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Equipment Discussions >> Reflectors

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Jim_Smith
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Reged: 11/16/12

1/10 wave?
      #5533781 - 11/22/12 11:24 AM

Is there any visual difference at the eyepiece between 1/4.....1/9...1/10 wave mirrors? Thanx Jim

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Mirzam
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Reged: 04/01/08

Loc: Lovettsville, VA
Re: 1/10 wave? new [Re: Jim_Smith]
      #5533808 - 11/22/12 11:41 AM

You may want to peruse this:

http://stellafane.org/misc/links.html#Quality

JimC


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Mike B
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Reged: 04/06/05

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Re: 1/10 wave? new [Re: Mirzam]
      #5533817 - 11/22/12 11:48 AM

Here's some more to read!


The short answer is, apparently, *yes*, there can be, if all the other aspects that affect optics are properly handled, and the seeing cooperates reasonably well.


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Pinbout
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Re: 1/10 wave? new [Re: Jim_Smith]
      #5533820 - 11/22/12 11:54 AM

watch this, it's easy to understand, and its entertaining so get some popcorn.

Validating Your Mirror Quality


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Mirzam
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Re: 1/10 wave? new [Re: Mike B]
      #5533863 - 11/22/12 12:23 PM

I was looking for that! Thanks.

JimC


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dpwoos
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Re: 1/10 wave? new [Re: Jim_Smith]
      #5534002 - 11/22/12 01:59 PM

In my experience, the difference is huge. Folks will toss around all kinds of numbers, but the bottom line for me is what I see and the difference between a "diffraction limited" optic and a significantly better one is striking. In fact, I believe that a lot of eyepiece mythology stems from not recognizing this, i.e. that sooper-dooper eyepieces can close the gap, because the mirror can't be the problem. Well, they can't, as a mediocre (diffraction limited) mirror will never produce top-notch views regardless the eyepiece.

I think that some caveats are in order. Any mirror will only perform as good as the collimation, thermal control and seeing allow, and as everyone agrees. However, a factor that maybe is less recognized is the variability in folk's visual acuity. It is not unusual that, at our local club events, not everyone can see whatever we are looking at, and sometimes only a few can. For some of these folks, it may be that the difference between a mediocre and a great optic will go unnoticed. In fact, a friend of mine who was in attendance at the famous Stellafane mirror shoot-out has said that it was surprising to him how many folks claimed to be unable to detect even the most gross differences. In any case, it can be pretty awkward when some folks claim to see nothing while others are all oohing and aahing!

Edited by dpwoos (11/22/12 02:50 PM)


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ausastronomerModerator
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Re: 1/10 wave? new [Re: dpwoos]
      #5534471 - 11/22/12 09:05 PM

Quote:

In my experience, the difference is huge.




I can't quite agree with that comment entirely.

With smaller aperture telescopes (under say 12") the differences are fairly noticeable to a skilled observer under good observing conditions between say a diffraction limited mirror (1/4 wave) and a 1/10th wave mirror. However without star testing under excellent conditions it gets quite difficult to tell the difference between say a 1/7th or 1/8th wave mirror and a mirror which is 1/10th wave or better, for the simple reason a mirror that is a genuine 1/7th or 1/8th wave mirror is an exceedingly good mirror in any case. Any differences between mirrors at this level are very subtle at best and only detectable under the very best conditions.

With larger aperture telescopes (over say 15" aperture) it gets progressively more difficult to separate them, once they are better than diffraction limited, smooth and free of astigmatism. It is certainly still easy to pick a mirror which is worse than diffraction limited (1/4 wave). Larger aperture scopes are more noticeably affected by the subtleties of seeing and thermal equilibrium than smaller scopes. More often than not with a scope of this aperture you are limited by seeing and thermal equilibrium not optical quality, provided the mirror is better than diffraction limited, smooth and free of astigmatism.

What you will invariably find with these larger mirrors, from whichever premium maker you care to choose, is that most of them are no better than 1/6th or 1/7th wave across the full face of the mirror at the worst point. This of course still correlates to a true strehl in the very high 90's and will deliver excellent views.

Cheers,


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Asbytec
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Re: 1/10 wave? new [Re: ausastronomer]
      #5534867 - 11/23/12 04:33 AM

When the conditions are nearly perfect, the complete package including cooling to ambient, calm seeing, spot on collimation, and well corrected optics combine to form an image that is astounding. Great optics give the focused image that little, but noticeable nudge over the top. Combine that with some observing experience and you truly have a jaw dropping moment. It doesn't get any better than that. IMO, premium is worth it for those moments.

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Mark Harry
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Reged: 09/05/05

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Re: 1/10 wave? new [Re: Asbytec]
      #5534993 - 11/23/12 08:18 AM

A really good, smooth 1/10th wave mirror with no edge problems, is hard to beat, or fault with general purpose viewing habits. For planetary/lunar, I would recommend 1/8-1/10th as maximum error for the serious observers. IME, 1/8-1/10 with a good smooth surface and a touch of over/undercorrection can reach 50x/inch, with little to no bloat on stars occurring if conditions permit.
******
My 2 cents, with 1/4 wave and some experience under the belt, I think most observers would start to notice it won't be quite good enough- would lack the astounding characteristic that Norme relates above.
M.


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Asbytec
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Re: 1/10 wave? new [Re: Mark Harry]
      #5535032 - 11/23/12 08:47 AM

Mark, that's a good point. One might think better correction means it can stand a bit more power per inch. I suspect it does, as long as seeing permits and all the other performance variables are controlled. Not sure if that's accurate, though, since the Airy disc remains the same size. But it might due to less light scattered elsewhere. Contrast might hold up under a bit more magnification. But, higher powers are doable in practice and in good seeing.

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dpwoos
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Re: 1/10 wave? new [Re: ausastronomer]
      #5535096 - 11/23/12 09:22 AM

I haven't spent all that much time with mirrors bigger than 15", so your comments about big mirrors are interesting. Our club has recently purchased an 18" Obsession, and it disappoints me to say that I am disappointed. On the other hand, our club also purchased a 14" dob, and after a club member refigured the (awful!) mirror it now performs as well as any. A real pleasure to observe with.

Is the big 18" incapable of providing high power images that are as good as the 14", regardless of mirror figure? It may be so, but I won't believe it until I have the opportunity to see this for myself. I believe that the Obsession mirror is guaranteed to be "diffraction limited". There are those in our club who say that the 18" dob is a "light bucket", and not a "planetary scope". However, the same thing was said about the 14", and after that mirror was refigured there is no question that it is an excellent "planetary scope". So, for me the jury is still out on this question for bigger (> 14") mirrors, but I have to admit to being skeptical that something magical happens to collapse the perceived difference between a mediocre optic and an excellent optic when the size goes from 14" to 18", and beyond.

Edited by dpwoos (11/23/12 09:57 AM)


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tomharri
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Reged: 09/19/08

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Re: 1/10 wave? new [Re: dpwoos]
      #5535184 - 11/23/12 10:15 AM

I've had 3x 10" newts over the last 30 years, and you can easily see the difference between a Coulter 'diffraction limited', vs. a strehl .92 Hubble, vs. another Hubble at .98 strehl.

Also mirror mounts are critical especially for thin Hubble mirrors. A 3 point won't allow maximum viewing quality, must be a 9 point floater for 10". And for larger than 14-15" mirrors, must use a 18 point mount to support the mirror properly.


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BillFerris
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Re: 1/10 wave? new [Re: Mike B]
      #5535274 - 11/23/12 11:16 AM

Quote:

Here's some more to read!


The short answer is, apparently, *yes*, there can be, if all the other aspects that affect optics are properly handled, and the seeing cooperates reasonably well.




The bottom line takeaway for me from the Ceravolo, Dickinson & George article is that a diffraction-limited optic (1/4-wave peak-to-valley on the wavefront) will deliver in-focus views that, to the casual observer, are indistinguishable from those delivered by a 1/10-wave optic. In short, Lord Rayleigh's standard remains relevant as a measure of an optical system's performance.

As the authors found, an experienced observer who knows what to look for is able to discern a modest improvement in views delivered by a 1/10-wave system when compared, side-by-side, to a 1/4-wave system under very good to excellent seeing conditions. A 1-wave system should be obviously limited in performance even to the casual observer. A 1/2-wave system should reveal itself as limited after not much time or effort. But 1/4-wave and better systems require time and the right conditions to distinguish.

This is not to say a 1/10-wave system isn't worth the expense. For some observers, just the knowledge that the view delivered by their scope isn't indistinguishable from perfect is enough to limit their enjoyment. But for most observers, a diffraction limited scope is going to deliver crisp, detailed views and years of enjoyable use.

Bill in Flag


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dpwoos
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Re: 1/10 wave? new [Re: BillFerris]
      #5535452 - 11/23/12 12:57 PM

For sure that is what they say, and I don't agree at all. These three are very respected, and so it gives me no pleasure to disagree. However, I can only say that I see what I see, and what other folks that I know and respect as much as these three claim to see. I would advise anybody who cares about this to visit their local astro club, where they can use all kinds of scopes with all kinds of optical quality, and judge for themselves.

A club member with a commercial/Chinese 10" dob recently did just that, and he ended up having our club's best mirror maker do a refiguring job. I don't recall specifically looking through his scope before the work, but now it is a real joy to observe with and he seems elated with the improvement. In my experience this story is not uncommon. Anecdotal evidence only? For sure, but so is the evidence to the contrary even though it comes from respected folks.


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GeneT
Ely Kid
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Reged: 11/07/08

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Re: 1/10 wave? new [Re: Jim_Smith]
      #5535521 - 11/23/12 01:31 PM

Yes.

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ausastronomerModerator
Carpal Tunnel
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Reged: 06/30/03

Loc: Kiama NSW (Australia)
Re: 1/10 wave? new [Re: dpwoos]
      #5535886 - 11/23/12 05:16 PM

Quote:

I haven't spent all that much time with mirrors bigger than 15", so your comments about big mirrors are interesting. Our club has recently purchased an 18" Obsession, and it disappoints me to say that I am disappointed. On the other hand, our club also purchased a 14" dob, and after a club member refigured the (awful!) mirror it now performs as well as any. A real pleasure to observe with.

Is the big 18" incapable of providing high power images that are as good as the 14", regardless of mirror figure? It may be so, but I won't believe it until I have the opportunity to see this for myself. I believe that the Obsession mirror is guaranteed to be "diffraction limited". There are those in our club who say that the 18" dob is a "light bucket", and not a "planetary scope". However, the same thing was said about the 14", and after that mirror was refigured there is no question that it is an excellent "planetary scope". So, for me the jury is still out on this question for bigger (> 14") mirrors, but I have to admit to being skeptical that something magical happens to collapse the perceived difference between a mediocre optic and an excellent optic when the size goes from 14" to 18", and beyond.




Quote:

I have to admit to being skeptical that something magical happens to collapse the perceived difference between a mediocre optic and an excellent optic when the size goes from 14" to 18", and beyond.




This isn't the case at all. What happens is the number of nights where you can actually see that one mirror is better than the other reduces dramatically as the aperture increases. Mirror makers going to thinner 1.6" thick mirrors in larger apertures has helped dramatically in this regard but an 18" mirror under unfavourable thermal conditions can still take plenty of cooling, even if it is thin. Further as the aperture increases it takes progressively better seeing conditions for larger scopes to deliver their best images. For instance I have seen nights where my 14" scope will hold up at 400X but due to a combination of thermal equilibrium issues and variable seeing the 18" scope will not get past 250X. When everything comes together with the 18" scope it is an excellent planetary scope. I have had it to 1075X on the Moon and Saturn on a couple of occasions. It's just that as the aperture increases these occasions become less frequent.

We have 4 18"/F4.5 classic Obsessions in our 3RF arsenal of scopes. One of these has a .90 strehl mirror, the other 3 have mirrors with a strehl > .97. These mirrors have been interferometrically tested. On one occasion only in the 7 years we have had these scopes in our care, have I been able to pick the lower graded mirror apart from the others and that was a feature on Jupiter which was barely detectable; and that could have even been attributed to variable seeing as I changed scopes. That having been said that mirror is still a very good mirror. It's smooth with an excellent edge and slight undercorrection. In terms of peak to valley its worst point on the mirror face is 1/4.5 waves. That mirror is in fact the mirror in my scope and it can still push 1075X under favourable conditions. Would I have gained anything with 1 of the other mirrors, yeah something barely detectable once in 7 years.

Cheers,


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rguasto
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Reged: 11/18/10

Loc: Long Island, NY
Re: 1/10 wave? new [Re: ausastronomer]
      #5536097 - 11/23/12 07:05 PM

Quote:

yeah something barely detectable once in 7 years.




Well said. Very nice.

Image quality IMO is more dependent on seeing which we have no control over. The best optics and eyepieces still won't help bad seeing conditions. We're at the mercy of the atmosphere. The best views I've ever had were with telescopes of "average" (no apo's, all commercial mirrors) optical quality, 8" aperture and under but the atmosphere was very, very forgiving. 1/4 wave, 1/10 wave?.....................

-Rob

Edited by ausastronomer (11/24/12 06:09 PM)


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dan_h
Carpal Tunnel


Reged: 12/10/07

Re: 1/10 wave? new [Re: rguasto]
      #5536237 - 11/23/12 08:47 PM

Quote:

Image quality IMO is more dependent on seeing which we have no control over. The best optics and eyepieces still won't help bad seeing conditions. We're at the mercy of the atmosphere. The best views I've ever had were with telescopes of "average" (no apo's, all commercial mirrors) optical quality, 8" aperture and under but the atmosphere was very, very forgiving. 1/4 wave, 1/10 wave?.....................

-Rob




But if you were to get out of NY and relocate somewhere that has routinely excellent seeing, your point of view would change. Under premium conditions, premium optics rule.

dan


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ausastronomerModerator
Carpal Tunnel
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Reged: 06/30/03

Loc: Kiama NSW (Australia)
Re: 1/10 wave? new [Re: dan_h]
      #5536303 - 11/23/12 09:32 PM

I don't live anywhere near New York. In fact I have some premium optics, observe under Bortle I skies day in day out and haven't changed my opinion on this when it comes to larger optics.

Cheers


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ed_turco
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Re: 1/10 wave? new [Re: ausastronomer]
      #5537121 - 11/24/12 11:49 AM

True, under mediocre seeing, once in a great while, the seeing, by accident, becomes much better. And then you will thank your lucky stars that you have 1/10 wave mirror! Images become close to miraculous!

Ed


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rik ter horst
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Reged: 11/01/10

Loc: Ewer, the Netherlands
Re: 1/10 wave? new [Re: ausastronomer]
      #5537282 - 11/24/12 01:54 PM

Quote:

We have 4 18"/F4.5 classic Obsessions in our 3RF arsenal of scopes. One of these has a .90 strehl mirror, the other 3 have mirrors with a strehl > .97. These mirrors have been interferometrically tested. On one occasion only in the 7 years we have had these scopes in our care, have I been able to pick the lower graded mirror apart from the others and that was a feature on Jupiter which was barely detectable; and that could have even been attributed to variable seeing as I changed scopes. That having been said that mirror is still a very good mirror. It's smooth with an excellent edge and slight undercorrection. In terms of peak to valley its worst point on the mirror face is 1/4.5 waves. That mirror is in fact the mirror in my scope and it can still push 1075X under favourable conditions. Would I have gained anything with 1 of the other mirrors, yeah something barely detectable once in 7 years.

Cheers,




Interesting, your experience! I believe you should be very happy with your mirror (and you are I understand!). The mirrors have been tested with an interferometer in a laboratory under stable conditions I guess, and your particular mirror is undercorrected. Couldn't be much better because a 100% corrected large mirror tends to become slightly overcorrected during cool down because the edge of the mirror cools faster than the rest. Although your mirror has been measured to be Lambda/4.5 wave P-V undercorrected I expect it to be better than that during observation, at least as long as the temperature drops....

This might explain the almost invisible difference in performance.


Cheers,
Rik

Edited by ausastronomer (11/24/12 06:11 PM)


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dan_h
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Reged: 12/10/07

Re: 1/10 wave? new [Re: ausastronomer]
      #5537283 - 11/24/12 01:54 PM

Quote:

I don't live anywhere near New York. In fact I have some premium optics, observe under Bortle I skies day in day out and haven't changed my opinion on this when it comes to larger optics.

Cheers




The worst mirror you referenced was a Strehl 0.90 which is already a pretty decent optic. Do you think you would see the difference between it and a 0.8 Strehl? Under excellent skies, I think it would stand out to all but the most casual of observers.

dan


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csrlice12
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Re: 1/10 wave? new [Re: ed_turco]
      #5537325 - 11/24/12 02:30 PM

Regarding 1/10 wave mirrors in a Diagonal for a refractor. Celestron has a 1/10 wave XLT diagonal, any idea how it compares to other diagonals?

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Asbytec
Guy in a furry hat
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Re: 1/10 wave? new [Re: csrlice12]
      #5537376 - 11/24/12 03:15 PM

With the amount of light cast into the rings with 1/4 wave spherical, one should notice a difference when seeing is calm enough and all other variables controlled. Especially doing a side by side comparison of the focused image. When seeing, cooling, and collimation are combined with better correction, IMO, you will be astounded. The better correction just gives it that nudge over the top during those moments. The difference between Strehl 0.8 and perfect 1 should be apparent while 0.95, give or take, is quite good enough to show improvement.

Edited by Asbytec (11/24/12 03:20 PM)


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ausastronomerModerator
Carpal Tunnel
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Re: 1/10 wave? new [Re: dan_h]
      #5537648 - 11/24/12 06:32 PM

Quote:

The worst mirror you referenced was a Strehl 0.90 which is already a pretty decent optic. Do you think you would see the difference between it and a 0.8 Strehl? Under excellent skies, I think it would stand out to all but the most casual of observers.

dan




Hi Dan,

I discussed this in one of my earlier posts.

Quote:

With larger aperture telescopes (over say 15" aperture) it gets progressively more difficult to separate them, once they are better than diffraction limited, smooth and free of astigmatism. It is certainly still easy to pick a mirror which is worse than diffraction limited (1/4 wave).





The point I am trying to make is that with larger mirrors it's easy to detect a mirror that is less than good to very good, but you need progressively better seeing and thermal stabilisation to detect any gain once the mirror gets to this level and even then the gain is very subtle.

The difference with smaller mirrors (say under 14" to 15") is a lot more noticeable and on a lot more occasions.

Cheers,


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GeneT
Ely Kid
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Reged: 11/07/08

Loc: South Texas
Re: 1/10 wave? new [Re: ed_turco]
      #5545658 - 11/29/12 02:46 PM

Quote:

True, under mediocre seeing, once in a great while, the seeing, by accident, becomes much better. And then you will thank your lucky stars that you have 1/10 wave mirror! Images become close to miraculous! Ed




This is where I come out.


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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: 1/10 wave? new [Re: GeneT]
      #5545745 - 11/29/12 03:52 PM

I use a large enough scope that it is very rare for the seeing to actually exceed the resolution of the mirror.
What I do notice, though, is that it seems the seeing is consistently better with a better mirror. I suspect what that means is that the tightness of the star images is better on the better optic, and the occasional flashes of truly good seeing are more likely to be seen and caught because the better optic has less scattered light.
Whatever the reason, I seem to be seeing a lot more excellent seeing than I used to be since the Zambuto mirror.


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pstarr
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Re: 1/10 wave? new [Re: Starman1]
      #5545894 - 11/29/12 05:40 PM

Quote:

I use a large enough scope that it is very rare for the seeing to actually exceed the resolution of the mirror.
What I do notice, though, is that it seems the seeing is consistently better with a better mirror. I suspect what that means is that the tightness of the star images is better on the better optic, and the occasional flashes of truly good seeing are more likely to be seen and caught because the better optic has less scattered light.
Whatever the reason, I seem to be seeing a lot more excellent seeing than I used to be since the Zambuto mirror.




This has been discussed on the Zambuto mirror forum. A top notch mirror seems to cut through the bad seeing. The best way I can describe it is, you see the waves of turbulence across the image but underneath the image is still sharp.


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Asbytec
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Re: 1/10 wave? new [Re: pstarr]
      #5546932 - 11/30/12 10:27 AM

I think that's interesting because seeing adds to total RMS in the wavefront. In good seeing, the wavefront is pretty much planar. As seeing worsens, the wavefront is further misshapen. As it hits the optic, the timing is all off generating larger P-V errors (and ray trace no longer parallel generating speckles, as I understand it.)

I tend to think optics with very good RMS can handle a bit worse seeing conditions before the image breaks down. For example, say your scope has an RMS of 0.030 and a Strhel of >/= 0.95. It would take average seeing on the order of RMS 0.40 to drag the final image to (or below) the diffraction limit (~0.070.) Where as a scope that is diffraction at best might not produce a good image even in better seeing.

But, when seeing permits a diffraction limited scope to operate at or near it's theoretical MTF, can it be distinguished from a scope that is better corrected in those same conditions? I believe so. Both scopes would perform near their ideal MTF curve, and a better scope simply has a better curve (obstruction aside.)

Personally, I believe my own scope to be near RMS 0.40 (~1/6th or better SA) and when seeing is good, it's simply jaw dropping (smaller aperture and CO, aside.) I doubt I could tell much difference if it were 0.030 RMS or better. But, the loss of contrast could be seen if it were closer to 0.07 RMS with more light in the rings.

I do think Starman is onto something. Imagine a good scope's image in good seeing, then the same scope's image in slightly worse seeing and at the diffraction limit for the final image. The light scatter (diffraction and speckling) is better in the former and the contrast should be noticeably better than the latter.

Dunno, just an ejumecated, working hunch and a little experience with better than average seeing.


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Mike Lockwood
Vendor, Lockwood Custom Optics
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Reged: 10/01/07

Loc: Usually in my optical shop
Re: 1/10 wave? new [Re: Asbytec]
      #5547006 - 11/30/12 11:27 AM

It's not too complex.

A mirror is supposed to focus light into a tiny spot. A poor mirror makes a larger spot. The larger spot can happen on a mirror with transverse/slope error even though it has a small P-V error, even 1/10th wave. (A smooth mirror will not have the transverse error, though.)

Seeing further enlarges the spot in a random, time varying manner.

So, a good mirror will still make a smaller spot, on average, than a poorer one.

The difference in the tightness of star images and detail resolved on objects IS noticeable even under somewhat bad seeing, and even on large instruments.

A good reflector of any focal ratio, properly equilibrated and collimated, small or large, should produce good images on most nights, and superb images frequently.

I often hear of instruments that have "one spectacular night per year". Immediately this sets off an alarm in my mind that something is wrong.

For any poor instrument, thermal conditions will vary, but chances are that infrequently, once in a great while, those conditions will help the poor mirror perform better than it actually is!

However, the number of nights that a better mirror will perform well is FAR greater than the poorer one.

In my experience, any test claiming high Strehls should be taken with a grain of salt. Evaluation under the stars on a good number of nights, and comparisons with other instruments on the same night, under the same conditions, is a test method that I recommend for telescope owners.


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

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Re: 1/10 wave? new [Re: Mike Lockwood]
      #5547154 - 11/30/12 12:50 PM

Mike, when you say larger "spot", you are not referring to the airy disc?

To me that's a gray area I don't fully comprehend. It's easy enough to see how at least one ray could miss the Airy disc or some target "spot", but really the Airy disc forms when the entire wave comes to focus. And if one ray is not cooperating (the wavefront P-V), then it get's sent to the diffraction rings when the diffraction pattern comes to focus. Right?

The Airy disc is determined by aperture, not figure. A poor 6" figure does not make an Airy "spot" larger than 0.92" arc, it redistributes light throughout the pattern. Is not the result of a poor optic a normal Airy pattern with more light spread into the rings? Or does that offending ray actually show as blurring somewhere in the first minimum?

Edited by Asbytec (11/30/12 12:52 PM)


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ausastronomerModerator
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Re: 1/10 wave? new [Re: Mike Lockwood]
      #5547807 - 11/30/12 08:21 PM

Quote:

It's not real complex.

The difference in the tightness of star images and detail resolved on objects IS noticeable even under somewhat bad seeing, and even on large instruments.

A good reflector of any focal ratio, properly equilibrated and collimated, small or large, should produce good images on most nights, and superb images frequently.

For any poor instrument, thermal conditions will vary, but chances are that infrequently, once in a great while, those conditions will help the poor mirror perform better than it actually is!





Hi Mike,

It's more complex than you give it credit. I can't agree entirely with your comments. Your comments really only apply to areas that do not experience dramatic temperature changes from day to night.

Let me give you this example:-

Yesterday in Sydney the temperature peaked at 29.1 C at 1:40pm (84.4F) and fell to a low of 23.5 C (74.3 F) at 2:40am this morning. That is a very low temperature change of .4 C / hour or .8 F / hour. I have no doubt that those people who observed in Sydney last night, with scopes of all apertures, had really nice views, provided they had decent mirrors. All the high grade mirrors would have shone like beacons.

Two weeks ago when I observed at Coonabarabran with a group of 22 Canadian and Asian observers who were eclipse visitors, the temperature change from 3pm Sunday afternoon to 1am Monday morning was dramatic. The temperature dropped from 33 C (91.4 F) at 3pm to 8 C (46.4 F) at 2am. this is a drop of 2.5 C / hour or 4.5 F /hour.

I can guarantee you at no stage did any of the scopes deliver exceptional images. My 14" with a thin Zambuto mirror was certainly the least affected and gave the best planetary images, because it was the smallest scope on the field with the thinnest mirror (1.3"); but the 18" and over scopes with 2" plus thicknness mirrors never had a hope of catching up and stabilising at any stage of the night.

In Australia (and I am sure in some parts of the US) we frequently observe under these types of conditions. It's not uncommon in many places to go from high 30's C to well under 10 C at night. I observed one night at about 3000 feet elevation where the temperature dropped from 42 C (108 F) at 5pm to 4 C (39F)at 1am. A fall of 4.75C or 8.6 F per hour, over 8 hours. The rapid temperature drop was sensational for the body and soul, pretty ordinary for the telescopes and local seeing conditions. Granted with people like you and Mark Suchting who are now capable of producing top quality large thin mirrors, things have improved dramatically. But irrespective of mirror thickness an 18" plus aperture mirror can still struggle to stabilising under rapidly falling temperatures. It just does a lot better than a thick one of the same aperture. These rapidly falling temperatures also have an effect on local seeing conditions as well as on thermal equilibration of the mirror.

You also need to keep in mind that in regards to the number of 18" plus aperture mirrors running around the world, there are an infinitely greater number of them 2" thick at minimum, than there are < 2" thick. For that matter there are also a lot more mirrors in the 12" to 18" aperture class mirrors which are 2" think running around the world than there are which are less than 2" thick. 2" thick was the norm with 10" to 20" mirrors for decades. At one time it was even a selling point to have a "full thickness mirror". Thin mirrors and "honeycomb" mirrors are a recent innovation, thick mirrors are still in proliferation.

Quote:


In my experience, any test claiming high Strehls should be taken with a grain of salt. Evaluation under the stars on a good number of nights, and comparisons with other instruments on the same night, under the same conditions, is a test method that I recommend for telescope owners.




I couldn't agree more. You need to remember that I am more than capable of properly star testing a telescope to confirm its optical quality. In addition, a very skilled optician is a friend of mine and I have seen some of these mirrors on the test stand, under bench test conditions.

You and I both know the accuracy of the interferometer results is totally dependent on the honesty of the optician. In some cases the skill of the optician to use his test equipment and the software properly also comes into play. There are 101 ways to fudge the interferometer results as you and I both know. Clipping the edges of the fringes and/or removing astigmatism are two pretty common ones. There are plenty of others. I have seen several examples of it over the past couple of years, where the interferometric test certificate and calculated numbers were clearly inconsistent with the "actual" mirror quality. I am not going to comment any further on that, but I have no doubt you have seen it many times too.

Cheers,


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Re: 1/10 wave? new [Re: ausastronomer]
      #5547836 - 11/30/12 08:40 PM

John's post brings up the fact I recently went from a 2" thick mirror with 1 fan to a 1.25" thick mirror with 3 fans, so I am at the ambient temperature a lot more.
This time of year, my observing site goes from a daily high of 58F degrees (15C) to a low night time temperature of 15F (-10C)
And it does so in the first 3 or 4 hours of the night, then stabilizes to a 1F degree per hour fall after that.
If I'm seeing better images, the thermal characteristics of the mirror have a lot to do with it.


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Re: 1/10 wave? new [Re: Starman1]
      #5547991 - 11/30/12 10:35 PM

Quote:

I recently went from a 2" thick mirror with 1 fan to a 1.25" thick mirror with 3 fans




how did that effect your balance?


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Re: 1/10 wave? new [Re: Pinbout]
      #5548040 - 11/30/12 11:26 PM

Quote:

Quote:

I recently went from a 2" thick mirror with 1 fan to a 1.25" thick mirror with 3 fans




how did that effect your balance?




Different scopes go along with those different mirrors. He now has a 12.5" Teeter and used to have a 12.5" Discovery Truss. Or do you mean does Don fall down now? ; )

Bill


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Re: 1/10 wave? new [Re: Bill Weir]
      #5548048 - 11/30/12 11:33 PM

Quote:

Or do you mean does Don fall down now?




No, i think he was asking about his checking account since getting the Teeter.


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Re: 1/10 wave? new [Re: Mike B]
      #5548092 - 12/01/12 12:07 AM

Quote:

No, i think he was asking about his checking account since getting the Teeter.







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Starman1
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Re: 1/10 wave? new [Re: Mike B]
      #5548153 - 12/01/12 01:18 AM

Quote:

Quote:

Or do you mean does Don fall down now?




No, i think he was asking about his checking account since getting the Teeter.




Amen.


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Re: 1/10 wave? new [Re: Starman1]
      #5548356 - 12/01/12 07:14 AM

Agree with John; +1. There are several favorite objects I frequent, and one of them I've seen certain resolution, contrast and definition only once in the last 3 years on a spectacular transparent night when everything cooperated. Several have said it was theoretically impossible- but later substantiated as doable with fine conditions and excellent equipment.
M.


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Re: 1/10 wave? new [Re: Starman1]
      #5549309 - 12/01/12 07:36 PM

Quote:

What I do notice, though, is that it seems the seeing is consistently better with a better mirror. I suspect what that means is that the tightness of the star images is better on the better optic, and the occasional flashes of truly good seeing are more likely to be seen and caught because the better optic has less scattered light.
Whatever the reason, I seem to be seeing a lot more excellent seeing than I used to be since the Zambuto mirror.




Hi Don,

We both mention one of the main reasons for this in subsequent posts. That is the better thermal equilibrium of the new scope with much thinner mirror and a better fan setup.

The other reason is the fact that the Zambuto mirror is exceptionally smooth and totally eliminates any micro ripple or surface roughness, which contribute to increase light scatter.

While your new Zambuto mirror is likely better than 1/10th wave, I have little doubt you would still see a greater frequency of nights with "better seeing" if you had a Zambuto mirror that was only 1/5 or 1/6 wave.

Cheers,


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Re: 1/10 wave? new [Re: ausastronomer]
      #5550978 - 12/02/12 08:21 PM

Quote:



I can't quite agree with that comment entirely.

With smaller aperture telescopes (under say 12") the differences are fairly noticeable to a skilled observer under good observing conditions between say a diffraction limited mirror (1/4 wave) and a 1/10th wave mirror. However without star testing under excellent conditions it gets quite difficult to tell the difference between say a 1/7th or 1/8th wave mirror and a mirror which is 1/10th wave or better, for the simple reason a mirror that is a genuine 1/7th or 1/8th wave mirror is an exceedingly good mirror in any case. Any differences between mirrors at this level are very subtle at best and only detectable under the very best conditions.

With larger aperture telescopes (over say 15" aperture) it gets progressively more difficult to separate them, once they are better than diffraction limited, smooth and free of astigmatism. It is certainly still easy to pick a mirror which is worse than diffraction limited (1/4 wave). Larger aperture scopes are more noticeably affected by the subtleties of seeing and thermal equilibrium than smaller scopes. More often than not with a scope of this aperture you are limited by seeing and thermal equilibrium not optical quality, provided the mirror is better than diffraction limited, smooth and free of astigmatism.

What you will invariably find with these larger mirrors, from whichever premium maker you care to choose, is that most of them are no better than 1/6th or 1/7th wave across the full face of the mirror at the worst point. This of course still correlates to a true strehl in the very high 90's and will deliver excellent views.

Cheers,




My 16 inch Meade was tested at 1/4 wave. I believe 1/4 wave is the lower limit for a diffraction limited performance. Certainly not the best optic I own, however over the 18 years I have owned it, I must say that it has given me the best views of Jupiter and Saturn that I have ever seen, but only on rare occasions when the atmosphere was not the limiting factor.
Unfortunately around my part of the world, the atmosphere rarely lets the 16 inch operate at it's theoretical limit of resolution.

Steve


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Re: 1/10 wave? new [Re: Starman1]
      #5552338 - 12/03/12 04:01 PM

Quote:

the occasional flashes of truly good seeing are more likely to be seen and caught because the better optic has less scattered light.




I have found this to be true. Often, the seeing moves in and out from average, then for a few seconds sharpens up to excellent, then back to mediocre.


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Re: 1/10 wave? new [Re: ausastronomer]
      #5552659 - 12/03/12 07:30 PM

Quote:

It's more complex than you give it credit. I can't agree entirely with your comments. Your comments really only apply to areas that do not experience dramatic temperature changes from day to night.



No, it's not more complex - please re-read my original statement - I specifically stated my assumption that the mirror was equilibrated.

I was not considering the case of rapidly falling temperatures. I was trying to address the original question that started the thread, which made no reference to cooling.

I have seen mirrors with relatively low P-V error that had significant slope/transverse error. After rework, the P-V error was slightly lower, but the transverse error was significantly lower. Stars were perceived to be "tighter" after the rework, in a variety of seeing conditions. Of course the effect will be more noticeable when the seeing is good.

Quote:

You also need to keep in mind that in regards to the number of 18" plus aperture mirrors running around the world, there are an infinitely greater number of them 2" thick at minimum, than there are < 2" thick. For that matter there are also a lot more mirrors in the 12" to 18" aperture class mirrors which are 2" think running around the world than there are which are less than 2" thick. 2" thick was the norm with 10" to 20" mirrors for decades. At one time it was even a selling point to have a "full thickness mirror".



That's true, and a bit unfortunate.

Quote:

You need to remember that I am more than capable of properly star testing a telescope to confirm its optical quality. In addition, a very skilled optician is a friend of mine and I have seen some of these mirrors on the test stand, under bench test conditions.



Since I was not replying to any of your comments, so I am not sure why you are telling me what I should remember and keep in mind. Have we corresponded before?

Quote:

Mike, when you say larger "spot", you are not referring to the airy disc?
To me that's a gray area I don't fully comprehend. It's easy enough to see how at least one ray could miss the Airy disc or some target "spot", but really the Airy disc forms when the entire wave comes to focus. And if one ray is not cooperating (the wavefront P-V), then it get's sent to the diffraction rings when the diffraction pattern comes to focus. Right?



Yes, I'm talking about the Airy disc. The Airy disc is the smallest image that a perfect mirror can form. This is due to the size of the mirror and the geometry of image formation.

Quote:

The Airy disc is determined by aperture, not figure.



The minimum possible size of the disc is determined by aperture. The pattern and energy distribution are determined by the central obstruction and figure.

Quote:

A poor 6" figure does not make an Airy "spot" larger than 0.92" arc, it redistributes light throughout the pattern. Is not the result of a poor optic a normal Airy pattern with more light spread into the rings? Or does that offending ray actually show as blurring somewhere in the first minimum?



As the spot loses amplitude, the rings lose or gain amplitude depending on how the energy is moved around, and the various minima grow less minimal. So, yes, the blurring does move into the minima, effectively enlarging the spot and making it less distinct from the surrounding energy.


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bartine
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Re: 1/10 wave? new [Re: ed_turco]
      #5552667 - 12/03/12 07:35 PM

For me there is a difference.

I had heavily light polluted skies in Atlanta. Often very turbulent. Had a Meade Starfinder 16.

Scope was OK for viewing, but not great. Re-figured the mirror from 1/4 wave to 1/9. The difference was striking.

Period. Regardless of conditions.


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Re: 1/10 wave? new [Re: bartine]
      #5552847 - 12/03/12 09:13 PM

Quote:

For me there is a difference.

I had heavily light polluted skies in Atlanta. Often very turbulent. Had a Meade Starfinder 16.

Scope was OK for viewing, but not great. Re-figured the mirror from 1/4 wave to 1/9. The difference was striking.

Period. Regardless of conditions.




If that's the case I would have no hesitation in saying that the original mirror wasn't even close to a 1/4 wave, in reality it was probably 1/3rd to 1/2 wave, with a poor level of polish, despite what it might have been advertised or sold as. I have seen a couple of those 16"/F4.5 Starfinder mirrors that aren't nearly a genuine diffraction limited mirror. On the other hand of course some of those 16" Starfinder mirrors are 1/2 decent and certainly 1/4 wave.

Further, the refigured mirror will have a much higher degree of polish than the Meade original, irrespective of its P to V wavefront error, thus reducing light scatter dramatically and giving better images each and every night.

Cheers,


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Re: 1/10 wave? new [Re: ausastronomer]
      #5553069 - 12/03/12 11:11 PM

You seem to know a lot about a mirror that you have never been near? Really, no anecdotal evidence is going to change your mind, and in fact that is the only kind of evidence that we have! I think that this discussion reaffirms what I have come to realize: when all is said and done everybody will form opinions based on their experiences - climate, seeing, optics, eyesight, observing experience, etc. One person's absolute certainty is another person's no way in a million years, and given all of the variables it may be that both are equally correct. Which is why the best advice is, finally, to get out and observe with friends and club members, and see what is what for oneself. Personally, I have not been dissuaded from my belief that a substantially better (than diffraction limited) optic results in (frequently) substantially better views, though I have found your explanations thought-provoking.

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Re: 1/10 wave? new [Re: dpwoos]
      #5553087 - 12/03/12 11:24 PM

Quote:

You seem to know a lot about a mirror that you have never been near?




Because I know that if it was a genuine 1/4 wave mirror he would not see a difference each and every night, having regard to the fact those Meade mirrors are 2" thick and take plenty of cooling because of the closed tube, the cell design and no fan in most cases. I am very confident what he started with was a pretty average mirror, despite what it might have been sold as.

I have an 18" mirror that is just better than 1/4 wave at it's worst point, so it is a "genuine" 1/4 wave mirror (across the full face of the mirror) and very smooth and the times it can be picked apart from other mirrors with a higher P to V wave rating you can count on one hand each decade.

Cheers,


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Re: 1/10 wave? new [Re: dpwoos]
      #5553125 - 12/03/12 11:46 PM

Quote:

Personally, I have not been dissuaded from my belief that a substantially better (than diffraction limited) optic results in (frequently) substantially better views, though I have found your explanations thought-provoking.




Some part of the gain you are attributing to a better wavefront error, will in fact be attributable to the fact the mirror is smoother, has a better polish and reduced microripple. This reduces light scatter which in every case will offer an improved view.

However, a better polished mirror generally goes hand in hand with a mirror having a better wavefront surface accuracy. Both stem from the skill and time that goes into the mirror.

Cheers,


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Asbytec
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Re: 1/10 wave? new [Re: Mike Lockwood]
      #5553163 - 12/04/12 12:16 AM

Quote:

So, yes, the blurring does move into the minima, effectively enlarging the spot and making it less distinct from the surrounding energy.




Thank you. I guess that makes sense, but hard to imagine the minima being brighter. Maybe it's a relative thing, set the mimima to zero then measure the fall off in the airy disc from that point.

But, what you're saying is the Airy disc is the same diameter in a given aperture regardless, it's just some of the light encroaches nearer to the first minima that defines it. That's hard to grasp because the peak intensity falls off and so would the size and brightness of the spurious disc. Seems light near the edge of the Airy disc would fall off equally with a more shallow curve onto the first minimum instead of an increase.

Anyway, thanks for replying. Got me thinking...an aberrated and diffracted wave front collapsing to focus is a mysterious thing.

There is little doubt smoothness is critical. One might wonder what a mirror with 1/20th wave micro ripple (pretty smooth, I guess) on the surface would look like so long as the edge was not turned back by L/4 on the wave front.

I dunno, someone said it best above. IME, better optics are just better up to a point, and it becomes increasingly harder to tell beyond that point (all other variables, including experience, held constant and assuming perfect seeing.) Maybe it is the associated smoothness that makes the largest difference. But, certainly a less fuzzy minimum and brighter peak/Dimmer rings are important, too. That's resolution, that can be seen. IME, anyway.


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Starman1
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Re: 1/10 wave? new [Re: Asbytec]
      #5553712 - 12/04/12 10:34 AM

The Airy disc includes the spurious disc in the center (the point) and out to the middle of the first minimum in between the spurious disc and the first diffraction ring.
Throwing more energy out of the spurious disc reduces its height faster than its diameter. By the time the spurious disc is 10% smaller in diameter, the intensity of the spurious disc has fallen to 50% or less (from the theoretically perfect 84%).
So improved resolution on double stars does not come from greater diminishment of the intensity of the central spike by throwing more intensity into the rings. If it did, double star observers would all use 50% secondaries to throw more light into the rings.
What does diminish the apparent size of the spurious disc is to reduce the height of the central spike and diffraction rings (i.e. view a star of fainter magnitude).
The apparent, visible, width of the spurious disc does diminish as more of the star image (spike and diffraction rings) falls below the threshold of visibility. As I understand it, the distance to the first ring stays the same even as the star image grows fainter, so the true size of the Airy disc does not diminish with magnitude, only the apparent size of the spurious disc. Since the visible width of the gaps between diffraction rings will seem to grow as the image gets fainter, this could lead to a better resolution of double stars.
Indeed, I have found that I can see closer gaps between double stars in faint doubles than I can in bright doubles.
It should be noted that as the intensity of the central spike diminishes, so does the intensity of the diffraction rings.


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Re: 1/10 wave? new [Re: Starman1]
      #5554012 - 12/04/12 01:35 PM

Quote:

As I understand it, the distance to the first ring stays the same even as the star image grows fainter, so the true size of the Airy disc does not diminish with magnitude, only the apparent size of the spurious disc.




First, I may be misunderstanding what you are saying. If so, I apologize in advance.

Having said that, I think "in theory" that as the central obstruction increases, the radius of the first minimum decreases, the intensity of the central peak decreases and the intensity of the rings increases.

I think the calculation is actually done (to some extent) in Arfken, Mathematical Methods for Physicists (and many other places as well).

I'll look it up when I get home and either provide more details or apologize for a bad memory.


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Asbytec
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Re: 1/10 wave? new [Re: Starman1]
      #5554564 - 12/04/12 08:11 PM

Don, thank you. It's interesting topic. IME, a difference can be noted, but why? Is it the inherent smoothness or is it the transverse aberration?

Yes, I suspect the Airy disc remains the same diameter (pending any article by Sirchz), but does it change appearance other than being dimmer? Does it get mushy and surrounded by faint light at lessor correction and more tight at better correction. If so, is this responsible for the improvement we can see? No reason to doubt that, really, but is a mushy appearance the reason?

The diffracted wavefront coming to focus is full of interference, some points on the disc are canceled while others are augmented (CO aside for the moment.) If the optic is perfect, then it adds no more disruption to the phase nor the pattern of interference. However, if the optic does not produce a spherical wavefront, then the greater optical path from different rays changes the pattern of interference.

But, the radii of the Airy disc, first minimum and second maximum, etc., should remain constant and based solely on the wavelength and aperture. So, as I understand it, the first minimum is a point of maximum interference cancelling all energy from that point (or at that radii) regardless of the figure. So, the Airy disc will still fall to zero at a set first minimum distance even as the peak intensity falls off. The PSF curve at the very edge of the Airy disc remains very steep, hence the Airy disc well defined. (I was wrong above thinking the slope toward minimum would be more shallow.)

So, I am not sure transverse error has a role in giving the Airy disc a mushy appearance. This diffracted and aberrated wavefront still cancels energy at the first minimum, just the pattern (at set radii determined by wavelength and aperture change in relative brightness.) However, it does redistribute the light across the pattern as the total energy must remain the same.

Similarly, micro ripple across the entire surface is an aberration, it sends rays here and there and induces some very fine changes in the wavefront. But they tend to cancel over the entire wavefront. Some points are, say, +1/10th and other points are -1/10th from the perfect reference sphere. Without doing the math, I think total RMS is not affected. In other words, what's left to affect the average is the overall wavefront deviation (back to transverse ray's again ) But, I think at focus ray's do not define the pattern seen, they are simply geometrical representations of self interfering wavefront and not a portion of the wave providing energy into the first minimum. As I can understand it, anyway.

So, what would give the in focus Airy pattern that washed out look and is this what makes the difference between a better optic and a lessor one? Or is it the redistribution of light across the pattern through interference induced by diffraction, surface deviation (phase), and the CO? I dunno, but I suspect it's the redistribution of light into the rings that makes the difference. CO induced diffraction would simply add to the wavefront's diffraction making the redistribution more pronounced (and a tiny bit more so if the secondary is not perfectly flat?)


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Re: 1/10 wave? new [Re: dpwoos]
      #5554673 - 12/04/12 09:28 PM

Quote:

Really, no anecdotal evidence is going to change your mind, and in fact that is the only kind of evidence that we have!




All the evidence isn't anecdotal. Ceravolo, Dickinson and George conducted an experiment in which one person figured four 6-inch, f/8 mirrors, respectively, to 1-, 1/2-, 1/4- and 1/10-wave accuracy. The figures were confirmed with an interferometer.

The finished paraboloids were installed in identical optical tube assemblies and Dobsonian mounts. Then, labelled only A, B, C and D, the four scopes were shipped to Terence Dickinson for multiple nights of testing and evaluation. When Dickinson was done with his evaluation, the scopes were sent to Douglas George, who performed his own independent tests.

Both Dickinson and George knew the four scopes included mirrors figured to different levels of accuracy. Neither knew which was which. They had to rely on their own evaluations of images at the eyepiece to distinguish one from another. In the end, both were in agreement that the 1/10-wave primary could be distinguished from the 1/4-wave primary, but only when conditions were excellent and careful attention was paid to subtle differences in the view.

Ceravolo, Dickinson and George chose to perform their experiment using small mirrors to limit the impact atmospheric seeing would have on the results. It seems to me a similar test could be performed using larger mirrors. And with the substantial number of large aperture Dobsonians in use that also have documentation of the quality of their mirrors, it should be possible for an interested party to organize such a test.

Bill in Flag


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Re: 1/10 wave? new [Re: Asbytec]
      #5554869 - 12/04/12 11:45 PM

Quote:

Don, thank you. It's interesting topic. IME, a difference can be noted, but why? Is it the inherent smoothness or is it the transverse aberration?

Yes, I suspect the Airy disc remains the same diameter (pending any article by Sirchz), but does it change appearance other than being dimmer? Does it get mushy and surrounded by faint light at lessor correction and more tight at better correction. If so, is this responsible for the improvement we can see? No reason to doubt that, really, but is a mushy appearance the reason?

The diffracted wavefront coming to focus is full of interference, some points on the disc are canceled while others are augmented (CO aside for the moment.) If the optic is perfect, then it adds no more disruption to the phase nor the pattern of interference. However, if the optic does not produce a spherical wavefront, then the greater optical path from different rays changes the pattern of interference.

But, the radii of the Airy disc, first minimum and second maximum, etc., should remain constant and based solely on the wavelength and aperture. So, as I understand it, the first minimum is a point of maximum interference cancelling all energy from that point (or at that radii) regardless of the figure. So, the Airy disc will still fall to zero at a set first minimum distance even as the peak intensity falls off. The PSF curve at the very edge of the Airy disc remains very steep, hence the Airy disc well defined. (I was wrong above thinking the slope toward minimum would be more shallow.)

So, I am not sure transverse error has a role in giving the Airy disc a mushy appearance. This diffracted and aberrated wavefront still cancels energy at the first minimum, just the pattern (at set radii determined by wavelength and aperture change in relative brightness.) However, it does redistribute the light across the pattern as the total energy must remain the same.

Similarly, micro ripple across the entire surface is an aberration, it sends rays here and there and induces some very fine changes in the wavefront. But they tend to cancel over the entire wavefront. Some points are, say, +1/10th and other points are -1/10th from the perfect reference sphere. Without doing the math, I think total RMS is not affected. In other words, what's left to affect the average is the overall wavefront deviation (back to transverse ray's again ) But, I think at focus ray's do not define the pattern seen, they are simply geometrical representations of self interfering wavefront and not a portion of the wave providing energy into the first minimum. As I can understand it, anyway.

So, what would give the in focus Airy pattern that washed out look and is this what makes the difference between a better optic and a lessor one? Or is it the redistribution of light across the pattern through interference induced by diffraction, surface deviation (phase), and the CO? I dunno, but I suspect it's the redistribution of light into the rings that makes the difference. CO induced diffraction would simply add to the wavefront's diffraction making the redistribution more pronounced (and a tiny bit more so if the secondary is not perfectly flat?)



Well, a larger secondary throws more energy into the rings, so lessens contrast. Remember that every extended object is a series of Airy discs surrounded by diffraction rings. The brighter those rings are (the more energy they possess) the lower the contrast in the image (the more damage to the MTF).
Scattered light can create its own wavefront errors, throwing more light into the rings, and even, at worst, creating multiple overlapping Airy discs. Ever see the "hairy" disc created by seeing conditions? It's actually overlapping discs coupled with reflected light that smears the perfect diffraction pattern. It's worse than the problems caused by less-than-perfect optics.
Seeing is a major contributor to optical problems. I've seen nights where each star was a point with a single diffraction ring. And I've seen nights in the same scope where each blobby star was surrounded by a thick retinue of diffraction rings.

[Aside: you are correct to think the diameter of the Airy disc is determined by wavelength and aperture--not magnitude.]

Some time we'll talk about diffraction spikes and how seeing modifies their visibilities, but that's a discussion for another time.


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dpwoos
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Re: 1/10 wave? new [Re: BillFerris]
      #5554930 - 12/05/12 12:41 AM

You have cited this test more than once, and I think it is interesting and the results are interesting. However, the most accomplished mirror maker in our club (and a fine observer) participated in the well-known mirror test at Stellafane, and he has a very different story to tell. Also, I and others in our club have experiences that also don't agree with those of Dickinson. You say that the Dickinson test isn't anecdotal, but as much as I respect all of the folks involved to me this is far from a scientific test. Do you think that a paper based on this test would be accepted in a scientific publication? I think not, and I bet that you don't either. Bottom line - it is anecdotal evidence.

But as I have said, no one should accept what I have to say, or what Dickinson or Ceravalo or you has to say. In our club there are plenty of opportunities to observe with all kinds of optics of all kinds of quality, and one can see what one can see and make up ones mind on how to achieve good and great views. I am certain that most other clubs offer the same opportunities. We can (and must) decide for ourselves.

Edited by dpwoos (12/05/12 12:43 AM)


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Asbytec
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Re: 1/10 wave? new [Re: Starman1]
      #5554982 - 12/05/12 01:40 AM

I've seen some pretty bad Airy discs, I suppose they can be called Hairy.

I wasn't so much interested in seeing effects on the Airy disc, but more trying to understand the difference between micro ripple and transverse ray aberration. To me, both of those contribute to the pattern of interference and the phase of the wavefront - hence light distribution.

But, total RMS should be less affected by micro ripple averaged over the wavefront than large areas of P-V error. And the difference is brighter rings, dimmer disc. That in turn is what differentiates a better figure from a lessor one as opposed to a Hairy disc. Again, in turn, this difference can be seen in contrast resolution supporting the case that 1/10th wave (RMS) can be noticeably better.

I guess what I am driving at is, image quality depends on how that complex series of interference collapses to focus. That complex diffraction and aberrant interference both augments and destroys energy at specific locations (determined by wavelength and aperture) leaving the radii unchanged and energy simply distributed differently. The result is more dependent on total RMS and less so on "tightness" (or lack of) in the Airy pattern itself.

In other words, if I understand it correctly, worse correction should not turn the Airy pattern into a continuous, washed blob of light. It will still maintain its patterns of dark minima and brighter maxima defined only by wavelength and aperture, not transverse error or any other aberration.

Ah, I am rambling...sorry. Just trying to understand why better mirrors are indeed better.

Edited by Asbytec (12/05/12 03:07 AM)


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mark cowan
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Re: 1/10 wave? new [Re: Asbytec]
      #5556807 - 12/06/12 02:13 AM

Quote:

Similarly, micro ripple across the entire surface is an aberration, it sends rays here and there and induces some very fine changes in the wavefront. But they tend to cancel over the entire wavefront. Some points are, say, +1/10th and other points are -1/10th from the perfect reference sphere. Without doing the math, I think total RMS is not affected.




No, this is definitely an RMS error. But then, ALL errors are RMS errors. A whole lot of small errors can add up to significant wavefront error, and they don't cancel like you're suggesting. Small scale ripple casts a veiling haze that lowers overall contrast.

Quote:

I guess what I am driving at is, image quality depends on how that complex series of interference collapses to focus. That complex diffraction and aberrant interference both augments and destroys energy at specific locations (determined by wavelength and aperture) leaving the radii unchanged and energy simply distributed differently. The result is more dependent on total RMS and less so on "tightness" (or lack of) in the Airy pattern itself.




You're leaving out the whole point of minimizing transverse error, which is to tighten the FOCUS. Transverse error describes slope deviations on the glass, and slope deviations translate to errors in focal length, if you want to think about it that way. So no, the radii are not unchanged, and these changes in energy distribution are always detrimental, never favorable.

In general, the edge of the airy disc may remain well defined, but the result of dumping energy into the rings is mush to the eye.

Quote:

Both Dickinson and George knew the four scopes included mirrors figured to different levels of accuracy. Neither knew which was which. They had to rely on their own evaluations of images at the eyepiece to distinguish one from another. In the end, both were in agreement that the 1/10-wave primary could be distinguished from the 1/4-wave primary, but only when conditions were excellent and careful attention was paid to subtle differences in the view.




Aside from anything else, though, all the mirrors were very well made with the sole deviation of greater SA on the lower rated ones. I.e., they were smooth with well controlled figures. This is not what you normally get for mirrors that don't test well.

Best,
Mark


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orion61

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Re: 1/10 wave? new [Re: mark cowan]
      #5556904 - 12/06/12 05:31 AM

I saw an article about 20 yrs ago in astronomy mag or was it S&T?? wehere they set up 1/4 1/8 and 1/10th wave systems
even people walking down the sidewalk could clearly see the difference..


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Asbytec
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Re: 1/10 wave? new [Re: mark cowan]
      #5556928 - 12/06/12 06:13 AM

Mark, thanks. Its an interesting topic, can one see a difference? I believe so. Then the question is, what properties of a better mirror make that better image happen. How does diffraction limited SA differ from better correction? Yes, less light into the rings and better contrast is the answer.

At L/4 SA has much lower RMS (Strehl ~.0.8) and large transverse errors across the entire surface. At 1/10th (Strehl >/~ 0.95.), an improvement that large in Strehl should be noticeable. So, I argue the positive results are not surprising.

Some minor misunderstandings, though. By radii, I didn't mean radius of curvature or that of a reference sphere - transverse and longitudinal error. I meant the radius of the diffraction rings and Airy disc, trying to describe the diffracted and aberrant pattern comes to focus, phase, etc. I just don't see how the first minimum can grow in brightness (and the Airy disc get larger) with interference cancelling at that point in the image space, if that is indeed what happens. Both of those are set by aperture and wave length. I'm almost positive there is no other variable that affects them.

Anyway, it was fun exploring the topic. I appreciate you engaging in good debate.

Edited by Asbytec (12/06/12 06:23 AM)


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Re: 1/10 wave? new [Re: Asbytec]
      #5557008 - 12/06/12 07:56 AM Attachment (24 downloads)

Any comments on the attached test?

I do have my opinion about it, but I am interested to hear comments from you folks since the discussion here is really interesting and people lurking in this thread are much more knowledgeable than me...


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Starman1
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Re: 1/10 wave? new [Re: nicknacknock]
      #5557319 - 12/06/12 11:27 AM

1) the surface is rough on a fine scale.
2) the Strehl is unbelievable. To me it merely indicates an inadequate number of points selected.
3) It would be interesting to see this mirror in a Lyot test.
I suspect it would show to be very rough.


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dpwoos
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Re: 1/10 wave? new [Re: nicknacknock]
      #5557426 - 12/06/12 12:19 PM Attachment (25 downloads)

My knowledge of interferometry and interferograms is very limited, but the premiere mirror maker in our club has made and regularly uses his own Bath interferometer, and he says that it is very easy to convince yourself that you are seeing a level of detail that the test simply doesn't support. I have been told the same thing by Bryan Greer (ProtoStar) in a discussion about claimed secondary quality. So, I would be hesitant to ascribe a great deal of meaning to the apparent fine detail portrayed. Of course, others with more Zygo experience might say something different.

Here are two images (using the Bath) that depict a before and after of a mirror from a club scope, where the image on the right depicts the mirror at around Strehl .94 and the image on the left depicts the mirror when it was awful (including obvious astigmatism). BTW, I was the one who first checked out the mirror (actually Saturn was the first target) and the astigmatism was obvious. I think that the fellow who did both the testing and the refiguring believes that even this level of detail is a bit unrealistic, but he knows his rig and so knows what he is seeing, and how to address it.

Edited by dpwoos (12/06/12 04:51 PM)


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mark cowan
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Re: 1/10 wave? new [Re: Asbytec]
      #5557505 - 12/06/12 12:59 PM

Quote:

Some minor misunderstandings, though. By radii, I didn't mean radius of curvature or that of a reference sphere - transverse and longitudinal error. I meant the radius of the diffraction rings and Airy disc, trying to describe the diffracted and aberrant pattern comes to focus, phase, etc. I just don't see how the first minimum can grow in brightness (and the Airy disc get larger) with interference cancelling at that point in the image space, if that is indeed what happens. Both of those are set by aperture and wave length. I'm almost positive there is no other variable that affects them.




Aberrations will affect the radii you're talking about. The Airy disc retains the same angular size pretty much (it's least affected by aberrations until it vanishes entirely into the haze) but interference effects change the maxima and minima of the resultant ancillary rings. Download "aberrator" and experiment with the star image displays while changing the wavefront errors to see this.

Best,
Mark


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mark cowan
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Re: 1/10 wave? new [Re: Starman1]
      #5557515 - 12/06/12 01:06 PM

Quote:

1) the surface is rough on a fine scale.




Not necessarily from that graph - note the vertical scale. This probably amplifies residual noise in the system.

Best,
Mark


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Asbytec
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Re: 1/10 wave? new [Re: mark cowan]
      #5557547 - 12/06/12 01:25 PM

Well, maybe I am wrong or maybe we're saying the same thing talking past each other. For example, when you say the Airy disc is least affected by aberration, I immediately think of the energy that is robbed from it by aberration. Then you state correctly, "until it vanishes into the haze" meaning, I think, it fades but retains it's diameter set by the first minimum.

By the way, ever see the Poisson spot darken at the very center, forming kind of a hole, as you scroll through focus? I think that's interference alternating in and out of phase. So, the Airy disc should actually contain the sum of all the noise that comprises it. If so, then it seems to be more affected by phase differences caused by greater surface error.

It's been a while since I played with Aberrator. If memory serves, the rings stayed the same radius but the relative brightness changed. So, yes, aberration changes the maxima in relative brightness, at least. Maybe I need to look closer at the minima. CLICK! Yea, the contrast changes, as well. Hmmm...

Still, how can the minima not be zero? There must be some points along the diffraction pattern that fall to zero. It's interference. There have to be points that cancel, I believe. If not, then the Airy disc would not be constrained to a given size as the Airy disc vanishes, "into the haze." It would have no minima at that point. Or would it?

Now you got me thinking how that can be so. Anyway, thank you, I am beyond my knowledge at this point. But that's okay, it spurs inquiry.


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Re: 1/10 wave? new [Re: Asbytec]
      #5557559 - 12/06/12 01:31 PM

There has to be cancellation, but not necessarily to 0. If you have multiple waves overlapping, it is quite easy to end up with no point where they add up to exactly 0. The minimum could be >0.

Jarad


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mark cowan
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Re: 1/10 wave? new [Re: Asbytec]
      #5557638 - 12/06/12 02:21 PM Attachment (12 downloads)

Quote:

For example, when you say the Airy disc is least affected by aberration, I immediately think of the energy that is robbed from it by aberration.




I mean it retains its shape the longest in the face of aberration, nothing more.

Quote:

If memory serves, the rings stayed the same radius but the relative brightness changed.




Uhm, no. Crank up the diameter and power, add some CO to induce more rings, then add a wave or so of various aberrations. Measure the radii and you'll see the changes, as well as seeing how the central peak can vanish completely under sufficient wavefront error. Here's a simple example, showing a contrast stretched zero error + 20% CO against a wave each of astig, primary spherical, and 5th order SA. It's just an extreme example but it illustrates what I'm talking about...

Best,
Mark


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mark cowan
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Re: 1/10 wave? new [Re: mark cowan]
      #5557664 - 12/06/12 02:43 PM Attachment (26 downloads)

Possibly more germane, here's 1/5th wave of defocus. Mush.

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ausastronomerModerator
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Re: 1/10 wave? new [Re: mark cowan]
      #5557974 - 12/06/12 05:48 PM

Link to test results for 10"/F6 Zambuto mirror

The above link will take you to an independent interferometer test of a 10"/F6 Zambuto mirror by Wolfgang Rohr in Germany.

The mirror has a true strehl of .987 and an RMS error of 1/54.7 waves. Wolfgang claims it is one of the best mirrors he has ever tested.

PLEASE NOTE THE MIRROR HAS A PEAK TO VALLEY ERROR OF 1/7.3 WAVES, so it is "only" a 1/7th wave mirror

So which one of you can pick this lowly "genuine" 1/7th wave mirror from a "genuine" true 1/10th wave mirror ?

I rest my case.

Cheers


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dpwoos
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Re: 1/10 wave? new [Re: ausastronomer]
      #5558040 - 12/06/12 06:34 PM

Quote:

I rest my case.




I don't follow you - what case does this post about the Zambuto mirror support? I don't read anything that bears on whether or not it is possible to differentiate between this mirror and a significantly more accurate one. If you think that the "Zambuto" name alone is going to end the discussion, then I think you are mistaken.


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Mike Lockwood
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Re: 1/10 wave? new [Re: ausastronomer]
      #5558047 - 12/06/12 06:40 PM

Quote:

The mirror has a true strehl of .987 and an RMS error of 1/54.7 waves. Wolfgang claims it is one of the best mirrors he has ever tested.

PLEASE NOTE THE MIRROR HAS A PEAK TO VALLEY ERROR OF 1/7.3 WAVES, so it is "only" a 1/7th wave mirror



Please note that another set of tests gave a value of less than 1/8th wave, and the error has a completely different form.

Also note that the peak values occurred near the top and bottom of the mirror, almost certainly due to gravity distorting it, or possibly an artifact of fringe tracing on an aspheric mirror. These raised areas are small, so they have a small effect on the Strehl calculated by the software. (A high area like this all the way around a zone would have lowered the Strehl more.)

(Just to be clear, I'm commenting on the issues of optical test results and procedures, not another vendor's product, so by the TOS I should be allowed to do so.)

However, let's not allow the separate issue of test errors and indiosyncrasies cloud the original question, which was the performance of mirrors of various qualities.

Quote:

So which one of you can pick this lowly "genuine" 1/7th wave mirror from a "genuine" true 1/10th wave mirror ?
I rest my case.



Well, since with the effects of gravity removed it's probably 1/10th wave or better anyway, the specific question is probably not valid, but this goes back to the original question - can better mirrors be distinguished from poorer ones?

The short answer is yes. However, it must be realized that one number/test is not enough to completely characterize a mirror or its performance.

I advocate testing telescopes side-by-side, under the sky, over multiple observing session. You'd think that it would be possible to find the good one in the bunch, right? Well, yes, the better scope should rise to the top after allowances are made for equilibration and other factors.

However, in my experience, the larger the mirror, the less likely one is to find a truly good mirror, properly supported, to compare others too, so it is possible that you might spend nights comparing mediocre optics to each other without knowing it.


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freestar8n
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Re: 1/10 wave? new [Re: Mike Lockwood]
      #5558121 - 12/06/12 07:22 PM

The importance of optical quality comes up often, and comparison tests are cited - but I think there are several factors that get ignored. First, it's one thing to be able to tell one from another at the eyepiece, but it's much harder to demonstrate that one actually performs better than another. There may be a softer "snap to focus" - but the real issue in terms of image quality is - how well does it convey the information of the object to the observer. I don't know of a single empirical test with observers that focused on realized performance rather than simply "telling them apart." In an extreme example, you can tell a refractor from a reflector by the secondary shadow and spider vanes - but that has nothing to do with the image quality. Of course - if people cannot tell them apart at all, even with defocus, then that does point to it not mattering - for those observers and those conditions anyway.

The other issue is the role of statistics in getting a brief, clear view of an object through the atmosphere. This is usually viewed in terms of having a perfect optical system and waiting for moments of perfect atmosphere. But just as good would be to have an imperfect optical system, and wait for a brief time when the atmosphere exactly cancels the imperfections of the optical system. This may seem crazy or pedantic - but for small errors in the optical system that roughly match the power spectrum of turbulence, a moment of perfect atmosphere is not a lot less likely than a moment of exactly complementary atmosphere. As the error in the optical system increases the probability of a match goes down - but in all this it is a matter of statistics rather than - you need perfect optics and perfect atmosphere to get a perfect image.

Frank


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Re: 1/10 wave? new [Re: ausastronomer]
      #5558184 - 12/06/12 07:57 PM Attachment (6 downloads)

Quote:

PLEASE NOTE THE MIRROR HAS A PEAK TO VALLEY ERROR OF 1/7.3 WAVES, so it is "only" a 1/7th wave mirror






when I modeled that mirror up in Jim Burrow's Diffract program his ronchigram looks better than 1/14th~ since autocollimation doubles the error.

for the model i used 1/7~ error undercorrected sphere to get the 1/14th~ overcorrection parabolic in autocollimation.


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maknewtnut
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Re: 1/10 wave? new [Re: ausastronomer]
      #5558264 - 12/06/12 08:56 PM


The mirror has a true strehl of .987 and an RMS error of 1/54.7 waves. Wolfgang claims it is one of the best mirrors he has ever tested.

PLEASE NOTE THE MIRROR HAS A PEAK TO VALLEY ERROR OF 1/7.3 WAVES, so it is "only" a 1/7th wave mirror

So which one of you can pick this lowly "genuine" 1/7th wave mirror from a "genuine" true 1/10th wave mirror ?

I rest my case.

Cheers




That's a great way to substantiate one of the biggest differences of opinion in this thread. Namely where one person implied that CZ mirrors are by and large, much better than 1/10 wave P-V. Uhh...no. Carl's work is consistently fantastic, but one has to learn more about differences in testing methods and results before jumping to that conclusion (which many do).

From personal experience, there are also a considerable number of folks who will argue that a 1/6 wave system (which is different from a 1/6 wave primary mirror)isn't worth their money when they can pay more for a 1/8 wave system...or haggle about receiving a refractor with a maker's test result indicating .96 Strehl rather than their buddy's which is .97.

1/10 wave optics are rare. What many involved in this thread seemed to have overlooked is the term 'casual observer' used in the Stellafane test. It was even added that the seasoned observer CAN see the difference, so why the debate?


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bartine
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Re: 1/10 wave? new [Re: maknewtnut]
      #5558402 - 12/06/12 10:34 PM

Interesting discussion.

Nothing like this forum to have someone
1. who doesn't have a clue who you are
2. knows nothing about your level of observing experience and who
3. never looked through your telescope ONCE

tell you you are wrong.

Geez.

Gentlemen - I had the scope, I had the mirror refigured, and I can swear to the results. There was a visible difference. Clusters resolved more clearly. In the Trapezium it was easier to see detail and e and f stars. Planets had different levels of detail.

In this world, you generally get what you pay for. Astronomy and optical precision is no different. 1/4 wave and a high strehl ratio will not compete with 1/10 wave and a low strehl ratio.

Otherwise, I guess all those guys who pony up the big bucks for high quality mirrors are nuts.

Why do people swear about the views through a zambuto mirror? Why do refractor nuts swear about views through their AP Physics or Televue refractors? Because they are better. Better oprically, mechanically and visually.

Some nights will be better than others, but side by side the better scope will always win out.


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Asbytec
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Re: 1/10 wave? new [Re: maknewtnut]
      #5558435 - 12/06/12 11:01 PM

Thanks, Mark and Jerad, your explanation is appreciated. I'm sure the short answer is "yes," too. It's fascinating to understand why that would be.

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ausastronomerModerator
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Re: 1/10 wave? new [Re: bartine]
      #5558455 - 12/06/12 11:15 PM

Quote:



Gentlemen - I had the scope, I had the mirror refigured, and I can swear to the results. There was a visible difference. Clusters resolved more clearly. In the Trapezium it was easier to see detail and e and f stars. Planets had different levels of detail.





I am not disputing the quality of the refigured mirror or the fact you noticed a visible difference. What I am disputing is the quality of the mirror you started with before refigure. ie was it a genuine 1/4 wave mirror that was smooth, free of astigmatsm, with a good edge correction and it's only aberation was 1/4 wave of correction error?

I would almost be prepared to bet money that what you started with prior to refigure was a pretty ordinary mirror.




Cheers,


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Re: 1/10 wave? new [Re: ausastronomer]
      #5558461 - 12/06/12 11:18 PM Attachment (24 downloads)

Jupiter through aberator with 1/4 wave SA and no other aberrations. 10" newtonian 15% CO

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Re: 1/10 wave? new [Re: ausastronomer]
      #5558466 - 12/06/12 11:21 PM Attachment (24 downloads)

Jupiter through aberator with 1/10th wave SA, 1/10th wave stig and 1/10th wave air turbulence. 10" newtonian 15% CO

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freestar8n
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Re: 1/10 wave? new [Re: ausastronomer]
      #5558639 - 12/07/12 02:48 AM

Quote:

Jupiter through aberator with 1/10th wave SA, 1/10th wave stig and 1/10th wave air turbulence.




It is also common in these discussions to provide simulations of wavefront error in the presence of turbulence - but that ignores the statistical nature of the realtime imaging process I describe above. I don't know what model is used for the turbulence in aberrator, but it is not trivial to do it properly - and either way it does not capture the realtime fluctuations in seeing that combine with the optical system to provide moments of clarity. This is where both the nature of the surface figure error cannot be captured by a single number (either P-V or RMS) and the atmospheric turbulence cannot be represented by a single static wavefront used to calculate a single PSF.

So - I think simulations like this have a pedagogical value for conveying the potential impact of aberrations on an image - but when it comes to experienced observers under variable seeing conditions, who are judging based on moments of clarity, they don't tell the whole story - which involves statistical optics in addition to aberration theory.

Frank


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mark cowan
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Re: 1/10 wave? new [Re: ausastronomer]
      #5559534 - 12/07/12 04:07 PM

Quote:

ie was it a genuine 1/4 wave mirror that was smooth, free of astigmatsm, with a good edge correction and it's only aberation was 1/4 wave of correction error?




Well, it could have astigmatism, a poor edge and not be smooth and still show "genuine" 1/4 wave performance. Errors accumulate.

But in the presence of those sort of errors, low resolution testing might easily rate it as better than it actually was (Foucault being blind to all three for the most part) and so be labeled better; in which case improving it by fixing those errors could produce dramatic results.

Best,
Mark


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nicknacknock
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Re: 1/10 wave? new [Re: mark cowan]
      #5578478 - 12/19/12 07:10 AM

Well, I posted my soon-to-be-here-if-the-end-of-the-world-doesn't-come-first 12" Orion Optics 1/10 primary mirror Zygo report and reading this thread I have come to the following conclusions:

1. Mirror quality is a subject that causes friction.
2. People can have diametrically opposite views and everything in between on the subject matter.
3. Zygo reports are to be taken with a pinch of salt.
4. Nothing beats testing the mirror in the field and then experience plays a huge role in understanding if you got a good mirror or a lemon.

I will spy with my little eye and see how my dob performs, paying no heed to the Zygo report even though the test indicates a decent mirror

Nicos


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