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Equipment Discussions >> Cats & Casses

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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: Opinion on Star Test new [Re: Eddgie]
      #5527329 - 11/19/12 04:44 AM

Quote:

I will be the first to admit that your saga has been very interesting...




Thank you for being part of it, Eddgie. For following along over the last few months, replying with your views, and putting up with occasional PM's.

Edited by Asbytec (11/19/12 08:02 AM)


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Pinbout
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Reged: 02/22/10

Loc: nj
Re: Opinion on Star Test new [Re: Asbytec]
      #5527435 - 11/19/12 07:52 AM

Quote:

Not sure how the KE would "spin."




its a test to rule out stig. if you have stig it will spin as you go thru focus.


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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: Opinion on Star Test new [Re: Pinbout]
      #5527441 - 11/19/12 08:01 AM

Ah! Okay, thanks. Considering your suggestion, how to proceed and when. Is there a web site somewhere that talks about this in some detail?

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Eddgie
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Reged: 02/01/06

Re: Opinion on Star Test new [Re: Asbytec]
      #5527587 - 11/19/12 09:59 AM

Quote:

If the model is balanced, as Vlad often says it "must" be, then the correction is very good.




And this is the issue with all SCTs and MCTs with 30% or larger obstructions.

When the obstruction is very large, it puts a lot of pressure on the optical quality. If the scope has very good to excellent optics, it can still produce a very nice image, and despite the biggish central obstruction, the damage is far less than many people seem to think.

But often when people have reported that the SCTs lack contrast, I suspect that their sample is not in the very good to excellent catagory. A small defect like 1/6th wave of SA in a refractor does little real harm to the image. Most people would not easily see it.

But because the SCT already starts with a large obstruction, the small optical defects pile on to the sag in the MTF chart and cause the scope to perform only about as well as a telescope with half the aperture.

And that may explain why some people have said they owned 8" SCTs that could not match their 4" APO, and others have reported owneing 8" SCTs that compare well with 6" and even 5.5" APOs.

Even 1/6th wave of SA combined with some surface roughness and a minor zone could make this difference.

To work well, these scopes have to have very good to excellent optical quality. Below that, and for me personally, they are not satisfying to use as compared to one with excellent optics.


Celestron seems to have stepped up in recent years though, so hopefully we are seeing serially high levels of quality. My fingers are crossed. Using a very large SCT with good optics can give a wonderful viewing experience. Of coures an 8" APO might be slighly better, but who can afford that?


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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: Opinion on Star Test new [Re: Eddgie]
      #5527757 - 11/19/12 11:32 AM Attachment (10 downloads)

You know, Suiter talks about encircled energy and scopes with large obstructions being, "on the brink." So, yes, a scope of good optical quality can "act", if you will, like a scope of less than 0.8 Sthrel.

Two things can bring it back to his "good" criteria, better correction and a smallish CO. For that, you have to have 1/8th correction and 30% CO (or 1/6th wave and 25% CO) for 84%(?) of the light to be within the Airy disc (I believe is his EER(1) radius.)

A 33% CO falls just short of 0.8 at 1/8 P-V, so maybe that's what's affecting some comparisons. MTF (total) = MTF(1) + MTF(2), etc.

Now, that's theory. I am still evaluating any improvement with a reduced CO and increased effective aperture. Very optimistically, it improved from 37% effective to about 27% actual. Immediately noticeable were improvement in star diffraction patterns. It lost one dim ring on the outer margin. That light had to go somewhere, probably back into the Airy disc. Also, lunar diffraction effects are noticeably improved.

At first, I could see no real change in planetary viewing. There is no apparent degradation, the image is at least as good. However, there has to be some improvement with a smaller CO. Question is, can it be detected. I am seeing a tad bit more in the SEB than last year. That could be experience or excellent seeing. So, the definite answer is..."maybe."


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Ed Holland
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Reged: 06/16/10

Loc: San Jose, CA and Oxford, UK
Re: Opinion on Star Test new [Re: Asbytec]
      #5528070 - 11/19/12 02:17 PM

A great thread, well worth reading

There is such a lot of information here that it is impossible to absorb everything in one pass. However, the patient amongst us will be rewarded. Rarely is there one single factor at work in an assembly with many optical surfaces, yet we are lucky these days that good instruments are available and affordable.

Based on one or two other threads and personal experience, I note one further factor that can be responsible for questions over performance - accurate mechanical assembly. One can have "perfect" optics, yet with a Mak or SCT there are many degrees of freedom that, if not well controlled, can really upset an instrument's performance. I've seen this at work in my Mak 127mm and C8, and was able to identify and correct issues in each. There must be some unlucky owners out there who have been let down by compromised assemblies and simply either accept the received wisdom that "SCT's are soft", or pass the 'scope on.

Cheers,

Ed


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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: Opinion on Star Test new [Re: Ed Holland]
      #5528890 - 11/19/12 09:48 PM

Ed, good point. Makes sense that mechanical alignment should hold the optics in alignment and the incoming focused wavefront orthogonal to the axis - to the best extent possible. If not, then as the wavefront collapses on itself, it would do so in a disorderly fashion (relative to to the mechanical axis holding the eyepiece) and induce some aberration. One side of the wave reaches focus while the other is just ever so slightly off. (Unless the Airy disc is spherical, then it would not matter what angle you look at it, right?)

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Pinbout
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Reged: 02/22/10

Loc: nj
Re: Opinion on Star Test new [Re: Asbytec]
      #5529265 - 11/20/12 01:14 AM

Quote:

how to proceed and when.




Dick Parker's testing optics eludes to KE testing from a star.

I always like KE test, you can see a lot about the surface instead of trying to interpret rings from a defocused star, although there is all the info there in a star test it's not very intuitive.


from this page


Quote:

Star Spot test



The Spherical Spot test is wonderfully sensitive because it is a null test. This means that when you do the test every part of the mirror is (supposed to be) at the same zone. Any part of the mirror which is at a different zone stands right out. You can get the same results from a complete telescope by using a real star as the light source.

Assemble your telescope, wait for some great seeing, and point to a star. You will need a sturdy mount for your telescope. Keep the star centered and remove the eyepiece (dob users usually prefer Polaris). You can start with a knife edge cutting in the cone of light, move the focuser to find the focal point of the telescope. This is a 1D null test and it is very sensitive. Many people have done this before. Now merely replace the knife edge with a very small spot. Since the star is a true point source, you will be able to use a spot as small as 0.001" for greater sensitivity.

Bring the spot to the focal point (keep the star centered). There is a null point there which will show you the quality of the wavefront exiting the telescope in graphic detail. But be aware, this test is severe. It tests the entire optical system. Not just the mirrors and/or lenses, but also the air the light travels through. It might even be as brutal as the regular star test (with an eyepiece; it shows the change in the diffraction pattern due to image aberrations). Certainly they should be used together. The star test can tell if you have errors, the Star Spot test will show you most of those errors directly.







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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: Opinion on Star Test new [Re: Pinbout]
      #5529270 - 11/20/12 01:20 AM

Danny, that looks fascinating. I understand the basic concepts behind the knife edge, the procedure might be a bit of a challenge. Maybe Polaris will help, since my mount is not driven. The KE moves continuously. But, if it can be done, and some accurate measurements made, hey...star test heaven!

Lemme read up on it. Thank you, Danny.


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Pinbout
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Reged: 02/22/10

Loc: nj
Re: Opinion on Star Test [Re: Asbytec]
      #5529277 - 11/20/12 01:27 AM

I do it on a non driven mount, no problem, use a bright star, not polaris. it's not like your 600x mag. it will move slow enough.

Edited by Pinbout (11/20/12 01:37 AM)


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freestar8n
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Reged: 10/12/07

Re: Opinion on Star Test new [Re: Asbytec]
      #5529365 - 11/20/12 04:26 AM

Hi-

I think if you are really interested in characterizing this thing (and it sure seems like you are) then - yes - the natural step would be to do a Roddier test. But when I tried Roddier on my C11 I did not get very consistent results. I think that shows how hard it is to estimate the aberrations present based on studying the star through focus - particularly when high order terms are involved.

In addition - since there is still uncertainty regarding any clipping done by baffles inside the scope - the through-focus behavior may not match any of the simulations in texts or software tools you are referring to - since they don't include it in the model. It's a classic case where simulation may not capture the true phenomenon and you need real data.

So - I would aim to capture digital images of a star at focus and going through focus. For Roddier you need two images on exactly opposite sides of focus, so the size of the spot is identical. I don't think the Roddier software can handle a situation where the secondary obstruction is at a different location from the aperture stop (as it would be if the baffle were the AS) but it would at least be something to try.

Either way - it would be nice to see actual images of the star spots on a linear scale, without saturation. Preferably where the pixel scale is known accurately so you can tell exactly how far you are from focus.

Frank


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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: Opinion on Star Test new [Re: freestar8n]
      #5529403 - 11/20/12 05:34 AM

Hi Frank. Absolutely, very interesting subject. The Mak inspired a lot of, well, refractor-like love. It's intrigued me since. Then Ed's effective aperture measurements spurred further study. Especially into SA, Mak design, and the baffling baffle.

It is very difficult to quantify a star test accurately, IMO. It's been said, "the interferometer takes no prisoners." You might be able to say it's good, or bad, or very nice. But only after filtering out biases as well as aberrations, seeing, what have you. Even then, it's hard to get simulations to present the data accurately. Close enough, maybe, a ball park figure. So, basically we're left kind of guessing. Hopefully well, along with some experienced opinions, too.

I'd love some real data, heck even a bench test. Roddier or KE test data. Barring any quantitative data, though, what's left is subjective visual, anecdotal, and some actual tests from Mak samples (on Rohr's site, mostly.) The latter is kind of like figuring out T-Rex behavior based on how a Komodo dragon behaves.

Truth is, it seems to be at least "good" according to Suiter and from what I can gather from Amateur Telescope Optics. And pretty darn good according to Jupiter. I think all the "circumstantial evidence" is pretty consistent, that's good enough for the time being.

You seem to have some interest in the design as well. I hope not to miss your thread if you find the time to publish your thoughts. Should be a very learned discussion.


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freestar8n
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Re: Opinion on Star Test new [Re: Asbytec]
      #5529429 - 11/20/12 06:15 AM

Well - do you have a web cam or any imaging equipment? It only takes a few seconds to video the star spot, which seems faster than drawing it as you have done - and the result is linear as long as you avoid saturation. Or you can use MetaGuide and just push a button to get a stacked image of the star spot - which is one of the goals I had in writing it (so people could just post images of their scope's performance on star spots). So there are tools to make it much easier nowadays to get quantitative info.

As for my own write up on maks - I will probably get back to it - but the discussion in the atm/optics section wasn't particularly satisfying since, ironically, it was hard to keep people focused. But my main conclusion about maks is the same - they are very hard to make using pure spheres and good results require final touch up in an autocollimator to aspherize one of the elements a bit. The faster maks today, and even the Meade 7", are built around a strongly aspheric element, and often include a large secondary obstruction. So it is incorrect to say they are inherently good performers because they are all-spherical - and it is often incorrect to say they are refractor-like because the secondary obstruction is small.

Frank


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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: Opinion on Star Test new [Re: freestar8n]
      #5529474 - 11/20/12 07:02 AM

I do have metaguide and a web toucam scrapped and mounted in an eyepiece barrel tucked in a box somewhere. Maybe GF's android can use the web cam. Lap top is shot.

Yea, I wish your previous topic had picked some steam. We may disagree a bit on what it means to be good, but smaller spot Maks can be pretty good (better than diffraction limited correction) with all spheres. I see it, just gotta let you have a peek. Of course they are much better with aspherics. Still, when you get back to it, a ray trace would be interesting to study at some point in the future. If I knew all the radii of mine, I'd give you the data.

Danny sent me on a KE journey, after a break to just enjoy observing for a week. Maybe some metaguide images then, too.

Cheers, Frank...scope is cooling.


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