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Is a masked SCT like an APO for viewing Mars?

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#51 Rich N

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Posted 14 October 2005 - 12:14 AM

Nor will this classic review, where a 7" Newt beats a 5" AP in image quality (mystique doesn't count).


Well, it sounded like it was pretty much of a draw on planetary viewing. And, the Newt was considerably larger than the refractor. They were also having to work around those nasty Newt defraction spikes. ;)

Darrell, bring you CAT out to Montebello OS some night. Let me know when you are coming and I'll do my best to get up there with my APO. I think my APO will give your CAT a run for its planetary money. :)

Clear skies,
Rich

#52 snorkler

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Posted 14 October 2005 - 12:15 AM

You're serious about the SN-10? I went so far as to check Meade's Factory Outlet store to look for myself, figuring that was your tongue-in-cheek part.

Yes, there was agreement that a masked SCT produces APO-like sharpness, but the thread switched to the fact that the unmasked SCT shows more detail.

#53 sixela

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Posted 14 October 2005 - 06:49 AM

The article does reinforce the consensus that it only takes an 8" reflector to beat a 6" APO in seeing detail, both in DSOs and in planetary viewing.


But those reflectors (with central obstructions of around 20%) are entirely different beasts from the SNs and SCTs that we were discussing in this thread, are they not?

I myself hinted at the important distinction between these two previously...


From Thierry Legault's site:

http://www.thrushobs...truction_files/

Posted Image

The *green* line is about an SCT. The *blue* line is a Newtonian reflector. They don't look the same.

#54 sixela

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Posted 14 October 2005 - 07:20 AM

You clipped my comments, which should have left no doubt that I didn't mean "the" MTF. The part you clipped said

The obstructed scopes have consistently higher resulting MTFs once you drop below 3 arc second detail, and the larger Newtonian beats the APO all the way on absolute MTF.


I'm glad we're in violent agreement; I think the devil is in the interpretation, as I took the first part of your sentence as unnecessarily reductionist (and thus bolder and different) statement not entirely explained by the rest of the paragraph, and it's the reductionism I objected to.

In the same vein, your statement:

Unless I'm reading this wrong, it indicates that APO superiority is only perceived, not quantifiable by MTF in any case.


is strange: to say the exact way in which an APO *can* be superior is not quantifiable, on the basis of a graph where the APO is superior on more than half of the length of the graph plotted is puzzling to say the least.

Yes, there is ample context, and I'm sure you didn't mean to write this, but the paragraph still does look like an incorrect and inflammatory (pseudo-)summary not justifiable by the context.

On the matter of the extra context, I also happen to think that the input pattern used to determine the MTF at 3 arcseconds (A) is not the same as any planetary "3 arc second detail" (B).

Going from B to A requires the application of some Fourier transforms that will change the points of the MTF function that are important in determining the system response, unless you define "detail" as something that must look like the ubiquitous CTF/MTF bars on TV test cards with a 3 arcsecond periodicity ;).

I'm sorry if I appear to be bordering on the insistent, but I really hate incorrect generalisations with a vengeance, though, I can assure you, not their authors. That's because these incorrect generalisations have a habit of turning into very tenacious myths.

Just to put things in perspective: I write all of this in tempore non suspecto, as I have zero refractors and two reflectors, and immensely enjoyed looking at Jupiter through C9.25s before I had my large aperture Newt.

However, I find it entirely unnecessary to deny *any* real advantage other designs might offer in certain contexts.

#55 JerryWise

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Posted 14 October 2005 - 07:25 AM

You're serious about the SN-10? I went so far as to check Meade's Factory Outlet store to look for myself, figuring that was your tongue-in-cheek part.
..........


Sure. They sold out night before last but one of the buyers backed out. I had already ordered one of the SN-6s as a result of this thread. They upgraded me to the one the buyer backed out on and it should be here shortly. The tongue in cheek part was referring to the 10 inch beating an 8 inch APO. I think my RCX 10 inch has outstaning views. So do the SCTs. But they just seem to take a lot more work to get the outstanding views.

I think the diffrent styles suit different preferences and conditions. Like last night. I mounted up the C-14 for the first time in months. First thing I do is go to Vega and put a Hartman Mask on. Then slew to M-13. Detail is a little weak. So then its find a suitable star and start collimating. Tweak, check, tweak, check and so on. Finally it's at the best it will get. Go to Mars and the first thing I think, "I wonder if collimation is really dead on or do I maybe have a tube current". With the Tak or Meade 102ED I just secure the OTA on the mount, do a quick star test and we are off and running. It's all relative but I think it's fair to test all affordable alternatives. The SN-10 has a lot of solid supporters and you can get in the game for a remarkable $449 so let's give it a try.

Meade Email:
------------------------------------
Meade Confirmation:
We received the additional payment and the order for the SN-10 is all clear to ship. Enjoy the scope!

Meade Instruments Corporation
The largest-selling telescopes in the world.

#56 snorkler

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Posted 14 October 2005 - 07:38 AM

But those reflectors (with central obstructions of around 20%) are entirely different beasts from the SNs and SCTs that we were discussing in this thread, are they not?


They're only different to a degree, in these theoretical discussions. Your link was only to the diagram that was clipped from http://www.thrushobservatory.org/ , so the text that accompanies it is informative. It says

The figure below gives the curves corresponding to reflectors obstructed at 20 % and 33 %. The extension of the left part of each curve shows to which unobstructed telescope the reflector is equivalent in contrast in the low frequencies. It appears that in these frequencies a reflector with an obstruction of 33 % is equivalent to an unobstructed instrument whose diameter is 33 % less (170 mm for 250 mm). A reflector with an obstruction of 20 % is equivalent to an unobstructed instrument whose diameter is 15 % less (210 mm for 250 mm).

. So it only means that instead of an 8" Newtonian equaling or bettering the performance of a 6" APO, you have to go to a 10" SCT to better the 6" APO. In the context of this discussion, where we were talking about a C11 masked to 3 or 4", my original assessment that the unmasked SCT would better the performance of the unmasked 3-4" APO is still quite valid.

#57 sixela

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Posted 14 October 2005 - 08:46 AM

So it only means that instead of an 8" Newtonian equaling or bettering the performance of a 6" APO, you have to go to a 10" SCT to better the 6" APO.

We're in violent agreement.

In the context of this discussion, where we were talking about a C11 masked to 3 or 4", my original assessment that the unmasked SCT would better the performance of the unmasked 3-4" APO is still quite valid.


Of course, and I won't deny that's perfectly true.

#58 snorkler

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Posted 14 October 2005 - 09:13 AM

We're in violent agreement....

Of course, and I won't deny that's perfectly true.


Thank goodness. I thought I was going to have to fly to Belgium, learn to speak Flemish or Walleroon, and buy you a beer to smooth things over between us :D :o

#59 snorkler

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Posted 14 October 2005 - 09:26 AM

You got yourself a couple of bargains. I paid nearly $100 more to have my SN-10 _repaired_, as used SN-10 OTAs typically sell for $550-600. I bought my SN-6 from Meade's factory outlet site a little over a month ago, and paid $40 or $50 more than they're selling for now.

I've found the SNs to be nearly ideal for astrophotography. Stars are round to the corners, they're free of chromatic aberration, they show little vignetting, and they're f/4 to f/5 for short exposure times. Although I've heard some complaints that they don't hold up to high magnifications visually, that's not an issue for prime focus astrophography.

#60 Rich N

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Posted 14 October 2005 - 02:34 PM


"So it only means that instead of an 8" Newtonian equaling or bettering the performance of a 6" APO, you have to go to a 10" SCT to better the 6" APO."

I think the above statement needs a little qualification. You are talking about two "paper" telescopes. I don't think you are taking into consideration the usual quality of SCTs from the big two SCT makers and you are assuming the SCT is having no problem with "cool down".

I mention this because I think it is a disservice to the readers to let them expect a common 10" SCT to equal or better the planetary views in a fine 6" APO on a regular basis (frequently). I base my opinion on many nights at local star parties with my APOs and looking through other telescopes at these star parties, including SCTS.

I think SCTs are a great value. I own a C8. A few times I've seen a few C8s show nearly as much planetary detail as my 6" APO. I've yet to see a larger SCT come as close to showing as much planetary detail.

Rich

#61 sixela

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Posted 14 October 2005 - 02:45 PM

I don't think you are taking into consideration the usual quality of SCTs from the big two SCT makers and you are assuming the SCT is having no problem with "cool down".


And collimation. But having looked through a few Cats, I think the issues are:

1. cooldown (very hard to control)
2. collimation (once you know how to do it and have Bob's knobs, it really isn't that hard)
3. (very distant third) Strehl ratio of optical system when collimated and cooled.

1. is very dependent on climate, *and* on the size of the SCT. It's a lot easier to cool a C9.25 than it is to cool a C14, especially if you don't have a Lymax cooler.

I'm actually surprised at how many 10-12" SCTs have no active cooling in the field, despite the fact that most larger Dobsonian Newt users these days *know* they need a fan somewhere, even though their open tube design makes it *less* hard to cool.

But I've observed through a perfectly collimated C9.25 cooled with a Lymax cooler, and on good nights it delivered superb planetary images despite the 36% central obstruction (of course, it does have a slower and easier to figure primary, and is relatively less hard to collimate as a result).

Though not images *better* than those from a very good 6" APO, one which doesn't need sacrificing to the collimation and cooldown gods to deliver its best images (it merely requires sacrificing an arm and a leg to the scope's manufacturer ;) ).

I almost ended up buying a C9.25 before I picked a large Dob (which, on good nights and after even more substantial sacrifices to the God of Cooldown Bearing Four Fans, crushes a C9.25 and any APO on planetary detail, but only after more than an hour or so, and only in stupendously good seeing).

I don't think we're doing anyone a disservice by stating that a well-sized SCT is capable of beating an APO, as long as we mention what it takes (and that the APO does *not* require the same level of effort).

Mentioning these issues even may help the large body of SCT owners out in the field that have scopes that don't perform to their limits.


#62 Rich N

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Posted 14 October 2005 - 05:14 PM

"I don't think we're doing anyone a disservice by stating that a well-sized SCT is capable of beating an APO, as long as we mention what it takes (and that the APO does *not* require the same level of effort)."

"Mentioning these issues even may help the large body of SCT owners out in the field that have scopes that don't perform to their limits."
--------------------------------------------------------

Yes, I agree with both statements. I also agree with your comment about active cooling and seldom seeing it used in the field. I think it is a great idea to use an active cooling system for the larger SCTs.

Rich

#63 snorkler

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Posted 14 October 2005 - 08:38 PM

Well, it sounded like it was pretty much of a draw on planetary viewing. And, the Newt was considerably larger than the refractor. They were also having to work around those nasty Newt defraction spikes. ;)

Darrell, bring you CAT out to Montebello OS some night. Let me know when you are coming and I'll do my best to get up there with my APO. I think my APO will give your CAT a run for its planetary money. :)

Clear skies,
Rich


Rich,

I'm sorry for not responding earlier. I'd love to get together to view through your APO, but I use the SCT mostly for astrophotography. Almost all of my visual observing is with my Newtonians. In fact, I don't use my SCT much at all. Its focal length is too long for most astrophoto targets and it vignettes a lot. Plus f/6.3 or f/10 apertures require 4-16 times the exposure of the f/4 SN.

#64 wh46gs

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Posted 14 October 2005 - 10:37 PM

Hello Jerry,

There was just a bit of tongue in cheek in there.



I'd say so. I only used it as a starting point to
express similar views (to yours) in regard to inappropriate use of MTF graphs for perfect obstructed/unobstructed apertures as an illustration of actual telescope performance.

Btw, it's more like "welcome back" - thanks...

Vlad

#65 Rich N

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Posted 15 October 2005 - 01:42 AM

Well, it sounded like it was pretty much of a draw on planetary viewing. And, the Newt was considerably larger than the refractor. They were also having to work around those nasty Newt defraction spikes. ;)

Darrell, bring you CAT out to Montebello OS some night. Let me know when you are coming and I'll do my best to get up there with my APO. I think my APO will give your CAT a run for its planetary money. :)

Clear skies,
Rich


Rich,

I'm sorry for not responding earlier. I'd love to get together to view through your APO, but I use the SCT mostly for astrophotography. Almost all of my visual observing is with my Newtonians. In fact, I don't use my SCT much at all. Its focal length is too long for most astrophoto targets and it vignettes a lot. Plus f/6.3 or f/10 apertures require 4-16 times the exposure of the f/4 SN.


Ok. But if you feel like driving down to Montebell OS bring one of your Newts. It's always fun getting out for a look at the stars. In the spring you can see Omega Cen. from MB (Montebello). It has a low southern horizon.

Rich

#66 snorkler

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Posted 15 October 2005 - 08:30 AM

I'd say so. I only used it as a starting point to
express similar views (to yours) in regard to inappropriate use of MTF graphs for perfect obstructed/unobstructed apertures as an illustration of actual telescope performance.


I found the discussion of MTF quite enlightening. It was a good theoretical explanation of why we see what we see. Apologies if I gave the impression that the theoretical discussion was the actual facts. It merely explains the actual facts - the reason we see more detail with more obstructed aperture than with less unobstructed apterture.

I don't recall seeing anyone who disputes that more aperture wins. The disagreeent only comes regarding how much more CAT/obstructed aperture is needed to get more detail v. unobstructed aperture.

There are, of course, the inevitable digressions into refractor views being contrastier and more pleasing, and of CATs and Newtonians costing far less than refractors that give comparable views of bright objects. Those digressions, like refractors themselves, rarely enter the minds of those of us who pursue dim DSOs.

So IMHO the MTF discussion enlightened me on why a refractor performs better than an equal aperture obstructed scope. It gave me a "why" to replace the "Doh, because it dosen't have something blocking the view, stupid" explanation.

#67 wh46gs

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Posted 15 October 2005 - 01:24 PM

I found the discussion of MTF quite enlightening. It was a good theoretical explanation of why we see what we see.



The MTF presented only show contrast in perfect apertures,
obstructed and unobstructed. Any extention of this perfect
scenario into our (real) world is arbitrary, without including the effect of wavefront aberrations. This factor
is far from negligible.

I don't recall seeing anyone who disputes that more aperture wins.



Well, I do. Everything is relative. Put enough of wavefront
error into large aperture, and it will show less than 2-3 times smaller well corrected aperture. When talking about
planetary details, limiting resolution is only about half
as good as the cutoff frequency (113.4/Dmm arcsec angular equivalent), or at about the middle of MTF graph - for a perfect aperture. How aberrations and c.obstruction affect
this treshold is explained by Rutten and Venrooij, as mentioned. If you want to save time of going through it,
simply draw a line connecting 0.2 contrast level on the left
side of MTF graph with 0.65 contrast level on the opposite side. Where the plotted MTF intersects this line, there is
the resolution treshold for (MTF standardized) brightly illuminated low-contrast details. In general, an aperture
D with c.obstruction C will have contrast lowered to a
(D-C) aperture level for details about 4 times and more larger than 113.4/D arcsec. For smaller details, the MTF recovers rather quickly, resulting in considerably smaller loss in the planetary resolution treshold. For instance, a 30% c.obstruction would only lower this treshold for about 10% vs. perfect aperture.

Main disadvantage of the "perfect aperture scenario" is that
it misses to address a number of important factors that determine how actaul telescopes perform. An enlightening MTF
needs to include error sources from sherical correction, seeing, thermal imbalance, miscollimation, surface irregularities, scatter, baffling, pinching, etc. These
can differ significantly with both, telescope type and aperture size (of course, they vary with individual telescopes too). It is the only way to really understand - there is no easy way around it.

#68 sixela

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Posted 15 October 2005 - 02:36 PM

It is the only way to really understand - there is no easy way around it.


No arugment there. On the other hand, things like spherical aberration have an influence on the MTF graph that look surprisingly like the influence of central obstruction, so even this simplified discussion is pretty enlightening.

Of course, thermal effects are a different kettle of fish, and so is seeing - but we're usually interested in how scopes perform when the atmosphere cooperates.

#69 snorkler

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Posted 15 October 2005 - 08:31 PM

Vlad,

Theoretically perfect is a lot more understandable to me than putting a bunch of variables into the equation. After assuming scope A has optical fault X and scope B has optical fault Y, it approaches blue smoke and mirrors, and you can skew the data to favor either viewpoint at the extreme end of the discussion. If you're saying that a larger aperture scope with a poor mirror won't beat a perfect smaller refractor, that's all well and good for the poor soul who deliberately goes shopping for a scope with a poor mirror. I don't see how it helps the rest of us.

#70 Rich N

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Posted 16 October 2005 - 01:29 AM

Vlad,

Theoretically perfect is a lot more understandable to me than putting a bunch of variables into the equation. After assuming scope A has optical fault X and scope B has optical fault Y, it approaches blue smoke and mirrors, and you can skew the data to favor either viewpoint at the extreme end of the discussion. If you're saying that a larger aperture scope with a poor mirror won't beat a perfect smaller refractor, that's all well and good for the poor soul who deliberately goes shopping for a scope with a poor mirror. I don't see how it helps the rest of us.


Hi Darrell,

I understand your point. But, none of us are buying perfect telescopes, of a particular design, and using them under ideal conditions.

You don't seem to want to accept the reality that you aren't likely to find a virtually perfect SCT from one of the "big two" SCT makers. They make a great telescope... for the money.

AP refractors are more expensive for several reasons. One reason is the time and care spent making sure their optics are very well crafted. Astro-Physics is marketing to a different group of buyers than the "big two" SCT makers.

Beyond the quality of the optics, good sized SCTs from the "big two" have problems "cooling down" that are often more noticable than the cool down problems with a 6" AP APO refractor. And, SCTs as they come from the store may well need a little touch up of their collimation. AP refractors are dead on in collimation virtually every time you receive them from AP and they hold their collimation very, very well.

Again, I'm a little concerned about someone reading this thread and getting the wrong impression from a discussion of "paper telescopes" under ideal conditions.

Rich




#71 JerryWise

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Posted 16 October 2005 - 09:19 AM

And now for something totally different (courtesy Monte Python):

Thought I would put the "Paper Telescopes" to a real world test. I plotted a VTF (visual transfer function) on Mars and Andromeda over the last two nights testing different OTAs.

MOUNT
Celestron CGE, Polar aligned and refined.
Dovetails: All Losmandy securely attached to Losmandy “Dual” dovetail plate allowing side by side comparison of views.

TELESCOPES:
OTA 1: Celestron C-14
OTA 2: Orion ED80
OTA 3: Takahash FS-102II
OTA 4: Celestron 9.25

EPs:
Televue 13, 11 T6s, 3-6 zoom, Panoptic 35 and 19. Meade UWA 14, 8.8 and 6. Pentax 20mm XWA. Televue Powermate x5.

Cool down time: All OTAs placed in observatory with a fan blowing over them for 3 hours after sunset.

Celestron C-14 and C-9.25 collimated to highly refined level between 11:00 and 12 midnight using Televue x5 Powermate and various decreasing FL EPs.

Observations started shortly after midnight with stable air and temperature. Mars at around the 11:00 o’clock position. Losmandy dual dovetail side by side mounting plate was used to place a reflector and refractor side by side.

MARS:

(Worst OTA views to best of mars)

Worst: Orion ED80. Some detail coming in and out. Nice crisp background and stars were clear but when magged enough to see detail Mars was dim and washed out.

Next: Celestron C-14. Mars was very bright. Seemed hard to hit the "sweet spot". Mounted FeatherTouch two speed and placed Triangle and then Circular pattern Hartman mask to refine focus. Occasional detail would come and go but still looked washed out and too bright. :bawling:

Next: Takahashi FS 102-II. Bright round stars on black background (even with full moon). Mars nice bright round globe. Some detail coming and going.

Winner: Celestron 9.25. Mars bright and clear. Banding and clouds visible nearly all the time. Some polar caps coming in and out that were never seen in the other OTAs. Appeared similar with full suite of EPs with Televue 13mm T6 really holding the image. :grin:

ANDROMEDA:

Worst: Orion ED80. Small blur in field. Nice clear stars on dark background

Next: Takahashi FS 102-II. Small blur in field. Nicer clear stars in nicer dark background

Next: Celestron C-14. Nice big blur on milky background. :bawling:

Winner: Celestron C-9.25. Nice medium blur with some faint swirl details even with the full moon. Clear, crisp view. :grin: :grin:


HUMBLE CONCLUSION:

This is an actual test of OTAs on the night sky. The OTAs were ranked on real world results with a fine set of EPs by a somewhat experienced observer. The observations were all done in a sequence time frame on a side by side mount under identical conditions. The C-14 was a flyer in the results.

Argument 1: There was something wrong with the OTAs causing the results to not agree with the charts.

Response 1: Charts are not valid for real world OTAs

Argument 2: The charts perfectly predicted the results.

Response 2: Well, they kind of did until you get to the C-14s performance. Charts perfectly predicting the results is clearly not a valid response to this test.

Argument 3: This was only one test, and the scopes weren’t… and the moon wasn’t….and you didn’t look at the pink line at point .0005…..and……and……

Response 3: Hello wall.


Bear in mind, I have an SN-10 on the way as a result of this thread. I think it has convinced me, at least, to explore the pure reflectors. My own efforts to validate the charts and conclusions have made it very clear the charts can only be considered a very rough and possibly unreliable guide in scope selection. Construction, particular sample, and a host of other variables ultimately determine performance. It will be interesting to see how this plays out. “Charts right, scopes wrong”, “Then Charts wrong, can’t predict scopes”. :foreheadslap:

#72 wh46gs

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Posted 16 October 2005 - 11:29 AM

Darell,

Theoretically perfect is a lot more understandable to me than putting a bunch of variables into the equation. After assuming scope A has optical fault X and scope B has optical fault Y, it approaches blue smoke and mirrors, and you can skew the data to favor either viewpoint at the extreme end of the discussion.




Oversimplification is always easier. That doesn't make it
good. Understanding actual telescopes inevitably requires
inclusion of actual factors. It may surprise you, but it is
not complicated at all. Aberrations and other wavefront
deformations, c.obstruction, seeing - they all can be with ease incorporated into MTF as degradation factors. It results in a realistic MTF.

If you're saying that a larger aperture scope with a poor mirror won't beat a perfect smaller refractor, that's all well and good for the poor soul who deliberately goes shopping for a scope with a poor mirror. I don't see how it helps the rest of us.


Poor mirror (or optics) is not a prerequisite for poor telescope or performance.
Nobody goes shopping for a poor telescope, but not a few people do get it, sometimes with
claims of good or even excellent quality. Larger apertures
are prone to larger errors from all sources; even a perfect
primary is not a promise of good performance.

Some of routine extra error sources of a large
Newtonian are primary deformation induced by its cell, thermal imbalance,
miscollimation, diagonal inherent
error and cell/thermal deformations, seeing,
and, of course, central obstruction. Let it be 1/8 wave
p-v mirror (0.95 Strehl) to begin with. We can say it is
a very good mirror. What will, say, 20% c.obstruction do?
We can quantify this effect with the lowering of the energy
encircled within the Airy disc, given by (1-c^2)^2, "c"
being the c.obstruction relative size. In this case it gives
0.92 which, for the left-hand side of MTF graph (which is
what matters for planetary observing) can be taken
as an MTF degradation factor. Combined with the correction
error, it will result in ~0.87. Although not a Strehl,
strictly talking (since it doesn't all come from wavefront
deformation), it can be looked at as sort of it. It says
that average contrast loss over the range of MTF frequencies
is ~13%, resulting in performance level comparable to
~13% smaller (linearly) aperture.

Let's say that miscollimation is 1/20 wave RMS , and that cell induced and thermal error are 1/40 wave RMS on both
primary and diagonal. Let's throw in 1/50 wave RMS on each, primary and diagonal, on account of local surface irregularities. Since these are mostly uncorrelated errors, we can use the square root sum squared to obtain likely cumulative error. The square root of the sum of all the RMS
errors squared comes to 0.076, or 1/13.1 wave RMS combined
error. From S~1/2.72^(39.5RMS^2), the appropriate Strehl
comes to 0.79. This is the next degradation factor to add,
and it lowers the system Strehl-like figure to 0.69.

Adding a very moderate 1/14 wave RMS seeing error results
in 0.82 degradation factor, lowering system's figure to
0.57. Starting out with a very good mirror, we ended up with
a telescope performing at ~0.57 Strehl level, a 1/8.4 wave
RMS level (from RMS=0.241sq.rt(-logS)), an equivalent of
1/2.5 wave of spherical aberration.

If similar calculation would be done for a 4"-6" apo
with 1/8 wave objective, all the error sources would be considerably lower. Thus the nominal aperture difference
would shrink down to a considerably smaller actual
difference. The point is that c.obstruction alone,
even if we put it at 35%, would play only a small part in it.

Vlad

#73 JerryWise

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Posted 16 October 2005 - 12:19 PM

Anybody know if a C-14 stopped down to the same area as an FS 102 APO gives identical views? :question:

#74 Rich N

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Posted 16 October 2005 - 07:46 PM

Anybody know if a C-14 stopped down to the same area as an FS 102 APO gives identical views? :question:


Hi Jerry,

It looks like you are equipped to answer that question. :)
I think a very good test object would be Mars at relatively high power. Your FS-102 vs. your C14 with an off axis aperture mask.

Rich

#75 wilash

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Posted 16 October 2005 - 08:12 PM

Has anyone actually put any of these scopes on an optical bench and get a MTF result on the optics. Does anyone do off-axis plots for both radial and tangental resolution?

The science is fine. You can make calculations as complex and as simple as you want. No problems with that. We can look at charts and see a difference. But just because there is a difference, does not mean it can be percieved in the eyepiece. And MTF charts do not indicate if images are "pleasing" or not. BTW, square apertures can give a great MTF results, but we don't see manufacturers trying it and I am not sure we would want square Airy disks.

The thing that really bugs me about this is not that we can't enjoy the details of optics, but we never require the manufacturers to release actual MTF data on their products. Isn't it curious that all the optics sold today could have MTF data created for them. This is fairly common in photographic optics. I would like the telescope manufacturers to start supplying this. It is really bogus advertising when the performance of telescopes are limited to simple resolution calculations from aperture.


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