Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

SCT's don't make good planetary scopes.

This topic has been archived. This means that you cannot reply to this topic.
255 replies to this topic

#51 mblack

mblack

    Throw me a bone here

  • *****
  • Posts: 7,549
  • Joined: 31 Oct 2005

Posted 12 March 2008 - 08:34 PM

I love SCTs for their virtues - the unique combination of affordability, portability,mountability and availability. I don't count optical superiority by design as one of the virtues.


Who said that? Wet noodle lashes coming! ;)

#52 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 81,130
  • Joined: 16 Jun 2004

Posted 12 March 2008 - 08:41 PM

I cannot tell you how many dozens of times a member of the public (non-astronomer) will comment that the view though a 10" or 12" commecial Dob or 4" commercial-grade APO is "sharper", "clearer" or "better" than the planetary view through an adjacent 12", 8" or 6" SCT. Hardly "5%" or "Nth degree" differences when a complete neophyte with no pre-conceived notions, and no experience, easily sees the difference predicted by the science.



I am not a big fan of SCTs but I have owned a few. The differences that are the most noticeable to me are not the optical differences but rather the thermal differences. Most Newtonian owners are quite aware of both collimation issues and thermal issues and make an effort to address both.

In contrast, most SCT's owners I setup next to seem to pay no attention to thermal issues and often seem to believe that "my scope never needs collimation, I haven't had to collimate it for X years."

It's no surprise that a 12.5 inch F/4.06 with fans that have been running an hour and is collimated so it is right on the money can outperform a 12 inch SCT.

But I am sure that your guys and gals here pay attention to the collimation and do your best with the thermal issues. A great scope that is not properly setup is not going to perform. On the other hand, start with a decent scope and prepare it so it can do it's best, add some good seeing, that's what it takes.

Jon

#53 Make-it Better

Make-it Better

    Vostok 1

  • -----
  • Posts: 165
  • Joined: 30 Jun 2007

Posted 12 March 2008 - 08:45 PM

I've been very impressed with the planetary views of my c11 since I made a artificial star (placed 2 blocks away across a grassy field) and followed these instructions -- http://www.astrosurf...ult/collim.html Near perfect collimation is critical on an SCT for planetary viewing. Of all the SCT's I've looked through over the years at most 5% were anywhere near being perfectly collimated. I think Astronomy clubs would do their membership a great service by having a SCT collimation night using an artificial star.

My 2 cents,

#54 mblack

mblack

    Throw me a bone here

  • *****
  • Posts: 7,549
  • Joined: 31 Oct 2005

Posted 12 March 2008 - 08:53 PM

But I am sure that your guys and gals here pay attention to the collimation and do your best with the thermal issues.


:bow:

A globular cluster of stars looks great in just about anything over 10", regardless of design, collimation or seeing.

Planets require even more in SCT's. Beyond collimation done by an experienced eye, there is no getting around thermal currents in the closed system.

Lymax coolers, or any other device that brings the internal air friction to rest in a closed system is just as important.

Or move south ;)

#55 Joad

Joad

    Wordsmith

  • *****
  • Posts: 18,211
  • Joined: 22 Mar 2005

Posted 12 March 2008 - 08:59 PM

I'm coming to believe that "seeing" is king, especially with planetary viewing. On a series of nights last summer the seeing on Mt. Pinos was exceptional, and my 8 inch SCT produced marvelous Jovian views. But I will admit that generally it is not a strong planetary performer. On good seeing nights I have been regularly pulling in Trapezium E and F, though, with inexpensive eps, and this makes me very happy with what now counts as a very modest piece of equipment.

#56 scrubmonkey

scrubmonkey

    Explorer 1

  • -----
  • Posts: 66
  • Joined: 09 Feb 2008

Posted 12 March 2008 - 10:34 PM

Good night seeing, (for me) the bene's of the SCT over the NEWT are:

1 - Comfort, Comfort, Comfort
2 - Longer Focal Length (a bit more bang for the buck in mag)
3 - the capability of using a 2"ep vs being stuck with 1.25's (bad comp I know, but I am comp'ing the Astroview 6)
4 - Saturn: focal length helped out a lot.
5 - Moon: Fantastic, much more clear and in the proper orientation! (thanks to the diag)
6 - Double clusters were superb, 20% more clear (maybe the XLT starbright coatings?) than the Astroview 6. (same EP 25mm)
7 - Orion nebula, view was 20% more clear. (same EP 25mm)

The SCT was in Collumnation out of the box, I double checked the Newt with the laser.

You get what you pay for, a $700 setup (Celestron SCT) compared to a $420 (Orion Newt)

The SCT for me wins, but this was not an entirely fair comparison for the Astroview. I'm still getting a 12" dob this summer to add to my instrumentation (can't beat the price!)

Good Night and Good Luck

Mike

#57 Ken Hutchinson

Ken Hutchinson

    Ranger 4

  • *****
  • Posts: 341
  • Joined: 24 Apr 2003

Posted 12 March 2008 - 11:12 PM

FYI the Strehl tests I can easily find on line for SCTs come in at:

0.85
0.87
0.92
0.81
0.89

Those are all better than a quarter wave, not much worse. The last one was done by an interferometer, the others are Roddier results. I was out at a club observing session during the "once in 60k years" close approach of Mars. I'd done my homework, my NS11 was properly cooled and collimated. I had quite a few people tell me that I had the best view on the field. I could turn one way and see two 18"+ Obsessions and turn the other way and see two 150 mm AP refractors. I think the Dobs may have been crippled by warm mirrors, the need to collimate every time you set one up, and the fact that the Dob guys in my club always, always, always want to set up on nice superheated asphalt whereas I set up on grass whenever possible. The refractors were on the grass not far from me however.

Ken

#58 TG

TG

    Vanguard

  • *****
  • Posts: 2,397
  • Joined: 02 Nov 2006

Posted 13 March 2008 - 12:13 AM

I Most production SCT's have primaries of 1/4 wave at best (if you are lucky!), probably the same as production


*If* the manufacturers' claim of having at least 1/4 wave (at the wavefront) optics is to be believed (and there's ample evidence from various people testing SCTs using Roddier and interferometry that this is indeed the case for a significant fraction of SCTs), then your assertion can't be true unless manufacturers can magically turn out perfect secondary mirrors and correctors but somehow mess up the primaries.

I suggest you read Robert Piekiel's eBook about Celestron (he sells it on A*mart) to see how much work goes into making an SCT -- at least at Celestron. It's *not* a matter of grinding a bunch of optical components and throwing them together.

Regards,

Tanveer.

#59 TG

TG

    Vanguard

  • *****
  • Posts: 2,397
  • Joined: 02 Nov 2006

Posted 13 March 2008 - 12:18 AM

I've often thought that I wish I still had my NexStar 8i so that I could compare it side-by-side to my Intes-Micro 703D. Now that I understand more about collimation and seeing, I wonder if the soft images I was getting from the SCT, particularly on the planets, was due to imperfect collimation and/or lousy seeing. But here's the thing: I know my Mak is not yet perfectly collimated and the seeing is rarely good here. Still, the views I had of Jupiter last year through the Mak, when collimation was more off than it is now, were much more detailed than the views of Jupiter through the 8i.

So I throw out this question: is it possible that SCTs are more sensitive to imperfect collimation than Maks? If so, why? Just curious.


I believe the Intes Maks are of the Rumak design with a separate secondary. As such, they're coma-corrected scopes and much less susceptible to small miscollimations. An run-of-the mill SCT has the same amount of coma as an f/5 Newt, and it shows up if the scope is even slightly miscollimated, destroying contrast. Note that an SCT will have coma toward the edges of the field even when perfectly collimated but a Rumak should have none at all.

Regards,

Tanveer.

#60 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 81,130
  • Joined: 16 Jun 2004

Posted 13 March 2008 - 05:55 AM

FYI the Strehl tests I can easily find on line for SCTs come in at:

0.85
0.87
0.92
0.81
0.89



These would be the Strehl numbers for the optics themselves rather than for the telescope as a whole. As I remember it, the Strehl for a optically perfect telescope with a ~35% CO is about -.80 so these numbers above would have to be corrected for the CO to get the overall Strehl.

Jon

#61 amys

amys

    Mercury-Atlas

  • *****
  • Posts: 2,733
  • Joined: 12 Jun 2006

Posted 13 March 2008 - 06:33 AM

I think Astronomy clubs would do their membership a great service by having a SCT collimation night using an artificial star.


I have a new Astrozap articifial star. I'm going to propose this to my club this weekend. Excellent idea, especially since I've been struggling to reach collimation nirvana with my Mak.

#62 Eddgie

Eddgie

    ISS

  • *****
  • Posts: 25,406
  • Joined: 01 Feb 2006

Posted 13 March 2008 - 08:41 AM

I had a somewhat similar experience. On the occasion of the Mars close approach I went out to the local dark sky observing site one night. There were a dozen telescopes out. Large dobs to VERY high end APOs up to about 5".

On that particular night, I felt that the amount of detail I saw in my NexStar 11 was FAR better than any other telescope present.

Now the Big dobs were probably not collimated well.

The refractors were just to small to resolve the dark detail as effectivly as the C11. And ONE of those telescopes cost more for the OTA than I paid for the the complete NexStar 11 AND the Eyepieces I was using.

And on that particular night, Mars wasn't even up high yet, so the seeing conditions were not great due to the somewhat low altitude of Mars (45 degrees above horizon maybe).

But again, here we are talking about very small refractors compared to a fairly large SCT. As I mentioned earlier, I don't know if the C11 would have done as well up against my current 20+ year old 6" AP refractor. Ah, but once again, I think the the C14 WILL do a bit better. And interestingly, I paid about the same amount of money ($3200 for the C14 and the same for the 6" AP).

So, SCTs DO in fact make excellent planetary scopes when compared to refractors that cost the same or big dobs with excellent mirrors that can't be kept in good collimation.

That is my story and I'm sticking too it.

#63 walt r

walt r

    Soyuz

  • -----
  • Posts: 3,611
  • Joined: 13 Feb 2007

Posted 13 March 2008 - 12:37 PM

Now the Big dobs were probably not collimated well.

The big Dobs were probably not cooled well. That's the biggest down side and performance buster of my 18".

#64 bobhen

bobhen

    Soyuz

  • *****
  • Posts: 3,560
  • Joined: 25 Jun 2005

Posted 13 March 2008 - 02:02 PM

Okay, I’ll dip my toe in the water on this one.

I’ve had an AP155 and a C11 at the same for eight years. I have compared them (visually) many times on the same night.

I live in southeastern PA and the seeing is average or worse about 70% of the time, good 15% and excellent maybe 5%.

When it comes to planetary viewing, I now always reach for the refractor first - but it took a little while (read a few years) for me to come to this conclusion so a few quick looks here or there at a star party might not tell the whole story. Your living conditions and seeing will also have a major impact on your scope selection.

I do agree with Mr. Yoshida from Japan when he listed the AP155 slightly ahead of the C11 - again, not by much but enough. Also remember, we’re comparing a 6-inch scope to an 11-inch scope - that’s a huge difference in aperture. Also, in “A C14 Confronts an AP 180” by Jay Reynolds Freeman reviewed here on CN, the author concedes that the refractor was delivering more low contrast planetary detail that is so important in planetary viewing – again, close but observed. In the end, the SCT’s CO (yes it does have a negative impact), mass production, the folded light path, the light scatter of reflective optics, etc. all have a negative impact that results in “slightly” softer images with less low contrast details as compared to the APO.

In most cases (from my comparison and the 2 articles I referenced) I believe it would take an SCT of TWICE the aperture to deliver more planetary details than an apo. In other words, it will take a 14-inch SCT to deliver more planetary detail than a 6 inch APO. This is also confirmed in this thread by the poster Eddgie when he states, “But again, here we are talking about very small refractors compared to a fairly large SCT. As I mentioned earlier, I don't know if the C11 would have done as well up against my current 20+ year old 6" AP refractor. Ah, but once again, I think the C14 WILL do a bit better.”

The SCT’s greatest quality is its compact design allowing for a large aperture in the shortest tube. So you can overcome the inherent SCT disadvantages by getting a very large aperture - and for less money! But (there’s always a but) you start to need better and better seeing as you increase aperture – and most locations don’t have seeing that can support a14-inch aperture on a regular bases (it might only be a few nights a year) And even then the planets will need to be positioned high in the sky. So if you like to view the planets on a more regular bases and get as much as you can from each session, you start leaning toward the best quality optics of “reasonable” aperture that will deliver great views on the “vast majority” of nights – Apos in the 6 to 8-inch class (maybe 5-inch as well), long focus 8-10-inch (F8) high quality Newtonians, high quality 8-10-inch compound scopes with low CO (less than 25%) and 6-8-inch, long-focus F15 Achromats, All of these scopes will deliver better planetary views than the “equivalent” aperture typical SCT and will usually best the large aperture SCT on most “average” nights. But when given the chance and conditions are optimum, the large aperture SCT is no slouch.


Bob

#65 deezdrama

deezdrama

    Vanguard

  • *****
  • Posts: 2,144
  • Joined: 29 Mar 2005

Posted 13 March 2008 - 04:23 PM

Good reading! nice arguments from all aspects- I cant comment too much as the only sct ive had was a nexstar8i and the only apo a c80ed- I never had many good planetary views with my sct - but back then when i had it i never probablly collimated it properly and may of not gave it time to cool, i love the sharp contrasty images in my c80ed but definatelly not enough aperture for high power detailed views- My discovery dob is great and opticaly wonderful but the dob mount is a pain to track at high powers, so i sold the dob(this sunday actually) and for the past month had to decide on my next scope that would be used probally 70% of the time on planets- I narrowed it down to a mid sized apo or a large SCT- I ended up getting a meade 127ed that had issues so i returned it and bought a 12" meade lx200 that is suppossed to be optically "as good as they get" (former arkansas observatory sky patrol scope that was dr. clays scope with hand matched optics) it is flocked and had a fan mod done to it. I get the scope next week and hope it will be a great planetary performer but i realize seeing will play the biggest role. I hope i made the right descision- it was either go with a optically perfect 12" sct- or go with a mid size refractor that although smaller would do better in bad seeing conditions. Guess ill just have to save for a refractor again so i can compare the both.

#66 Ken Hutchinson

Ken Hutchinson

    Ranger 4

  • *****
  • Posts: 341
  • Joined: 24 Apr 2003

Posted 13 March 2008 - 04:24 PM

FYI the Strehl tests I can easily find on line for SCTs come in at:

0.85
0.87
0.92
0.81
0.89



These would be the Strehl numbers for the optics themselves rather than for the telescope as a whole. As I remember it, the Strehl for a optically perfect telescope with a ~35% CO is about -.80 so these numbers above would have to be corrected for the CO to get the overall Strehl.

Jon



The Strehl ratio for an optically perfect telescope of any degree of central obstruction is 1.00

These adjusted Strehl ratios that penalize an obstructed telescope for its central obstruction are an attempt to generate a single number figure of merit for comparing telescopes. That adjustment alone does not result in a useful number since it does not account for aperture. It is extremely popular with those who are determined to prove the only a refractor is worth having, for obvious reasons.

Ken

#67 deezdrama

deezdrama

    Vanguard

  • *****
  • Posts: 2,144
  • Joined: 29 Mar 2005

Posted 13 March 2008 - 04:29 PM

Speaking of strehl ratios- i was told the the strehl of my 12" lx200 that i just bought has a perfect strehl ratio- is this possible?

Also what is the % of CO in the older meade lx200's?

#68 Bill McHale

Bill McHale

    Viking 1

  • -----
  • Posts: 583
  • Joined: 07 Nov 2004

Posted 13 March 2008 - 04:47 PM

Sorry for jumping on this thread late...

My first statement is this; the best scope for any purpose is the one you actually use. A little 4" Apo that gets used regularly is better than an 8" Apo that gets used once every couple of years :).

So for those people who find Newtonians too uncomfortable to view through, an SCT might well indeed be the best choice for planetary viewing.

Overall though, a well executed Newtonian or Refractor should be able to beat it on the planets. The fact that many SCTs are not executed as well as they should be (either by manufacturing, or by user failure in cooling or collimation) is that so many people seem to think of MCTs as a real step up for planetary work. Granted the f/15 versions often have smaller secondaries which can help, but even the f/10s are often considered more desirable. On axis, which is what counts for planets, an f/10 SCT and MCT with similar size C.O. should perform nearly identically.

But put me in the camp who prefers a Newtonian. I have seen Newtonians come very close to matching Apos of equal aperature in terms of planetary performance. And they performed far better than any SCT I of similar aperature I have looked through.

--
Bill

#69 jrbarnett

jrbarnett

    Eyepiece Hooligan

  • *****
  • Posts: 30,138
  • Joined: 28 Feb 2006

Posted 13 March 2008 - 08:14 PM

Anything is "possible"...

Probable? No.

Regards,

Jim

#70 Ken Hutchinson

Ken Hutchinson

    Ranger 4

  • *****
  • Posts: 341
  • Joined: 24 Apr 2003

Posted 13 March 2008 - 08:46 PM

Here is a chart that illustrates why merely adjusting a Strehl ratio for the central obstruction does not give a useful result. Plotted are a perfect 155 mm aperture (refractor); a perfect 280 mm, 17% obstructed aperture; and a 280 mm, 33% obstructed aperture (C11) with three different Strehl ratios. Even the C11 made on a bad day should do well against the refractor, if yours doesn't it isn't cooled or collimated properly. Any sane telescope figure of merit would be based on the area under the MTF curve plotted against sky resolution not focal plane resolution because that will reward big scopes for being big while still penalizing obstructed scopes (but only to the degree warranted) for being obstructed.

Ken

Attached Thumbnails

  • 2256300-MTFcomparison.jpg


#71 Stephen65

Stephen65

    Viking 1

  • -----
  • Posts: 941
  • Joined: 14 Apr 2007

Posted 13 March 2008 - 10:47 PM

I'll qualify my comments by saying that my f/12 Mewlon 250 is not an SCT, it's a CT, but that said so far it has outperformed (at least to it's owner's eyes) on Mars and Saturn:

- a 5" APO (FLT-132)
- a C11 and a C14
- f/4-5 newtonians ranging from 8" to 16", including some with premium quality mirrors and custom designed baffling etc

What it does require though to achieve that performance is precise collimation, good seeing conditions and proper cooling. In poor seeing my 5" APO will do better.

#72 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 81,130
  • Joined: 16 Jun 2004

Posted 14 March 2008 - 02:48 AM

These adjusted Strehl ratios that penalize an obstructed telescope for its central obstruction are an attempt to generate a single number figure of merit for comparing telescopes. That adjustment alone does not result in a useful number since it does not account for aperture. It is extremely popular with those who are determined to prove the only a refractor is worth having, for obvious reasons.

Ken



My point is that there is a Strehl for the quality of the optic but there is also a Strehl for the system, what is the wave front error? That is the question the Strehl asks. When considering a measure of optical quality, it is necessary to include not only the optical precision of the execution of the design but also the effects of the design itself as they relate to a perfect telescope of a given aperture.

As you point out, this is most useful in comparing telescopes of the same aperture, typically this would be comparing Newtonians with SCTs. An 8 inch Newtonian with a Strehl for the optics of 0.95 and an 18% CO will provide quite different planetary images than a SCT with an "optical Strehl" of 0.95 but a Central obstruction of 35%.

Jon Isaacs

#73 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 81,130
  • Joined: 16 Jun 2004

Posted 14 March 2008 - 03:04 AM

Here is a chart that illustrates why merely adjusting a Strehl ratio for the central obstruction does not give a useful result. Plotted are a perfect 155 mm aperture (refractor); a perfect 280 mm, 17% obstructed aperture; and a 280 mm, 33% obstructed aperture (C11) with three different Strehl ratios.



The way I look at it, the chart shows that a good 280mm Newtonian with a 17% Central Obstruction will provide substantially better planetary performance than a C-11. This is the value of the Strehl when considered as a measurement of the system rather than the optic alone.

The old rule of thumb (Aperture-C0) suggests that the C-11 would provide planetary views comparable to a 187mm APO, that would be an interesting chart.

Personally I think all scopes are good, people enjoying the stars, the planets, that's what this is all about. I believe the goal in discussions like this one are to help people, myself included, the why's and what ifs...

jon

#74 Paul G

Paul G

    Cosmos

  • *****
  • Posts: 8,638
  • Joined: 08 May 2003

Posted 14 March 2008 - 06:41 AM

These adjusted Strehl ratios that penalize an obstructed telescope for its central obstruction are an attempt to generate a single number figure of merit for comparing telescopes. That adjustment alone does not result in a useful number since it does not account for aperture. It is extremely popular with those who are determined to prove the only a refractor is worth having, for obvious reasons.


Strehl is independent of aperture. It's the other way around -- the "adjusted Strehl" that removes the effects of the central obstruction is misleading. One can remove the central obstruction from the Strehl calculation, but pretending the co doesn't exist doesn't eliminate its effects on the image. The 35% obstructed scope still has a reduction of brightness of the airy disc from 84% to 64% with a resulting increase in brightness of the first and subsequent diffraction rings. There should be two terms: "True Strehl" where 1.0 puts 84% of the light in the central airy disc and can be used to compare scopes of any design, and "Adjusted Strehl" that removes the effects of the co and is really only valuable in comparing one scope with another of the same size co.

This effect of the SCT's large central obstruction is to lower planetary contrast. It's worsened in suboptimal seeing because it doesn't take much atmospheric turbulence to smear the dimmer airy disc and the bright diffraction rings together into a larger blob, whereas in the unobstructed scope the diffraction rings are so faint they do not visibly extend the size of the disc. In addition the rough (relatively) optics of a machine polished SCT cause scatter, further smearing out the image and causing a glow around the planet.

Numbers aside, it's what you see at the eyepiece that really counts. Like Bob, I had an AP 155 and a very nice C-11, a hand picked Company 7 scope that was well collimated. As a long time lunar and planetary observer with an interest in detail I spent a lot of time comparing them. What I saw between the two scope is that on the best of nights when the large aperture wasn't affected as much by seeing the 6" refractor still showed better low contrast planetary detail than did the SCT.

What might have been more surprising is that the 155 was a better scope for open and globular clusters, my other passion. Well, not so surprising when one looks at the efficiency of the airy disc. A 35% central obstruction reduces light transmission 18%. This was a mid to late 90's C-11 so being very generous figure a light transmission of 90% per mirror, another 5% lost at the corrector (absorption and reflection), and the added loss of airy disc brightness due to diffraction by the central obstruction. Adding all of the transmission losses results in an airy disc efficiency of about 52%. Now this assumes that the "Adjusted Strehl" of this scope is 1.0 or 100%, perfect optics. Being generous and assuming a 1/4 wave system scope, the airy disc efficiency drops another 25%.

I've also spent many hours comparing the C11 and several 10" SCT's with a 10" Mak-Cass with a 23% central obstruction. The MCT bested the SCT's by a considerable margin in every way (except price and availability :) ). Experienced observers typically comment first on the contrast in the MCT. But this is not an apples to apples comparison, either, since the MCT has very smooth optics figured to better than 1/10 wave system, no coma, and lacks the spherical abberation and astigmatism of SCT's. How would a high quality SCT fare? It would be interesting to do a side by side with a Tak SCT to see how the images compare. Unfortunately, the market wouldn't support an SCT at that price point and only a small number were sold.

In response to the original poster, I would not say that SCT's are poor planetary scopes. A good SCT is a good planetary scope. Are there better? Certainly, but every scope is a compromise in some way or another. For me, the C11 was the largest, most affordable scope I could comfortably carry by myself, mount on a reasonably priced mount, and observe seated in comfort. I recommend an 8" SCT on a GOTO mount as the perfect beginner's scope -- low frustration level while learning the sky, a good performer for all targets, enough aperture that they may never outgrow it, and its suitability for photography and other nonvisual pursuits.

YMMV greatly, of course...

#75 Ken Hutchinson

Ken Hutchinson

    Ranger 4

  • *****
  • Posts: 341
  • Joined: 24 Apr 2003

Posted 14 March 2008 - 08:54 AM

The way I look at it, the chart shows that a good 280mm Newtonian with a 17% Central Obstruction will provide substantially better planetary performance than a C-11.


We could quibble about "substantial" but other than that the chart is indeed telling you that the Newt is better. My point is that what would your "system Strehl" tell you? This goofball adjusted Strehl will tell you that the 155 mm refractor is the best of the three! Look at the chart. Is it the best of the three? You would expect that at the lower frequencies it would be as good as the C11, otherwise it is the worst of the three. So what is the value of this "system Strehl"???

Trying to capture the performance of a complex system in a single number is bound to be an exercise in frustration. Using the total area under the MTF curve would be a better choice than adjusting the Strehl ratios for only the central obstruction. However for planetary observing some sources claim that this low contrast detail that everyone is chasing is mostly in the lower frequencies. If that is true then the refractor truly is as good as the C11 because the frequency range where the C11 beats it doesn't come into play. The problem is that your "system Strehl" predicts a result that is not true.

Ken


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics