must have an uncoated corrector because even old
magfluoride coatings on both surfaces gives a throughput of
97%, even after allowing for a small (less than a percent)
bit of absorbtion.>>
My corrector is fully coated with a single layer of Mag-Fluoride on either
side. The reflection loss of each surface is 2%, and the plate glass
transmission loss is not negligeable, especially in the green. The actual
measured throughput was 93% which takes into account all transmission and
reflection losses. In order for reflection losses to be lower, the coatings
have to be carefully monitored during application. Generally more than 2 layers
are required to get the reflection losses to be less than 1%. Even
multi-coatings do not guarantee this if they are not properly monitored during
application. I have seen multicoatings that have more than 5% reflectivity
(that is more than uncoated glass). Proper application of coatings is an art
similar to figuring of lenses.
Roland Christen
Michael Edelman
Fcathell wrote:
> If properly collimated most 8" SCTs today will absolutely outperform a 4"
> refractor. Let's stop kidding ourselves.
There's nothing like blatent, unsupported opinion masquerading as fact to
convince me, by golly.
Most consumer priced SCTs have less than perfect figure and severe fall off
and distortion as you move towords the edge of the field. Variability between
samples is very high. Most purchasers don't notice the difference.
--
Michael Edelman http://www.mich.com/~mje
Telescope guide: http://www.mich.com/~mje/scope.html
Michael Edelman
AndersonRM wrote:
> In article <19990517151537...@ng-cc1.aol.com>, chri...@aol.com
> (Chris1011) writes:
>
> >I have a properly collimated 8"SCT with good optics and it does not
> >absolutely
> >outperform my good 4" refractor most of the time on the planets.
>
> So you contend your 4" is outperforming an 8" SCT "most" of
AndersonRM
(Chris1011) writes:
> I have seen multicoatings that have more than 5% reflectivity
>(that is more than uncoated glass). Proper application of coatings is an art
>similar to figuring of lenses.
That I have no doubt of. One look at Ruby coated binoculars
and you see half the light coming back at you.
-Rich
"Ontario liberals choose mentally-challenged man as leader;"
"News at 11:00"
AndersonRM
<m...@michdeletethis.com> writes:
>> >outperform my good 4" refractor most of the time on the planets.
>>
>> So you contend your 4" is outperforming an 8" SCT "most" of
>> the time? That's what I'd call bad seeing conditions on average....
>
>Let's see: We have Roland, who makes some of the best scopes on the planet on
>one
>side of the coin, and opposing him, we have Rich, who has apparantly never
>had an
>unexpressed thought on *any* topic, whether he has any ideas what he's
>talking
>about or not..
>
>Who to listen to? Wow, tough call.
>
>
DON'T believe what I say. Just remember never
to compare a 4" apo next to a decent 8"SCT under good seeing
conditions and you can keep this stupid myth going forever.
Some people might want to exercise some free-will and do the comparison
themselves..
Chris1011
I had the great 10" Aries Mak out again tonight against the 7"f9 refractor and
my very nice 5.5" Mak-Cass (25% obstruction). Starting at about 7 pm with the
sun on the horizon, all three instruments showed good detail on the planet. The
seeing really started to improve around 7:45 as Mars rose higher in the sky.
The 5.5" showed excellent detail at around 230x, but sort of pooped out at
350x. Detail was still there, but beginning to soften a bit.
The 7" refractor running at 400x with a 4mm Ortho showed a very sharp ball with
lots of surface detail. Since I'm an optician and not a planetary observer, I
don't know Chryse from Syrtis Major. Therefore I concentrated on the polar cap.
Where the 5.5" showed it to be a bright dot with a hint of ovality, the 7"
refractor clearly showed it as an elongated area with the western portion
seeming to taper off to a thin point.
The big Mak showed more detail than the refractor in short tantalizing moments
at 463x. The polar cap was also elongated, but the western portion seemed to
have a break or gap in it before tapering off. Can anyone verify what I saw?
These views lasted no more than 15 minutes. The seeing got steadily worse and
the big Mak started to fall behind the refractor. By 9pm, the Mak showed little
or no detail on the polar cap, just a white spot bobbing back and forth like a
crazy person on speed. Amazingly, the 7" was again hardly affected, still
showing almost as much detail as earlier. The 5.5 lost steam, and even at
only 200x showed that wooly edge as the jet stream tore up the image.
Since our seeing in the spring seems to be best around 8pm, I should get better
views as Mars rises higher in the early evening sky. Even though the size of
the ball will diminish, I believe there will be lots to see with the 10" Mak
for quite some time. Tonight I had my first tantalizing glimpse of the level of
detail the scope can provide, conditions being perfect.
Roland Christen
Robert
Great post, thanks for the info.
With descriptions like the one below, I think you'd write a great Mars Atlas. :)
"By 9pm, the Mak showed little or no detail on the polar cap, just a white spot
bobbing back and forth like a crazy person on speed."
Rob
AndersonRM
>After you have read a few more of Rich's posts, you will squirm with
>embarassment for supporting his opinions.
>
>Del Johnson
Yes in Del's black and white world, you are either completely
right all the time, or wrong all the time.
AndersonRM
(Chris1011) writes:
>Amazingly, the 7" was again hardly affected, still
>showing almost as much detail as earlier. The 5.5 lost steam, and even at
>only 200x showed that wooly edge as the jet stream tore up the image.
I don't understand how seeing could adversly effect a 5.5" scope, a 10"
scope but not a 7."
tomd...@my-dejanews.com
[snip]
> These views lasted no more than 15 minutes. The seeing got steadily
worse and
> the big Mak started to fall behind the refractor. By 9pm, the Mak
showed little
> or no detail on the polar cap, just a white spot bobbing back and
forth like a
> crazy person on speed. Amazingly, the 7" was again hardly affected,
still
> showing almost as much detail as earlier. The 5.5 lost steam, and
even at
> only 200x showed that wooly edge as the jet stream tore up the image.
>
Has anyone analyzed obstruction vs. seeing? Bad seeing degrades
the incoming wavefronts, distorting, advancing, retarding portions
of the wavefront. If a telescope has a large obstruction which blocks
the center of the incoming wavefront, then the portion of the wave-
front which contributes to the image might have a poorer spacial
correlation than if the center of the wavefront was not masked off.
--
Tom DeMary
tomd...@my-dejanews.com
--== Sent via Deja.com http://www.deja.com/ ==--
---Share what you know. Learn what you don't.---
Dan McKenna
tomd...@my-dejanews.com wrote:
> Has anyone analyzed obstruction vs. seeing? Bad seeing degrades
> the incoming wavefronts, distorting, advancing, retarding portions
> of the wavefront. If a telescope has a large obstruction which blocks
> the center of the incoming wavefront, then the portion of the wave-
> front which contributes to the image might have a poorer spacial
> correlation than if the center of the wavefront was not masked off.
>
The image quality is a function of D/r0 where D is the telescope diameter
and r0 is the Fried parameter.
For Visual or 0.5 um wavelengths the relationship between r0 and seeing
is
r0 seeing
10 cm 1.0 arc second
20 cm 0.5 arc second
5 cm 2.0 arc seconds
etc.
Thus for a 5 inch such as the Pickering seeing scale is based on
(see http://user.mc.net/arf/seeing.html)
seeing D/ro P scale
0.25 .31 9
0.5 .63 8
1.0 1.27 6
2.0 2.54 4
The p scale rating was done by looking at simulated seeing on a computer
screen and maybe rated different using a eyepiece etc.
So, a 16 inch would have a D/r0 3.2 time a 5 inch
for example 1 arc seeing in a 5 inch = D/r0 of 1.27= a P6
the 16 inch would operate at a D/r0 of about 4 which is less than P2
If I did the simulations correctly.
It is useful to look at the performance of a optical system in the spatial
frequency domain. Its kind of like the tone controls on a sound system.
With out a atmosphere and ideal optics the diameter is the high frequency
control and the central obstruction is the low frequency control. The
atmosphere is a high frequency control, a big one.
There are lots and lots of books on this.
Dan
David H. Roberts
(epoch 27april1999) at
http://oposite.stsci.edu/pubinfo/pr/1999/22/index.html
Has anyone seen this?
Chris1011 wrote:
> More observations with obstructed and unobstructed scopes:
>
> I had the great 10" Aries Mak out again tonight against the 7"f9 refractor and
> my very nice 5.5" Mak-Cass (25% obstruction). Starting at about 7 pm with the
> sun on the horizon, all three instruments showed good detail on the planet. The
> seeing really started to improve around 7:45 as Mars rose higher in the sky.
>
> The 5.5" showed excellent detail at around 230x, but sort of pooped out at
> 350x. Detail was still there, but beginning to soften a bit.
>
> The 7" refractor running at 400x with a 4mm Ortho showed a very sharp ball with
> lots of surface detail. Since I'm an optician and not a planetary observer, I
> don't know Chryse from Syrtis Major. Therefore I concentrated on the polar cap.
> Where the 5.5" showed it to be a bright dot with a hint of ovality, the 7"
> refractor clearly showed it as an elongated area with the western portion
> seeming to taper off to a thin point.
>
> The big Mak showed more detail than the refractor in short tantalizing moments
> at 463x. The polar cap was also elongated, but the western portion seemed to
> have a break or gap in it before tapering off. Can anyone verify what I saw?
>
> These views lasted no more than 15 minutes. The seeing got steadily worse and
> the big Mak started to fall behind the refractor. By 9pm, the Mak showed little
> or no detail on the polar cap, just a white spot bobbing back and forth like a
> crazy person on speed. Amazingly, the 7" was again hardly affected, still
> showing almost as much detail as earlier. The 5.5 lost steam, and even at
> only 200x showed that wooly edge as the jet stream tore up the image.
>
> Since our seeing in the spring seems to be best around 8pm, I should get better
> views as Mars rises higher in the early evening sky. Even though the size of
> the ball will diminish, I believe there will be lots to see with the 10" Mak
> for quite some time. Tonight I had my first tantalizing glimpse of the level of
> detail the scope can provide, conditions being perfect.
>
> Roland Christen
--
David H. Roberts
William R. Kenan, Jr. Professor of Astrophysics
Physics Department, Yalem 326, MS057
Brandeis University
Waltham, MA 02254 USA
d...@quasar.astro.brandeis.edu, http://www.astro.brandeis.edu
Ratboy99
>scope but not a 7."
>-Rich
I think it has to do with the configuration. The jet stream wreaks havoc on a
catadioptric. So a 5.5" could be hampered (fuzzed out) while a larger refractor
(7" in this case) would still be affected, but the image is still there like a
flag waving. The 10" looks through over half again the air column, the 7" has
the advantage by nature of its smaller size. How did I do?
rat
~( );>
WHALEN44
So Chris,
When are the rest of us going to get a chance? Also what size secondary
obstruction does your 10" have?
Richard Whalen
whal...@aol.com
Time spent observing the heavens is not deducted from your lifespan
Chris1011
When are the rest of us going to get a chance? Also what size secondary
obstruction does your 10" have?
>>
Before offering anything like this for sale, I had to make sure that it is a
worthwhile project. After all, you can buy 10", 11" and 12" SCTs dirt cheap
(with mounting yet). This Mak is very well corrected, not only for the middle
of the visual spectrum, but all the way from deep red to the far violet. This
is not what you get in an ordinary Mak-Cass or SCT. I had to make sure that
this would make an appreciable difference that would justify the much higher
price. For the 10"correctors, we are starting off with $600 blanks of pure BK7
crown of guaranteed high homogeneity. This then has to be shipped to Aries in
the Ukraine at a shipping cost that is quite exorbitant. Similar with the
quartz mirror blanks. Then the whole kaboodle has to be shipped back after
fabrication. We also have to pay the opticians so they can make a living.
Then it gets carefully assembled here into a precision tube. The costs are very
high. Of course the performance is also going to be very high. Is the amateur
community ready for this?
I'm convinced that these scopes will raise the bar a few notches. Speaking of
notches, I saw a definite notch in Mar's polar cap with the 10" Big Mak last
night. The cap had structure, it wasn't just an oval. It came to a point on one
side and had a gap. I just uploaded an image taken by Hubble on April 27 that
another poster pointed out this morning. It shows the polar cap with the gap as
I described on an earlier post last night. Also the dark area surrounding the
pole was easily visible in the 10".
We plan to offer 10" f13.5 and 12" f13.5 at first. Smaller sizes will come
later. Central obstructions will be very small, as small as possible. Aries is
gearing up to make the optics this year. As soon as they tell me that their
operation is ready, I will offer them. Until then, I can not accept any orders.
Roland Christen
Sue_and_Alan
My impression is that obstructed instruments are more affected by seeing
than those lacking central obstructions. And it certainly makes sense that
they would be.
Clear skies, Alan
AndersonRM wrote in message
<19990519024132...@ngol02.aol.com>...
>In article <19990518224648...@ng-cs1.aol.com>,
chri...@aol.com
>(Chris1011) writes:
>
>>Amazingly, the 7" was again hardly affected, still
>>showing almost as much detail as earlier. The 5.5 lost steam, and even
at
>>only 200x showed that wooly edge as the jet stream tore up the image.
>
Dan McKenna
Ratboy99 wrote:
> >I don't understand how seeing could adversly effect a 5.5" scope, a 10"
> >scope but not a 7."
> >-Rich
>
Some times the "tube seeing" causes noticeable differences in the optical
performance. Maybe the thermal design of the seven inch is better. ??
R. Kantrowitz
rings at the expense of the central disk. I purchased a 6-inch Intes MK-66
a couple of months ago and tested it simultaneously against my 1986 4-inch
F/6 Astrophysics scope. When the seeing was poor, the diffraction rings
merged with the central disk, increasing the minimum resolvable image.
Under these conditions, the 4-inch and 6-inch had comparable resolution.
When the seeing was good, the 6-inch was able to resolve closer double stars
and easily split 52 Orionis.
I also tested against my 5-inch AP Starfire, but not simultaneously. On a
night of very good seeing, the 5-inch easily showed the companion to Antares
at 300x and higher separated by dark sky. The MK66 also did this under very
good seeing conditions, but the split was more difficult because the
diffraction rings of Antares and the companion were bright enough to merge
together when the seeing worsened.
Ralph K.
tomd...@my-dejanews.com wrote in message <7hutlm$cld$1...@nnrp1.deja.com>...
>Has anyone analyzed obstruction vs. seeing? Bad seeing degrades ...
Brian Tung
> My impression is that obstructed instruments are more affected by seeing
> than those lacking central obstructions. And it certainly makes sense that
> they would be.
Could you kindly elaborate on that? It's not obvious to me why that is.
Let me make my question a bit more precise. Let's suppose you have an
obstructed instrument--say, an SCT--and an unobstructed instrument--a
refractor. Suppose that they are such that in a vacuum, they exhibit
equal image quality. If the seeing is bad, do I understand you right,
that the SCT will degrade more than the refractor?
If I do understand you correctly, can you explain how that happens?
Thanks!
Brian Tung / byron elbows
br...@isi.edu (What, me worry about spam?)
Astronomy Corner at http://gost.isi.edu/brian/astro/
C5+ Home Page at http://gost.isi.edu/brian/astro/c5plus/
Dan McKenna
Brian Tung wrote:
> Could you kindly elaborate on that? It's not obvious to me why that is.
>
> Let me make my question a bit more precise. Let's suppose you have an
> obstructed instrument--say, an SCT--and an unobstructed instrument--a
> refractor. Suppose that they are such that in a vacuum,
> they exhibit
> equal image quality.
That's the rub, they don't have the same image quality.The point spread function
and thus the contrast transfer function is a little different
for the two cases.
Dan
Dave & Frani Pisak
Chris1011 wrote in message <19990519183639...@ng-da1.aol.com>...
. Of course the performance is also going to be very high. Is the amateur
>community ready for this?
yes
Sue_and_Alan
A perfect, unobstructed telescope puts 84 percent of the light from a star
in the central disk and 16 percent into the rings (with 91 percent in the
disk plus first ring). Placing a central obstruction in the system moves
light from the central disk into the rings, thus reducing contrast. Image
abberations also remove light from the central disk.
From this it seems that....
1. An obstructed instrument must be of somewhat higher quality to work as
well as an unobstructed one. (Since the obstructed instrument starts with a
bit of a handicap.)
2. Seeing can be more of a problem in an obstructed instrument. Seeing
distorts the incoming wavefront, resulting in increased image abberations.
Again, the obstructed instrument had a bit of a handicap to begin with, so
increasing the image abberations is more likely to move it out of
diffraction limited performance.
Yes, I am saying that poor seeing will hurt an SCT more than a refractor.
But if the wavefront distortions caused by seeing are very large, the slight
advantage of an unobstructed system over one with a modest obstruction may
not be enough to help.
Clear skies, Alan
Brian Tung wrote in message <7hvkqh$3...@dot.isi.edu>...
>Alan French wrote:
>> My impression is that obstructed instruments are more affected by seeing
>> than those lacking central obstructions. And it certainly makes sense
that
>> they would be.
>
>Could you kindly elaborate on that? It's not obvious to me why that is.
>
>Let me make my question a bit more precise. Let's suppose you have an
>obstructed instrument--say, an SCT--and an unobstructed instrument--a
>refractor. Suppose that they are such that in a vacuum, they exhibit
Chris1011
Tonight the seeing improved over last night and the split in the polar cap
(Rima Tenuis as I was reminded by Steven Keene) was obvious in both the 10" and
7". Even the 5.5" showed it as a notch in the white oval. This gap is well
situated now at 9pm, being nearly in the middle of the cap. It should be
visible for the next week. It is really cool looking.
With the seeing now beginning to favor the 10", I can see the hair thin split
better in the 10"Mak than in the 7" refractor. Both instruments show it as an
intense dark line, with the Mak showing it a bit thinner at the same power. The
refractor shows surface detail with more contrast, however. It will take
another step up in seeing stability before the Mak closes that gap.
Roland Christen
WHALEN44
amateur
>community ready for this? >>
Roland,
I think you already know the answer to that one! As long as the cost's are kept
somewhat reasonable (say around 8k for the 10"and around 10k for the 12") you
should be able to sell quite a few. On the other hand, if a 10" is going to
cost 12k I doubt you will sell more than a few dozen.
>We plan to offer 10" f13.5 and 12" f13.5 at first. Smaller sizes will come
later. >>
Sounds like you have put the 8" project on the back burner for a while? I
thought perhaps with your new equipment, you would be able to turn out the
8" model without taking up so much of your time from your APO's.
Anyways, the 10" & 12" sound very exciting, perfect for the home observatory in
a location with excellent seeing. What do you have your 10" mounted on?
Martin Brown
Brian Tung wrote:
> Alan French wrote:
> > My impression is that obstructed instruments are more affected by seeing
> > than those lacking central obstructions. And it certainly makes sense that
> > they would be.
>
> Could you kindly elaborate on that? It's not obvious to me why that is.
It should be the case, and most noticable when the scale length of the
atmospheric phase screen disturbance is about half the diameter of the aperture.
> If the seeing is bad, do I understand you right,
> that the SCT will degrade more than the refractor?
Yes - but how easy it would be to see in practice is another matter.
> If I do understand you correctly, can you explain how that happens?
The central obstruction has removed some power from the low to middle of the
spatial frequency spectrum which is less affected by the seeing. So it
accentuates the visibility of the fluctuations in seeing in much the same way any
high pass filter enhances noise.
Regards,
Martin Brown
AndersonRM
<Sue_and_A...@email.msn.com> writes:
>2. Seeing can be more of a problem in an obstructed instrument. Seeing
>distorts the incoming wavefront, resulting in increased image abberations.
You could fill a big book with this stuff. All the variations in what
seeing conditions do to a star, planet or deepsky object, especially
in a large scope. How does it effect a diffraction image, let alone
extended detail on a planet. How aberrated optics interact with
seeing. How centrally obstructed telescopes deal with seeing.
How about a picture book, with all possible scope and seeing
combinations and results? Example: Given an 8" SCT with
1/4 wave of aberrations composed of this much x, x, x, aberrations,
with 3 year old multicoated aluminum on the primary and secondary
mirrors, subjected to this specific seeing type, using this eyepiece,
with a person whose eyes have "x" density of floaters....
I figure, 4,000-5,000 pages, maybe 10 volumes at a cost of
about $2,000! :)
AndersonRM
<Sue_and_A...@email.msn.com> writes:
>Rich,
>
>My impression is that obstructed instruments are more affected by seeing
>than those lacking central obstructions. And it certainly makes sense that
>they would be.
This would be so simple to prove using a high-res video
camera, a big refractor and a fabricated central
obstruction. But, I haven't got a big refractor, yet.
Jeff
Rima Borealis. The longitude of the split you see is near to 70
degrees, not 140 as the Rima Tenuis would be.
Jeff Beish
Chris1011
Rima Borealis. The longitude of the split you see is near to 70
degrees, not 140 as the Rima Tenuis would be.
Jeff Beish>>
Thanks, Jeff. Check out the Hubble photos on the Nasa web site. There are 2
splits showing on their photo. The prominent one is the one I saw. There is a
shorter one almost directly opposite which was not visible last night.
Roland Christen
tomd...@my-dejanews.com
Dan McKenna <mck...@as.arizona.edu> wrote:
>
>
> tomd...@my-dejanews.com wrote:
>
> > Has anyone analyzed obstruction vs. seeing? Bad seeing degrades
> > of the wavefront. If a telescope has a large obstruction which
blocks
> > the center of the incoming wavefront, then the portion of the wave-
> > front which contributes to the image might have a poorer spacial
> > correlation than if the center of the wavefront was not masked off.
> >
>
> The image quality is a function of D/r0 where D is the telescope
diameter
> and r0 is the Fried parameter.
relating the Fried coherence diameter to seeing. I don't see how this
relates to unobstructed aperture vs obstructed aperture for
the same seeing conditions. The effect of blocking the center portion
of the wavefront will certainly depend on the spacial coherence
of the incoming wave. If the wavefront is perfectly correlated, there
is no effect (not talking about diffraction here). If the correlation
diameter is 1 inch, then every one inch circle in the aperture is
seeing a different version of the wavefront, and an unobstructed
aperture might be expected to produce an poorer image than an
obstructed one because it sees more perturbations of the wavefront.
I am speculating, not pontificating.
Clive Gibbons
Ratboy99 <ratb...@aol.comet> wrote:
>>I don't understand how seeing could adversly effect a 5.5" scope, a 10"
>>scope but not a 7."
>>-Rich
>
>catadioptric. So a 5.5" could be hampered (fuzzed out) while a larger refractor
>(7" in this case) would still be affected, but the image is still there like a
>flag waving. The 10" looks through over half again the air column, the 7" has
>the advantage by nature of its smaller size. How did I do?
>rat
>~( );>
Then again, I think Roland was inferring in his post, that the central
obstructions of the 5.5 and 10" scopes contributed to the "wooly"
appearance of the image when seeing was less than optimal. This does make
some sense, since the obstruction throws light from the Airy disk into the
diffraction rings. When seeing is poor, the bright first ring will boil
and smear, which would really mess up planetary detail. OTOH, an
unobstructed scope has a faint first ring, so even if bad seeing fuzzes it
out, it's subtlety won't degrade contrast and detail as badly.
Howzat?? :)
--
Clive Gibbons
Technician, McMaster University,
School of Geography and Geology.
lude...@my-dejanews.com
fcat...@aol.com (Fcathell) wrote:them. I also> high quality
scopes. I personally am very interested in seeing some of the
> upcoming 4" APO vs Meade ETX-127 "shootout' comparisons.
>
> Frank
>
Hi Frank,
we have done such an comparation last weekend on our german starparty
at Mars with an good quality 4" Apo, an 6" maksutov Cassegrain with 34
% Central Obstruction and an 127F/6 Maksutov Newtonian with 23% central
Obstruction.
Seeing was excellent in 1 night
The 4" Apo have had all the time racersharp Marsimage at 250 power
The 6" Mak-Cass. showed same amount of details but with slightly less
stable image
The 5" Maksutov Newtonian also showed same amount of details and image
stability was better as in 6" Mak-Cass. and slightly less than the 4"
Apo.
Thats means an 4" Apo of normal quality will blow away behind the moon
any 5" Maksutov Cassegrain with an 33 % central obstruction.
best wishes
Markus
Michael Spooner
I've been doing fair video for years with several scopes and documenting
seeing has been one of my goals. I have a medium sized refractor (9")
but proabbly an APO would be better due to its color correction. F/15
and f/10 ratios seem more stable than f/5 at times. More confusing to me
have been the nights when my 10" f/5.5 Newts give better images than the
refractor but other nights the results are just the opposite. Local
seeing possibly explains some of this. There is apparently better
contrast in the refractor more often but some of this is due to the
uncoated lens and three reflections that cut light levels. On nights
when I can push 600x with the Newts, planetary images have superb
contrast so I wonder if some complaints of washed out images with
central obstructions may result from inadequate magnification to
moderate the brightness. If I can remember (a major problem at times
<g>) to try the obstruction on the refractor, I video some Mars and see
how it compares.
Mike Spooner
tomd...@my-dejanews.com
aperture has a messier wavefront to image after the center is
punched out.
--
Tom DeMary
tomd...@my-dejanews.com
lude...@my-dejanews.com
Total throughput = 70%.
A modern SCT will have 94% mirrors
Rich we measured at the university of Saarbrücken last year an brandnew
Meade 8" LX 200 mirror for reflectivity and it was 90%, to have an
94~96% reflectivity you need an dielectric coating.
and
> 98-99% at the corrector surfaces.
with matched modern multicoatings you are right for 98%, but 99%. An BK
7 corrector loss on 10 mm thickness about 1% and on each surface 0.5%.
Roland even not includet the big light loss due the spherical aberation
present in each modern and old Schmidt Cassegrain ( maybe excapt a few
luckyly good optics)
So a new SCT will give
> 76% or better. That gives it the throughput of a 7" refractor.
Yes 7" Refractor , and how do you explain that an MN61 with 6" show
better deep sky details ?
Markus
> On deepsky objects, that's how it will behave, minus some
> contrast due to the secondary.
And some contrast lost due oversized baffleling and some contrastlost
due spherical aberation and some contrast lost due vigneting from the
primary baffle, if the backfocus is to far outside.
> -Rich
>
> "GO SEE STAR WARS P.M.;
> PUT ANOTHER NAIL INTO THE
> COFFIN OF QUALITY SCIENCE FICTION"
Ratboy99
>
>
>
>
>--
> Clive Gibbons
Man, we're rockin'. Pretty soon we'll be up there with Ludes. Never catch
Roland though. Anyway I meant to say that the 10" looks through double the air
column of the 7", not half again. I know that the size of the air 'cells' has
to do with it (poor seeing), but I have a hard time conceptualizing the air as
being cells some inches in diameter. Is there a fluid (ie. water) example?
rat
~( );>
lude...@my-dejanews.com
Michael Edelman <m...@michdeletethis.com> wrote:
>
>
> AndersonRM wrote:....
>
> Let's see: We have Roland, who makes some of the best scopes on the
planet on one
> side of the coin, and opposing him, we have Rich, who has apparantly
never had an
> unexpressed thought on *any* topic, whether he has any ideas what
he's talking
> about or not..
>
> Who to listen to? Wow, tough call.
Hi Michael,
thats not fair.Rich have his experience and Roland have his experience,
both are right. Roland talk any time about bad cool down time, thats
mostly an wrong statement to advantage his apos and his new Mak-designs.
I have looked in germany and austria through very good optical quality
8" Schmidt cassegrains (which is of course very seldom) and this
particular good 8" SC have had no cool down problem and performed on
Planets similar as 6" Apos.
Addional I can tell you from my observings in the Florida's keys, where
any time any scope ( correspondance to Roland mostly Schmidt
cassegrains) must have cool down problems, because all the night
cooling down, that there was an superlative perfect 8" Celestron , made
in 1973 (polished by Bobb Geoff) with normal mechanics and this perfect
8" Celestron snapped in at 700 power and showed same or even slightly
more amount of details as an AP 7"F/7 EDF of excellent quality.
The seeing was almost perfect and there the 8" aperature wins.The
resolution is limited by aperature not by light throuput. of course the
7" Apo have had an brighter image, but the 8" SC have had the better
resolution.
Correspondance to Roland it is impossible, because this 8" SC should
have even more trouble with cooling down.
Whom to believe now? an manufactor who takes advertising for his own
design or somebody who speak just about his daily experience ?
Both are right about here impressions, yes, but Roland is not right
about his long time talking of any time cooling problems in Schmidt
Cassegrains.
Best wishes
Markus
lude...@my-dejanews.com
"Del Johnson" <a...@aaa.com> wrote:
> After you have read a few more of Rich's posts, you will squirm with
> embarassment for supporting his opinions.
>
> Del Johnson
>
Del,
"some" of Rich's postings give me more informations , than most of your
postings
Dan McKenna
Ratboy99 wrote:
The short answer is air is the fluid .
The fried parameter that I talked about is the result of the propagation
of light through the atmosphere. It does not correspond to cell sizes in the
air but rather the effect of all the air in the beam.
The speed of light or refractive index in air depends on the density.
so at a given altitude it's related to the air temperature.
Imagine if you will, a column of air all at the same pressure, with temperature
variations that are moving at the wind speed. A point source at infinity will
produce plane wave fronts that when traveling through the air will distort
due the the different velocities of propagation. So if you could measure the
phase of the light at two points as the air moved across your phase detectors,
like an interferometer, as you moved the detectors apart you would notice
that the correlation between the two would become less as you moved the
detectors apart. at some point they would be uncorrelated .
The "cell size" relates to this distance and a variation of this distance
is known as the Fried parameter after Dr. Fried. It is also known as
r0 or, r not, (are too).
At visual wave lengths r0= 10 cm for 1 arc second, 5 cm for 2 arc seconds.
Did I do better this time ?
most of the time I tend to be clear as mud.
Dan
lude...@my-dejanews.com
> I think you already know the answer to that one! As long as the
cost's are kept
> somewhat reasonable (say around 8k for the 10" and around 10k for the
12") you>
Hi Richard,
I think your price estimation is right close to what they will sell.
My 10" opticsets from quarz with Zygo tested 1/8 wavefront cost $ 5,500
and the 12" cost US $ 6,890, now ad the transportation cost, some
duties, the tubemechanics and some Roland's profit and , yes you will
be in that area.
If you are fanatic in quality, than I think the price is still low. An
12" Mak of superaltive quality for the price of an 7" Questar, thats
realy inexpensive.
best wishes
Clive Gibbons
>In article <19990518023423...@ngol03.aol.com>,> >shading.
>Total throughput = 70%.
> A modern SCT will have 94% mirrors
>
>Rich we measured at the university of Saarbrücken last year an brandnew
>Meade 8" LX 200 mirror for reflectivity and it was 90%
Somehow, I'm not surprised by that... ;)
Meade likes to sing the virtues of their "EMC" coatings, but I can't find
any stated % efficiency for their mirror coatings. Do the LX200 owner's
manuals go into such details?
Meade might give the *impression* that their mirrors *should* be approx.
94% reflective, but perhaps this is just another example of their
advertising implying something that really isn't the case.
Recall the S&T review of SCTs in Dec. '89 and Jan '90. I think those
measured throughputs opened some people's eyes, despite what the
advertising of the time suggested. If only a magazine would do the same
quantitative tests, today.
Ratboy99
>most of the time I tend to be clear as mud.
>
>Dan
So if your looking at the bottom of the pool through the water, are the cells
the light markings that form the reflected shapes? I mean like the reflections
that the water casts on the wall, or are those reflections of actual waves, or
both?
rat
~( );>
Fcathell
when they are available. In theory the ETX should have better images (note I
said in theory!).
Frank
Ratboy99
>8" Schmidt cassegrains (which is of course very seldom)
That's really the crux isn't it?
rat
~( );>
Dan McKenna
This is where the details get thick. The patterns or caustics on the pool bottom
is a very compelling image, especially on a hot arizona day, unfortunately
it does bring home the concept that r0 is the sum of all the phase shifts
through the column instead of surface ripples acting as lens like entities.
r0 is driven by temperature gradients stirred by the cascade of mechanical
turbulence. " big whirls make little that feed on their velocity,
little whirls make smaller swirls and so on to viscosity" that sort of thing.
When I started thinking about seeing while looking at out of focus images
I too thought they looked like the pool caustics. life was simpler then.
Dan
Ratboy99 wrote: So if your looking at the bottom of the pool through the water,
Del Johnson
to lower costs. A mildly distorted smaller Airy disk is no worse than a
perfect big one.
Del Johnson
Sue_and_Alan wrote in message <7hvsbj$p...@news2.newsguy.com>...
>1. An obstructed instrument must be of somewhat higher quality to work as
>well as an unobstructed one. (Since the obstructed instrument starts with
a
>bit of a handicap.)
Chris1011
said in theory!).
Frank>>
Which theory? Relativity? Seriously, why should this Mak design be any better
than a well made SCT of the same size?
Roland Christen
Ratboy99
>I too thought they looked like the pool caustics. life was simpler then.
>Dan
You're making my head hurt man.
rat
~( );>
Fcathell
Meade has a 30% or somewhere in that range (% of diameter). According to some
of your earlier posts - all other things equal, the smallest central
obstruction should have the best contrast.
Frank
AndersonRM
>Meade 8" LX 200 mirror for reflectivity and it was 90%, to have an
>94~96% reflectivity you need an dielectric coating.
Well, they use "enhanced aluminum" which is supposedly 94%
reflective. 90% is only 2% better than raw aluminum so I wonder
what they put over it?
> So a new SCT will give
>> 76% or better. That gives it the throughput of a 7" refractor.
>Yes 7" Refractor , and how do you explain that an MN61 with 6" show
>better deep sky details ?
That I'd have to see, especially which deepsky objects you mean.
Perhaps globulars (resolution of tighter star points) but galaxies
and nebula are another question. According to some, the
larger central obstruction should hurt deepsky images as
much as planets, but I don't find this to be the case.
An 8" SCT is still brighter than any 6" scope of any design
i've looked through, the image just isn't as "good" as a good
refractor, Newtonian, etc.
AndersonRM
(Ratboy99) writes:
>I know that the size of the air 'cells' has
>to do with it (poor seeing), but I have a hard time conceptualizing the air
>as
>being cells some inches in diameter.
Air what? :) The cells concept is back.
AndersonRM
writes:
>If I can remember (a major problem at times
><g>) to try the obstruction on the refractor, I video some Mars and see
>how it compares.
With a 9" refractor, it would be an ideal test subject since is
bigger than an 8" SCT. What I think the various experiments
do indicate is there are many different types of seeing problems
and they effect images differently in different scopes.
I even remember once putting my hand on the side of a tripod
and seeing the image of Jupiter "sharpen up" because the
vibrations imparted to the tripod and scope by my hand were
counteracting a particular amplitude and frequency of seeing
error.
AndersonRM
(Chris1011) writes:
>Which theory? Relativity? Seriously, why should this Mak design be any better
>than a well made SCT of the same size?
>
>Roland Christen
Probably because Celestron SCT's STILL suffer from
rough optics, though maybe not to the extent they did
in the past.
AndersonRM
(Fcathell) writes:
>Good report Markus. I wll be interested in a C-5 versus ETX-127 comparison
>when they are available. In theory the ETX should have better images (note I
>said in theory!).
>
What should also be included in these tests is another telescope
of the same aperture with known excellent optics.
AndersonRM
(Clive Gibbons) writes:
>Meade likes to sing the virtues of their "EMC" coatings, but I can't find
>any stated % efficiency for their mirror coatings. Do the LX200 owner's
>manuals go into such details?
Interestingly enough, I can see nice purple coatings on
Meade correctors, but zero coating colour on my C9.25"
I wonder what multicoating material yields no colour?
Steffen Kluge
say 33% obstruction? Percent of diameter or surface area?
Thanks
Steffen.
--
Steffen Kluge <kl...@fujitsu.com.au>
Fujitsu Australia Ltd
Keywords: photography, Mozart, UNIX, Islay Malt, dark skies
--
Jeff
and ice forming near the edge of the cap. The Rima Tenuis runs across
the NPC from 140 to 320 degrees longitude. It usually shows up early
in an Aphelic apparition around 65 degrees Ls, but will be reported
during this season as well. It was rediscovered in 1979 and the last
it was reported, at least given our blessings, was during the last
apparition. That is still in question also. The problem is that Rima
Tinuis has not been imaged by space machines and it is a mystery why
and why not. Bad timing I suspect. I will look at the NASA page.
Nice observation......
Jeff
On 20 May 1999 14:05:12 GMT, chri...@aol.com (Chris1011) wrote:
>>>Rima Tenuis has not been obsered this apparition. What you see is
>Rima Borealis. The longitude of the split you see is near to 70
>degrees, not 140 as the Rima Tenuis would be.
>
>Jeff Beish>>
>
>Thanks, Jeff. Check out the Hubble photos on the Nasa web site. There are 2
>splits showing on their photo. The prominent one is the one I saw. There is a
>shorter one almost directly opposite which was not visible last night.
>
>Roland Christen
zgse...@netwayz.com
>Excuse my ignorance but what are people referring to when they
>say 33% obstruction? Percent of diameter or surface area?
Hi Steffen:
Obstructions are usually expressed as a percentage of the diameter of
the primary mirror. (In the case of an 8-inch telescope, a 33%
obstruction would be about 2.6-inches across.) If you see a number
that is suspiciously low (10.6%), then you're likely seeing it
expressed as a percentage of area - which is how you do it when you're
trying to convince someone that the size of the obstruction isn't too
big. : )
Hope this helps,
Gary
orion501
have a Coulter Odyssey 10 inch Dob. Is Martian detail simply beyond the
capabilities of my scope, or do I just need a darker site and more patience?
Jeff <jbe...@earthlink.net> wrote in message
news:37457a3d....@news.earthlink.net...
Sue_and_Alan
It is generally stated as a percentage of diameter. Some manufacturers
state it as area, which sound better (but anyone reading carefully should
notice this).
Clear skies, Alan
Steffen Kluge wrote in message <7i395s$e...@newshost.fujitsu.com.au>...
tomd...@my-dejanews.com
Dan McKenna <mck...@as.arizona.edu> wrote:
snip
>
> Ok, I tend to be confusing lets break it down and put it back
together.
>
> A 10 inch at 0.5 microns has a point spread function with a .42 arc
second
snip
Thanks. The explanation was clearer that time.
--
Tom DeMary
tomd...@my-dejanews.com
lude...@my-dejanews.com
ander...@aol.com (AndersonRM) wrote:>
> >Which theory? Relativity? Seriously, why should this Mak design be
any better
> >than a well made SCT of the same size?
> >
> >Roland Christen
>
> Probably because Celestron SCT's STILL suffer from
> rough optics, though maybe not to the extent they did
> in the past.
> -Rich
>
Roland talks about an well made SCT 5", he not say any 5" Celestron. I
remember from the past time an very well made 5" SCT from japan. This
older japanese 5" SCT have been better in optics and better in
mechanics than the todays known SCT. Now comparaing this " well made
SCT" to the ETX, than Rolands statement is fully correct.
best wishes
Markus
lude...@my-dejanews.com
ander...@aol.com (AndersonRM) wrote:
> In article <7i1oik$2...@mcmail.cis.McMaster.CA>,
gibb...@mcmail.cis.McMaster.CA
> (Clive Gibbons) writes:
>
> >Meade likes to sing the virtues of their "EMC" coatings, but I can't
find
> >any stated % efficiency for their mirror coatings. Do the LX200
owner's
> >manuals go into such details?
>
> Interestingly enough, I can see nice purple coatings on
> Meade correctors, but zero coating colour on my C9.25"
> I wonder what multicoating material yields no colour?
> -Rich
>
I am not an expert of coatings, but my opinion is that the coating is
than the best, if you look from front to the tube and you can nearly
not see the lenses or schmidt plates. I fightet more than year with
INTES MICRO to exchange here bad reflection coating to an new green
multicoating. The visual result is now , that the moon looks white and
not as before with an touch green ( was noticable only by the most
crazy german guys i know). Now you can look into the tube easily. Just
see the pictures of my webside. Scopes with the old coating show nearly
nothing from inside, scopes with new coatings, looks like without Mak-
Corrector.
On of the best coatings I have seen in scopes have been the MC of the
Zeiss APQ's. Coatings from Meade looks like the coating from AP and
from ARIES and sounds to be excellent.
Best Wishes
lude...@my-dejanews.com
>
> That I'd have to see, especially which deepsky objects you mean.
> Perhaps globulars (resolution of tighter star points) but galaxies
> and nebula are another question. According to some, the
> larger central obstruction should hurt deepsky images as
> much as planets, but I don't find this to be the case.
> An 8" SCT is still brighter than any 6" scope of any design
> i've looked through, the image just isn't as "good" as a good
> refractor, Newtonian, etc.
Rich, okay thats your observing impression, my impression is diffrent.
Spherical abberation, present a lot in most of the todays SCT smires a
lot. The image maybe looks brighter to you, but how about the visible
details?
Several years ago an Peter Ceravolo HD145 impressed me so much, that i
ordered the HD 216. This little 145 mm scope showed at the bright Orion
nebula more and fainter details than an side staying 12" LX 200.
My 10" Mak-Newt. kick the 16" LX 200 on each object to the north pol.
Aperature is only usefull , if the optics are good. Contrast and
quality winns any time against aperature with low quality.
Markus
> -Rich
>
> "Ontario liberals choose mentally-challenged man as leader;"
> "News at 11:00"
>