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Equipment Discussions >> Reflectors

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Jim_Smith
super member
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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
Post Laureate
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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
Starstruck
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Reged: 04/06/05

Loc: shake, rattle, & roll, CA
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
Postmaster
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Reged: 02/22/10

Loc: nj
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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Reged: 04/01/08

Loc: Lovettsville, VA
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
Pooh-Bah
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Reged: 10/18/06

Loc: United States
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
Carpal Tunnel
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Reged: 06/30/03

Loc: Kiama NSW (Australia)
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
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
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
Vendor
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Reged: 09/05/05

Loc: Northeast USA
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
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
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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Reged: 10/18/06

Loc: United States
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
sage
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Reged: 09/19/08

Loc: USA
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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Reged: 07/17/04

Loc: Flagstaff, Arizona, USA
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
Pooh-Bah
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Reged: 10/18/06

Loc: United States
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

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

Yes.

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ausastronomerModerator
Carpal Tunnel
*****

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
professor emeritus
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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
Pooh-Bah
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Reged: 08/29/09

Loc: Lincoln, RI
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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