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Equipment Discussions >> Binoviewers

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Sarkikos
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Eddgie]
      #6157371 - 10/25/13 12:28 PM

Quote:

I had star tested my new dob when I go it, and SA was actually really nicely corrected. This is not a premium quality mirror, but way better than I had prepared for.




My 10" f/4.8 Dob is a Celestron C10NGT OTA ported to a Dob mount. (I can't stand any Newt OTA larger than a 6" on a GEM.) The optics are very nice. I see no difference in the diffraction pattern in-focus and out-focus.

Quote:

I am going to qualify this statement before I even make it. I was not using a 33% obstruction, so the test may not be fully accurate.. Anyway, I decided to rack though focus and to my surprise, the secondary shadow ratio was a lot different on the other side of focus!

This is the classic sign of spherical aberration.

Again, it is important to mention that I did not use a 33% obstruction. The problem is perhaps not really serious and maybe is some higher order spherical aberration (and that is why you use a 33% obstruction rather than the native 23%.. It greatly reduces any influence of HSA).




A larger CO reduces HSA? I had not heard of this. Maybe I should go back and reread my Suiter's?

But I would think that any advantage to a larger CO - and I'm really skeptical that there is any practical advantage, except maybe for double stars - would be offset by the diminished contrast for planet/lunar. Did you raise the primary in an attempt to binoview without an OCS, and so had to change to a larger secondary? Otherwise, I can't imagine willingly changing from a 23% CO to a 33%.

By the way, my 10" Dob also has a native 23% CO. I like it that way, because IME it places the scope in a Goldilocks Zone making it good for planet/lunar and deep sky.

Quote:

Will try to remember to follow up.




Please do, even if you have to start another thread to show your binoviewer experiences.

Quote:

Scope seems to perform well on the moon though. I would say at least ad good as my 6" APO and I think perhaps a bit better, but I really haven't done a side by side, so just working from memory of detail around the Triesnecker Rille. I know this area well from the C14 and the 6" APO, and I felt that it was showing detail right between the C14 and the 6" APO.




Sounds very much like my 10" Dob. That's why I'm not in such a hurry to upgrade to a 14" Dob - which I do want to do eventually. I'll just have to continue to ignore the condescending remarks and glances from my fellow observers at the dark site. It seems like for deep sky you're not really a member of the club unless you have at least a 14" Dob.

Quote:

I love the Mark V and coma corrector though.

But the coma corrector extends into the focuser tube 92mm. This is not much different than the OCS on the Binotron.

So, this may not work for you either..




Yep, this definitely needs more thought before I pull the trigger. I might even wait on a binoviewer upgrade until I get that 14" Dob. Looks like I'll have to design the Dob around the binoviewer so everything will work optimally.

I'm also concerned that the Mark V coma correction might not be as good as a TV Paracorr. After all, the Paracorr has built-in tuning to optimise coma correction. AFAIK, there is no accommodation like that for the Mark V corrector OCS. Unless the coma correction is just right, there is a definite possibility of making the image worse rather than better. That may be what you are finding out with your Mark V and Dob.

Mike


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Eddgie
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Sarkikos]
      #6157463 - 10/25/13 01:19 PM

Quote:

A larger CO reduces HSA?




No, not what I said. The larger CO does not eliminate or reduce HSA at all.

The problem is that the star test is super sensitive and if you do it with to small (or no) obstruction, it can give the impression that there is lower order spherical aberration.

This is the mistake that Roland Christen made when he condemmed the star test. He noted that his breakout was very unbalanced.

It was clear to me from his essay on the star test that he did not follow Suiter's instructions.

The 33% obstruction is part of the process. The purpose is to desensitize the star test for the presence of HSA.

Unlike LSA, the HSA component will normally not throw light outside of the in-focus Airy Disk until it is quite bad.

It will slightly expand the diameter of the Airy disk, but not usually enough to be disenable visually. Since the light remains in the Airy Disk, contrast is not lowered.

But, if you defocus, you quickly change that dynamic and now the light is going into the ring structure.

The 33% obstruction at 10 wavelengths just gets you so far away from best focus that the effects are not easily seen.

So, obstruction does not fix HSA. Only masks it for the star test.

HSA when present rarely does damage. Even the all spherical f/12.5 MCT only looses .05 Strehl to HSA (best computed Strhel will be .95). This is not enough effect to be seen visually.

So once again, CO does not reduce HSA, only reduces the star test sensitivity to it. All of the HSA is still there, but you simply have to defocus so much that its effects are not enough to influence the breakout.


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Sarkikos
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Eddgie]
      #6157474 - 10/25/13 01:27 PM

Quote:

So, obstruction does not fix HSA. Only masks it for the star test.

HSA when present rarely does damage. Even the all spherical f/12.5 MCT only looses .05 Strehl to HSA (best computed Strhel will be .95). This is not enough effect to be seen visually.

So once again, CO does not reduce HSA, only reduces the star test sensitivity to it. All of the HSA is still there, but you simply have to defocus so much that its effects are not enough to influence the breakout.




OK, thanks. I understand your meaning now.

Mike


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Eddgie
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Eddgie]
      #6157475 - 10/25/13 01:27 PM

I have to add one thing though. I was absolute in my answer, and it is really more complicated than that.

It is perhaps conceivable that a big enough obstruction does in fact lower HSA.

One of the forum members (Norme) removed the cone shaped secondary baffle from his MCT because he wanted to see if it improved the contrast.

When he did this, it lowered the obstruction by a meaningful percentage.

He immediately reported that HSA seemed more pronounced in the star test.

But again, the question is "Was there really more CA? Or did the bigger obstruction simply desensitize the test?"

Ant some point, the size of the obstruction takes so much energy out of the Airy disk that a slight change in diameter caused by HSA becomes so moot as to be purely academic because the far greaterr damage is done by the obstruction itself.

And this is why the f/12.5 MCT works OK. The HSA lowers the Strehl a bit, but the scope will star test well, and no one will complain that their scope has excessive LSA.

And I think Norme saw the difference.

Ask him about it in the Cats and Casses forum. He as probably done more with star testing and HSA than anyone else on the forum.

Nomre is his CN user name.

By now, maybe the best star tester on CN. His work with star testing his MCT pushed his understanding of HSA far beyond mine.


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Sarkikos
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Eddgie]
      #6157505 - 10/25/13 01:43 PM

Quote:

It is perhaps conceivable that a big enough obstruction does in fact lower HSA.

One of the forum members (Norme) removed the cone shaped secondary baffle from his MCT because he wanted to see if it improved the contrast.

When he did this, it lowered the obstruction by a meaningful percentage.

He immediately reported that HSA seemed more pronounced in the star test.

But again, the question is "Was there really more CA? Or did the bigger obstruction simply desensitize the test?"

Ant some point, the size of the obstruction takes so much energy out of the Airy disk that a slight change in diameter caused by HSA becomes so moot as to be purely academic because the far greaterr damage is done by the obstruction itself.




IIRC, there have been many numerous discussions about larger obstruction benefiting double star observing, all revolving around these questions. I'm not sure which side is correct. But I'm confident that for planet/lunar, no advantage will be gained by a larger obstruction, ceteris paribus.

Quote:

And this is why the f/12.5 MCT works OK. The HSA lowers the Strehl a bit, but the scope will star test well, and no one will complain that their scope has excessive LSA.




Now you're making me question the excellent star test on my C6.

Quote:

And I think Norme saw the difference.

Ask him about it in the Cats and Casses forum. He as probably done more with star testing and HSA than anyone else on the forum.

Nomre is his CN user name.

By now, maybe the best star tester on CN. His work with star testing his MCT pushed his understanding of HSA far beyond mine.




Yep, I know just the guy you're talking about. I've talked with him many times on the Planetary and Lunar Fora. I envy his retirement in the Phillipines. My choice is between Delaware and Florida.


Mike


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Sarkikos
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Eddgie]
      #6158285 - 10/25/13 09:56 PM

Eddgie,

Quote:

I am going to qualify this statement before I even make it. I was not using a 33% obstruction, so the test may not be fully accurate.. Anyway, I decided to rack though focus and to my surprise, the secondary shadow ratio was a lot different on the other side of focus!

This is the classic sign of spherical aberration.

Again, it is important to mention that I did not use a 33% obstruction. The problem is perhaps not really serious and maybe is some higher order spherical aberration (and that is why you use a 33% obstruction rather than the native 23%.. It greatly reduces any influence of HSA).

So, for now, I have to say that I have not had a chance to re-test with the 33% obstruction in place.




I have to ask you about this: Do observers usually star test their Newts with a 33% obstruction in place? I'd never heard of this before in actual practice. Suiter does mention it a few times in Chapter 10 of his Star Testing. But he doesn't talk as if this needs to be a standard phase of star testing and I've never read about it anywhere else. In fact, I've never heard of anyone actually doing it until now.

Mike


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Eddgie
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Sarkikos]
      #6158385 - 10/25/13 10:54 PM

Ah, good question.

For an instrument with no inherent HSA, (Newtonian) it is less important in the sense that if the pattern is perfectly balanced, the scope is fine.

But what happens if you do see some imbalance?

The 33% obstruction was chosen by Suiter because it is large enough that when used at 10 wavelength of defocus, if there is unbalance, it makes it easier to estimate how much spherical aberration is present.

This is the beauty of Suiter's method. If a 20% obstructed scope is perfect, then you will see it. But if it is not perfect, having the bigger obstruction makes it easier to see the ratio of the difference so you can make a better estimate of the actual SA present.

And again, in designs with HSA, or in APOs that can have HSA and color mixing that is different one either side of focus, the 33% obstruction can be necessary, but again, I believe he chose the 33% simply for the easy of quantifying the amount of SA. If the obstruction is smaller, seeing a difference of 20% is much harder than if the obstruction is a bit larger.

So this I think is why he came up with the 33% obstruction. It minimizes any HSA, and it provides a big enough shadow that it is easier to estimate any SA that is present.

Also, for the 3:1 breakout, it offers the same advantage. A 2 : 1 breakout would be to sensitive, and the 33% obstruction is once again just the right sensitivity. In particular, the breakout can be very imprecise even at 3 : 1 ratio. I have to often do this test several times and average my breakout point because it seems to be hard to find the exact spot where the inner ring comes out. I think at 2:1 it would be hard.

That is why I keep saying to people that the method is very specific, and my own results with all scopes have felt much more accurate when I use 33%.

But for an HSA free design, if the balance looks perfect, it is probably close to it. If it is not though, estimation is harder.

But I cheat. I always use a 33% obstruction but I use Aberrator to model what I see. I find it easier than estimating the secondary shadow size ratio difference. Finding a pattern with aberrator that looks as much like the pattern I see seems to work best.

But after doing it so many times, I have gotten pretty good at taking a wild guess for the initial settings for SA in Aberrator.

I also studied a lot of Rohr's tests where he has Fresnel patterns, and also the Aberrator site (which is where I learned how to do the modeling based on their 40 or so real telescope star tests and Aberrator models for those scopes.. A great web site for starting to learn about star testing.

Anyway, that is why I believe that 33% is best to use. It seems easier to estimate if there is SA present.


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Sarkikos
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Eddgie]
      #6158790 - 10/26/13 09:20 AM

Quote:

I have to tell you that recently I became aware that the Baader coma corrector might induce some spherical aberration.




Yes, I think this is exactly what is happening. A Paracorr can also induce aberrations if everything is not set up optimally. I have heard that unless the optics are closely aligned and that the field stop to Paracorr distance is just right, aberrations can be introduced into the system - SA and even more coma. I take for granted that your telescope optics are well collimated. So the odds are that the source of the SA is the Baader coma corrector.

When I first heard about the Baader coma corrector OCS I doubted the correction would be optimal and was concerned that it might even be harmful to the image. How can it be optimized if there is no mechanism such as the Paracorr's tuning top to finely adjust the corrector lens distance? Even if some sort of tuning ring were introduced into the system, how could the user easily gauge when the Baader OCS was optimally set? "Some coma correction is better than none" does not always work well in practice.

Especially for planet/lunar, we don't want SA induced into the optical system. Isn't this the same reason why the back-focus distance should not be overly extended when binoviewing a Cat?

Mike


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Eddgie
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Sarkikos]
      #6158982 - 10/26/13 11:27 AM

Well, some SA is not going to make much difference, so it is all a matter of how much SA changes, and I have not been able to measure that.

The Baader OCS is designed very specifically for the spacing provided by the Binoviwer and while it is only optinmal at one specific spacing, because the much longer design length (120mm minimum behind the corrector with the BV body in place) small variations will not be as big in terms of percentage as they would for correctors desgind for accepting just an eyepeice.

The Coma correction itself is quite excellent.

I won't know how much SA there is until I can use an obstruction and estimate it. I am not going to angst over 1/6th wave. It is really difficult to see much affect from this small a change.

But if it is larger than this, I may remove it and use a standard barlow for planets.

People forgat about this aspect of coma correctors. You don't have to use them when you don't want them.

For planets, I can just drop the OCA and stick in the BV with a 2" Barlow element screwed into the 2" nose piece.

Of course I have not tested that configuration to see if I can reach focus, but It worked in my 6" APO so I think it may work here as well.

And again, the only time SA really matters all taht much is for planets.

Yes, lots of back focus in an SCT can indeed induce SA. And not just a little.

I tested one configuration taht induced about 1/4th wave. That is getting to the edge of what I would tolerate.

But kept under about 200mm of light path and the change is pretty modest. Again, I don't angst over small changes in SA.

Years ago, I was totallly anal about it, but after much testing, I realized that it took more than 1/6th wave difference to really see, and even then, until it gets to 1/4th wave, it is not all that easy to see, though if there are any other issues like roughness, you start to push into soft view territory.

Anyway, hope to test tomorrow night if the weather clears.


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Sarkikos
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Eddgie]
      #6160468 - 10/27/13 10:24 AM

Quote:

Well, some SA is not going to make much difference, so it is all a matter of how much SA changes, and I have not been able to measure that.




Yes, as you - and Suiter - have said, degradation of the image from SA depends on whether it's HSA or LSA and how much is present. Also, effect of SA will depend on the object viewed and how it is viewed. I have a 4.5" f/4.4 Ball Newt with a spherical primary. The image is acceptable - to me - for low power views of open clusters, bright bright nebulae and large bright galaxies. It was the first telescope that showed me M78 under my light-polluted suburban skies. But for higher power observation of planet/lunar? Forgetaboutit.

This is why I am concerned about SA induced by the Baader coma correcting OCS. My binoviewing is virtually always for planet/lunar.

Quote:

The Baader OCS is designed very specifically for the spacing provided by the Binoviwer and while it is only optinmal at one specific spacing, because the much longer design length (120mm minimum behind the corrector with the BV body in place) small variations will not be as big in terms of percentage as they would for correctors desgind for accepting just an eyepeice.




Makes sense.

Quote:

People forgat about this aspect of coma correctors. You don't have to use them when you don't want them.




Some observers, though, apparently refuse to do any observing unless a Paracorr is in the focuser. I'm not that type of observer. I'd rather have coma correction at all times. But sometimes you do have to give it up for other advantages. For instance, at this time I cannot binoview or use filter wheels with a Paracorr in the system. So out goes the Paracorr. eh...

Quote:

And again, the only time SA really matters all that much is for planets.




And for lunar, and probably for any DSO that are better seen at higher power. But SA is the big bugaboo for planet observation. AFAIK, SA affects on-axis sharpness more than coma. So according to my understanding, for planet/lunar, correcting SA is more important than coma correction.

Mike


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Eddgie
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Re: Mark V or Binotron for 10" Dob - Planet/Lunar Only new [Re: Sarkikos]
      #6160532 - 10/27/13 11:03 AM

Quote:

for planet/lunar, correcting SA is more important than coma correction




Yes, this is when it really matters.

But even here, a small diffence is not possible to see visually. It really takes a bit more than 1/6th wave of SA before the contrast is lowered enough that it becomes detectable.

That being said, like you, I would prefer to drop the coma corrector and barlow for planets if the corrector induces any meningful SA.

Hope to measure tonight but clouds have been around the last several days (rain here this AM).

I will say again though that I am very happy with the Baader corrector. For general work, the performance is really great.

And what little lunar I did seemed to be pretty sharp, though I have not had very good seeing on the few ocassions I tried.

Will let you know when I have tested though.


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