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Equipment Discussions >> Cats & Casses

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Asbytec
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Opinion on Star Test
      #5523696 - 11/17/12 12:20 AM Attachment (133 downloads)

Please glance over the star test images below and offer your assessment.

The intra and extra focal images appear quite different at a glance, but on closer inspection are not that dissimilar. Of course, the most notable feature is the strong loss of contrast outside of focus. Inside focus patterns are clean and high contrast, outside are not. This made evaluating outside patterns a little difficult.

At 4 waves, the contrast difference is not that apparent despite the flaring of the outer ring. In both de-focus locations, the inner most rings are clean, well defined, and high contrast. However, inside focus, the inner most rings are noticeably dimmer than outside focus. The third ring is bright, clean and well defined. There is also a dim halo of two rings, the inside one is not too faint while the outer one is barely seen. Outside, the general pattern of dimmer to brighter still applies like the inside focus pattern.

However, the innermost rings are definitely brighter than the corresponding rings inside focus. They are well contrasted, too. The third ring is a bit dimmer than the same ring inside focus (probably due to the lesser contrast.) This third ring, in particular, contributes nearly all of the flaring seen at this de-focus. Again, the same ring inside focus is very clean and well defined. The difference of this is striking.

Amateur Telescope optics shows 4 waves balanced (at bottom), but they appear only similar in some aspects, possibly between 0.1 and 0.2 PV. Neh?

http://www.telescope-optics.net/Mak_spherical.htm

About 10 waves, the contrast loss outside of focus is very prominent. In fact, it's difficult to detect the ring structure in moderate seeing, so it did require great seeing to get a good look at them. Inside of focus has the same relative brightness patterns consistent with Suiter's representation of 1/8th wave spherical. Again, they are very clean with full contrast.

Interestingly, in the best moments, outside of focus the relative ring structure and relative brightness are strikingly similar to inside focus. Outside the rings are a bit dimmer than inside, but this is due to the light scattered across the outside focus pattern. In less than perfect seeing, the two rings bordering the shadow tend to blend into one fuzzy, tad brighter ring. But, if one looks deep into that fuzzy glow, there are indeed two rings there. The inner most ring on the shadow boundary is actually a little dimmer than the one just outside it, just like inside focus pattern. Both are very thin and hard to see without good contrast. But, having said that, the basic relative pattern still holds pretty well with Suiter's (first edition) 1/8th wave representation.

Inside of focus, the central shadow is also noticeably darker at both levels of de-focus. It also appears a bit larger inside focus, but that's difficult to evaluate because of the flaring and loss of contrast outside of focus. OUtside it's more difficult to see the defined shadow. I did not do the shadow break out tests, the CO is only ~29% and the test looks very difficult to perform accurately. Nor have I applied Aberrator at this point. Maybe some night I will do so.

Lastly, the in-focus image is shown. Of course, the spurious disc (Aldebaran at 383x) is bright and as perfectly round as I could make it out to be. The first ring is pretty bright. That all appears normal for a spherical, obstructed scope with some spherical aberration present. Outside the first ring, there are two much fainter rings. The first of which is fairly faint and the third, outer most ring is very faint. The latter can only be seen during the best moments of seeing (I hope it even shows in the sketch.)

One interesting artifact(?) exists just outside the first bright ring and between that ring an the next. In generally good seeing, a series of extremely thin, not too dim, arcs seem to dance between the major diffraction rings. In very steady seeing, those dancing arcs settle into two very, very thin rings of moderate brightness (the outer ring a tad brighter than the inner.) I am not sure that to make of those rings. Again, they appear to be consistent with Suiter's representation of 1/8th wave in-focus representation.

Anyway, my guess is, despite the loss of contrast, the correction seems pretty good. Surely they are not perfect, and I doubt they are 1/8th corrected. I cannot quantify it except to say it's consistent with Suiter's 1/8th wave, and much less so with 1/4 wave SA. I might wager the correction is near 1/6th P-V, with lower order and higher order contributing (and I suspect balanced to some degree.) Despite the contrast loss outside of focus, Jupiter come to focus rather snappy and shows a great bit of detail.

I am still trying to evaluate the HSA component, but that is very difficult to do. According to Suiter, I really gotta get down to 1 to 3 waves de-focus to see, "mouthpiece of the born pulled back through the caustic."

Anyway, what say you?

Edited by Asbytec (11/17/12 12:35 AM)


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thomas68
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Re: Opinion on Star Test new [Re: Asbytec]
      #5523968 - 11/17/12 08:59 AM

Asbytec, your collimation looks great! I bet it looks even better under those dark skies you have.

Tom


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Asbytec
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Re: Opinion on Star Test new [Re: thomas68]
      #5523981 - 11/17/12 09:15 AM

Tom, thank you. Yes, it's amazing, really. Very fortunate, tropical dry season is the best seeing I have personally seen. When you can get down to 1 wave de-focus and the Poisson spot sits still dead center, it's nice to get collimation spot on. Dark. Not bad, pretty dark. Especially when the power goes out.

Edited by Asbytec (11/17/12 09:21 AM)


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thomas68
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Re: Opinion on Star Test new [Re: Asbytec]
      #5524008 - 11/17/12 09:29 AM

Asbytec, once in a while I wish the power would go out! Only on the clearest night of the year.

Tom


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Asbytec
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Re: Opinion on Star Test new [Re: thomas68]
      #5524122 - 11/17/12 10:44 AM

Yea, one cannot cast stones at that idea.

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orion61

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Re: Opinion on Star Test new [Re: Asbytec]
      #5525011 - 11/17/12 08:09 PM

what brand/type OTA is that, I've found for whatever reason Maks fair better on the focused image with SA.

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Asbytec
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Re: Opinion on Star Test new [Re: orion61]
      #5525040 - 11/17/12 08:31 PM Attachment (37 downloads)

Orion, that's exactly what I am trying to figure out. They do focus well, but why? The images are from a modified Orion 150 Mak, f/13, ~28% CO.

I am sure there is some HSA, but I am having difficulty with determining how much or whether it's balanced. I suspect it is. According to Suiter, you can best see this at very close to focus. Apparently, there is a brighter central region with light falling off toward the edge of the pattern in the presence of SA. The condition gets worse with less correction. It's just so very hard to see any of that.

The 1 wave patterns are sketched below. I really do not know what to make of them other than to praise God for such favorable seeing (and perfect collimation. )

Last night observing Capella, outside focus a little brighter than Aldebaran, the two inner rings bordering the shadow appeared to be about equal brightness. So, maybe there is some revision to the description above, but not much.


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Pinbout
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Re: Opinion on Star Test new [Re: Asbytec]
      #5525100 - 11/17/12 09:07 PM Attachment (47 downloads)

here's what I get for a 150f10 1/6~ under balanced

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Asbytec
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Re: Opinion on Star Test new [Re: Pinbout]
      #5525131 - 11/17/12 09:24 PM

Thank you, Danny. Inside looks darn close, very darn close. Outside, the inner ring might be just a tad brighter than observed, but that's definitely ball park. The higher order is more visible in that image, with a more distinct dimming toward the edge as I understand the effect. Still, it might be accuracy in observing that might account for that difference.

Playing with Aberrator last year, I found it was pretty sensitive to CO. Change the CO by a couple percentage points, and the ring structure and relative brightness changed a bit. A CO of 30% is probably close enough. Gotta re-install Aberrator and give it a look see.

Thanks for posting, Danny.


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Eddgie
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Re: Opinion on Star Test new [Re: Asbytec]
      #5525146 - 11/17/12 09:34 PM Attachment (36 downloads)

I had difficulty modeling this.

The second diffraction ring visible in the in-focus image required .15 wave of spherical abberation to achieve. This is the primary cause for light to be pushed outside of the Airy Disk and first diffraction ring, and no HSA simulation I tried would generate an easily visible second diffraction ring, so I am going to guess that the contribution to this is some LSA.

This is consistent with what Suiter suggests for the design(that some LSA could be used to negate the rolled edge appearance that would otherwise result from the HSA component.

I think the model for 4 waves is about right, but at 10 waves, the model did not work out. It showed what looked like a turned edge. Was the secondary baffle in place? If not, I would think that the HSA could cause this appearance (of a turned edge) which again is what Suiter suggests.

Anyway, I included my settings. I had to flip the in/out focus patterns to match your provided drawing.

I think you should obstruct to 40% and try to model the lower order SA so you can get a read on it.

I think much of what you are seeing is the effects of some HSA, but there appears to me to be some LSA too.

Anyway, why don't you load my figures into Abberator and give it a try. You will see that if you correct the LSA, the outer ring will go out, even with .4 waves of HSA.

I modeled the HSA a bit stronger than necessary.. I think the .3 setting looked perhaps a bit closer so I would plugh that into abberator.


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Asbytec
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Re: Opinion on Star Test new [Re: Eddgie]
      #5525255 - 11/17/12 10:56 PM

Eddgie, I had a lot of trouble modeling it, too. Beginning with some initial assumptions, then hit and miss tweaking the patterns to get them close. Then, as you say, scroll through focus to check consistency.

It was a real chore, best I could do is get all de focus ranges close. The closest I could do (previously) was LSA at -.15 and adding about +0.1 to .2 HSA (about 1/5th wave that might be expected in a balanced system.) The result was close across 10 and 4 waves (and in focus) giving RMS of about 0.038, including a trace coma and astig added (but not observed.) So, the test proved pretty good at one point earlier this year.

Initially, your presentations brought an "oooh, he got it!" reaction. At 4 waves, though, the inner most ring brightness is reversed. If you reverse them back, it's closer. LOL I tell ya, it's tough to get it right. It's one reason I am hesitant to play with Aberrator, again. I am trying to observe it directly and infer some level of correction visually.

I do get that "turned edge" look, but outside of focus (with the secondary's baffle removed.) That was the one scary thing testing without the baffle, the star test was no longer "beautiful." When the star tests were beautiful, shadow break out occurred at about 1: ~1.5 to 2, (or a little less than 2.) I was shocked it turned out that well, but with the CO closer to 52mm (37%) of 140mm eff aperture, well that might be expected. With the excessive contrast loss outside focus, and the CO less than 33%, the break out test is much more difficult. So, I rely on the previous tests remaining somewhat accurate if not a little over estimation.

Thank you for chiming in, Eddgie. Always appreciate your views. Still mulling it over.


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Eddgie
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Re: Opinion on Star Test new [Re: Asbytec]
      #5525291 - 11/17/12 11:35 PM

Well, this is the one thing Suiter mentions in his book and this is perhaps the best example one could have.

Suiter is quick to mention that single abberations rarely appear alone, and that when you mix the effects of the abberations, it can be difficult to interpret the results of the star test.

We assume higher order Spherical Abberation (you and I do of course because you are one of the few people I know to have read his book) in this scope because we know that it is a consequence of a "Fast" aluminized spot all spherical MCT.

And we may conclude that the secondary baffle acted to mask the effects of HSA, and that when you removed it, it let those abberatios show through.

I believe that there is a bit of LSA, because I can't find a model using pure HSA that will give a result similar to what you observer.

But if there is HSA and LSA, finding the exact match may prove to be impossible simply because there are so many possible combinations of LSA and HSA.

And sadly, there is one more complication. A Turned Edge can cause some of the things you are obseving. Even a tiny turned edge could account for some of what you see, even if it was not enough to have a meaningfull effect on the image.

If you have a Rinchi grating, you may want to test for this.

The Ronchi when used single-pass is just not very good for detecting Spherical Abberation unless it is very bad. But it is very good at showing a TE.


So, perhaps the LSA component is milder than we have modeled and maybe there is a mixed in TE.

Again, just brainstorming, but I tried a half dozen models and could not get the 10 wave pattern to work.

With a tiny TE though, I could imagine that you might see your 10 wave pattern.

Sadly, Abberator 3.0 doesn't model turned edges..

Ah, but Abberator 2.0 does.. LOL...

But see if you can order a plastic Ronchi strip and check for TE. If you had a tiny bit, it would explain a lot.


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Asbytec
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Re: Opinion on Star Test new [Re: Eddgie]
      #5525315 - 11/17/12 11:55 PM

Edggie, no doubt there is some LSA and even residual HSA. I am trying to determine how much and whether the system is balances for 5th order.

Absolutely, that is the core of the entire exercise, to determine if the baffle was cutting off some marginal rays and improving the star test. And whether removing the baffle has been detrimental to the wavefront figure afterward. I tend to think not, Jupiter still "snaps" pretty well.

My Ronchi is not that sensitive at 50ln per inch, but at f29 I could detect no trace of a turned edge. Just some very weak under correction, pretty consistent with Ronchi for windows. The test is not conclusive, but I think the crisp inside patterns indicate no turned edge.

Studying HSA (including Suiter's book) seems to suggest it needs to be pretty severe to degrade performance (~0.4 waves at 0.075 RMS.) According to Vlad's site, the mere process of making a usable Mak requires reducing higher order SA to some degree. Balancing has minimal impact on the wavefront and can still be quite good RMS, wise.

But, I am trying to actually see this. Really, Jupiter is just as good if not better, but the star test is kind of ugly outside of focus (only.) This is much like Roland's description of asymetrical testing resulting from higher order SA. So, all this seems to point to well corrected and balanced SA.

Another test, very subjective, last night observing Capella in focus, so much of the light was centered near the Spurious disc and first ring. There was a little scatter outside the first ring, usually when seeing blurred the image. When it did, the fainter second and third rings would simply vanish. When they were visible, they were very faint. So, I conclude EER(1) is pretty good.

Using Suiter's EER(1) chart, and some optimistic estimations of 1/8th LSA (only) and a 37% CO, EER(1) was near 72%. Without the baffle, EER(1) improves to about 0.80. So, even though the actual Strehl (calculated from Aberrator generated RMS) is near 0.95, the EER(1) approaches "diffraction limited" performance, "good" according to Suiter and taking into account Strehl and CO effects.

But, I still cannot say conclusively the system is balanced for HSA, it just looks to be from all accounts and to the best of my knowledge. And if so, then removing the baffle had minimal impact, in fact might have improved performance.

Thanks, Eddgie...brain dead. More later...


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Asbytec
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Re: Opinion on Star Test new [Re: Asbytec]
      #5525603 - 11/18/12 07:02 AM Attachment (21 downloads)

Well, here are some star test patterns from Amateur Telescope Optics. Been focusing on the ones in the link above and forgot to look at these.

http://www.telescope-optics.net/diffraction_pattern_and_aberrations.htm

The 4 wave de-focus sketches above do resemble the balanced form (and nothing like the primary aberration alone.) That's good, very good.

Its very hard to tell which level of residual HSA remains, however. The patterns look very difficult to differentiate visually at the eyepiece. In 0.2 PV form, both outer rings are close to equal brightness on both sides of focus.

In the 0.4 PV form, the difference between that outer ring inside and out is a little greater. However, the inner most ring is noticeably dimmer inside focus. It's barely visible.

Both outside patterns look pretty similar, too similar to differentiate visually.

So, it looks like it boils down to evaluating how bright that inner ring is and evaluating how closely the outer rings are in terms of brightness on both sides of focus. Really hard to say, but it's pleasing to know they resemble the balanced form.


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Asbytec
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Re: Opinion on Star Test new [Re: Asbytec]
      #5525986 - 11/18/12 12:04 PM

Well, kind of sort of did the shadow break out, without the 33% obstruction. I know it's not the standard test, but was just a curious look see. What harm could it do?

I de-focused until I was sure it was broke out, then went way outside focus and moved back toward focus until it looked like it disappeared. So, I approached the break out from both directions. Did that several times.

It seems the shadow breaks out between 6 and 8 waves inside, and about 12 to 15 waves outside. And some of that error was subjective, was the shadow well defined or how large should it be. Surely with a larger CO, it would have held up closer to focus, but this look was just a wag.

Also, as I thought, 4 wave patterns were really difficult to discern. The outer ring was pretty close in brightness on both sides, but that was hard to estimate because outside of focus the ring spread light into some flaring. That tiny inner ring inside was, neh, fairly bright. So, truthfully, HSA probably lies somewhere between 0.2 and 0.4, probably closer to 0.2. Say, wagging it, 1/4 P-V HSA.


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Eddgie
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Re: Opinion on Star Test new [Re: Asbytec]
      #5526338 - 11/18/12 03:28 PM

Well, this is going to be the problem with even the 33% obstruction... It greatly reduces the influence of HSA, but does not eliminate it, and your result could in fact have been affected by HSA.

By my model suggested a bit of LSA, and this is consistent with your 33% obstruction test. The ratio was maybe 2:1, and that is consistent with about 1/6th wave of SA, which is almost exactly what I put in my model that I posted earlier. I tried to get the second ring to show using more HSA, but that caused the 10 wave test to not match your 10 wave drawing even a little.

Don't you have access to a camera" Even if you can do it afocal, you may be able to use the Roddier test????

I have not used the Roddier test, but I keep telling myself I need to learn how.

But honestly, when the abberations mix, it is not easy to know if your result is being influenced a bit. Maybe the LSA is a bit better than this.. Maybe 1/7th, and the HSA is influencing the test some (which I would think might happen if it is more than a very tiny amount).

As you are seeing, the star test is indeed sensitive to HSA at small defocus amounts, and that is why Suiter says to run the obstruction when testing LSA.


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Pinbout
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Re: Opinion on Star Test new [Re: Asbytec]
      #5526506 - 11/18/12 05:21 PM

how about laying a piece of black tape across half of your diagonal and using it as a knife edge test on a bright star. you go thru focus to see if the ke spins. you look to see is its a good null at focus where it nulls. if there are abberations you'll start to see donut images like testing a parabola @ center of curvature.

its an easy test and does require a lot of effort.


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Asbytec
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Re: Opinion on Star Test new [Re: Pinbout]
      #5526965 - 11/18/12 10:13 PM

Eddgie, yea, thought about trying the Roddier test. I have a old PC cam shoved into an eyepiece barrel and a lap top. Problem is, the scope is not driven.

You're correct, LSA still exists and HSA is residual. It's certainly not perfect. And it's very hard to model in Aberrator, I spent weeks pushing and pulling the simulation just to get it close. You have always said the star test is sensitive, it certainly is.

I guess the main point of this exercise was to evaluate balanced SA, or not. I am convinced by anecdotal and observational evidence it is, indeed, balanced. This, then, beggars the question why the baffle was vignetting when it need not. When the marginal rays lag the reference sphere at best focus in balanced scope, they focus a bit closer to best focus than with LSA alone and need not be cut out of the focus. The overall P-V and RMS improves due to the correction, not the baffle. In fact, the baffle would have very little effect at focus, but make the star test look pretty (it was pretty before.)

In lower order alone, the marginal rays precede the best focus reference sphere centered at best focus (0.7 zone) and the center lags it. The difference is a large P-V figure and gives the wavefront a parabolic(?) shape. When it collapses on itself, it delivers the classic under corrected caustic focus.

If you can imagine the balanced wavefront shaped like a Schmidt corrector figure, you can understand the paraxial wavefront is advanced with respect to the 0.7 zone giving the wavefront that central bulge (similar to the convex central figure on a Schmidt corrector.) And when the marginal wavefront is delayed a bit, it lags the 0.7 zone curling the lip back on itself (not unlike the thickening of a corrector's outer zones.) When this curve collapses on itself, all focal points are closer together improving correction with a shorter caustic. And since the curled lip is very small (while much of the central bugle is masked), the effect is better correction and complicated star test.

Interestingly, I believe the Schmidt corrector leaves the best focus rays untouched, corresponding to the concave section of the corrector. This zone behaves just as if there were no corrector at all, then the varying thickness delays the wavefront just enough to correct all other zones on the point of best focus (leaving the focal ratio untouched.) The Mak meniscus shifts all zones further from the primary, including best focus (giving a slightly longer focal ratio.) Lengthening the primary's radius, then, is partly responsible for the marginal timing "delay," and advancing the paraxial zone, producing the balanced higher order wavefront.

The result is, as Suiter explains, someone reached into the horn and pulled the mouthpiece back through. So, the Schmidt seems corrected at the corrector (which is probably why it's called a "corrector" LOL) and the primary's surface radius is left alone, where the Mak uses the primary's radius to "finish" the correction. Looking for the reason even fast food Maks are pretty darn good, I think balanced correction is it. With HSA corrected, and a bit longer focal ratio, there is no question they can reach a bit better than diffraction limited.

Danny, yea, forgot about the knife edge test. Might have to try that. Thanks. (In fact I played with it using the Ronchi grating as a knife edge to no avail, really.) Not sure how the KE would "spin." But, I can imagine cutting the cone at paraxial focus, for example, should give brighter outer zone (on one side?) Hmmm...been a while using knife edge.

Sorry for the length, fellas...just an interesting exercise to share.

Edited by Asbytec (11/18/12 10:58 PM)


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Eddgie
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Re: Opinion on Star Test new [Re: Asbytec]
      #5527073 - 11/18/12 11:32 PM

I will be the first to admit that your saga has been very interesting, and I would very much like to know the answers to your questions just about as much as you would. I enjoy star testing (and belive in it) so that is part of my interest, but like you, I would like to know the answer to the question about the secondary baffles you are posing.

But like you, I could not get Abberator to churn out a model that worked for me. I feel like I got kind of close at 4 waves, but the result did not work at 10 waves.

That is why I think that a Roddier test would be interesting to see.

We may never know the answer to the question about the baffle. Was it a design error, or was it intentially sized to minimize the influence of the HSA on the star test? We just won't know.

Suiter was apparently right in his own model when he said that the aluminized spot design would experience some HSA if made faster than f/15, but even his own f/12 model could still have a Strehl of .95, which in my book is still excellent.

One of the best mystery stories ever on CN, and the big flurry of informatoin on aperture testing that resulted was (I thought) one of the best threads ever posted here. I learned some really excellent things during that.

Keep us up to date if you make progress.


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Asbytec
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Re: Opinion on Star Test new [Re: Eddgie]
      #5527161 - 11/19/12 12:34 AM

You know, it has been a fascinating journey made possible by Ed Holland's monster thread.

Quote:

I could not get Abberator to churn out a model that worked for me.




Someone mentioned Aberrator HSA term was, in fact, the balanced form. So, there may be no need to add any LSA at all. That may be why it gives us a hard time.

On the baffle design, I can attest to it being very tight. By that, I mean absolutely impossible to see around it from the visual back. Maybe the design was just tight, tighter than it need be. Maybe it was designed so as not to scare the star test crowd (like it did me.) Dunno.

Yes, Suiter's account of his own model seems accurate. A pretty good Strehl and acceptable EER(1). If the model is balanced, as Vlad often says it "must" be, then the correction is very good. If it's not balanced, I suspect you will end up with a lot of under correction and performance like those very bad "China Maks" on Rhor's site - instead of the pretty good performance of the Skywatcher (and many SCTs.)

Will do, Eddgie, Danny. Cheers...


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Asbytec
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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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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
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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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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
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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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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
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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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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
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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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Re: Opinion on Star Test new [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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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
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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
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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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