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Distinguishing High Quality Optics

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#1 BillFerris

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Posted 08 July 2011 - 01:20 AM

In another thread, a statement was made that it is not possible under real world conditions to distinguish between a 1/8-wave peak-to-valley mirro and perfection at the eyepiece. In fact, it has been demonstrated that this is possible; not even all that difficult.

The best example that comes to mind is a March 1992 article published in S&T. Peter Ceravolo, Terence Dickinson and Douglas George collaborated on the article. Peter Ceravolo made four 6 inch, f/8 Newtonians. He figured the primary mirrors to near 1-, 1/2-, 1/4- and 1/10-wave peak-to-valley on the wavefront. In other words, the primary mirrors Ceravolo made were measured on the surface at 1/2-, 1/4-, 1/8- and 1/20- wave peak-to-valley.

Dickinson and George--independently of each other--used the four telescopes on multiple nights for both deep-sky and planetary observing. The challenge was to distinguish between the four instruments and rank them in order of quality. Both observers were able to identify the 1- and 1/2-wave instruments and saw obviously better deep-sky images through the 1/4- and 1/10-wave instruments. (Anyone who would suggest that quality optics are not needed for deep-sky observing clearly needs to rethink their position.) Both observers were able to distinguish between the 1/4- and 1/10- wave scopes during planetary observing. Dickinson compared both instruments favorably to his 5 1/2 inch Astrophysics refractor. George described the views as approaching those delivered by a 7-inch Astrophysics StarFire refractor. A year later, three of the four Newtonians were set up at Stellafane and attendees were invited to have a look and try to identify the best optical set. Not surprisingly, the 1/2-wave scope was easily identified as the worst of the group. About 3/4 of those participating in this informal test were able to identify the 1/10-wave instrument. Those familiar with the star test were able to distinguish between this and the 1/4-wave scope with relative ease.

So, the question has been answered. At the eyepiece, it is possible to distinguish between very good and outstanding optics. The star test makes this a fairly trivial matter. It takes a bit more time and effort to distinguish between such excellent optics when comparing in-focus deep-sky or planetary views but, again, it is possible. It has been done.

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#2 Mark Harry

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Posted 08 July 2011 - 06:16 AM

Nice observation, Bill. 'Bout time someone shot down the 1/4 wave myth!
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#3 UmaDog

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Posted 08 July 2011 - 07:23 AM

Yes, I think I remember that. And people couldn't distinguish between 1/8th and 1/10th wave, correct?

#4 Maverick199

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Posted 08 July 2011 - 11:22 AM

Sigh, another discussion on optics. :foreheadslap: I think any person with a bit of survey can quite understand how "quality optics" play their part in the seeing. Problem is, how much would one average astronomer is willing to spend to get that bit of an extra oomph between an average mirror to a quality one.

Me, if I had the resources and availability, most probably opted for Zambuto or similar.

#5 Pinbout

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Posted 08 July 2011 - 03:41 PM

At the eyepiece, it is possible to distinguish between very good and outstanding optics. The star test makes this a fairly trivial matter.



During nights of good seeing. :grin:

#6 gatorengineer

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Posted 08 July 2011 - 08:30 PM

Please take the time to get the quote right. It was 1/8 wave RMS. Please Google what the RMS means..

The test you mention was not I believe on RMS mirrors. I have the article at home and will look when I get there in a couple of days.

The mirror under the stars is as far as I know an unknown, as far as quality, what is known is that it was pinched, in an unflocked and in baffled tube. Had Chinese coatings as opposed to a new mirror with high quality coatings.

What we did learn is that a $1400 scope under 3 arc second conditions cannot do what a 60mm refractory can do under under great conditions.

I would also make the arguement that under the conditions of the referenced test, an 1/8th wave ems mirror would be diffraction limited, and if other mechanical conditions were equal no difference between the Scopes could not be discerned. That's what the math would say, but......

No slight to ZOC is intended or has ever been made. They are extremely fine mirrors. For someone with scope size limitations as the starter of
The original thread stated, it would be how I would go too. But as another famous optician states aperature rules..... and above that mother nature and seeing conditions .......

#7 GeneT

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Posted 08 July 2011 - 11:30 PM

Problem is, how much would one average astronomer is willing to spend to get that bit of an extra oomph between an average mirror to a quality one.


This is always the issue. Where do you believe the best value for the money lies?

#8 BillFerris

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Posted 09 July 2011 - 02:38 AM

Please take the time to get the quote right. It was 1/8 wave RMS. Please Google what the RMS means..


A more effective search would be to google the phrase, Marechal Criterion. The top-ranked results should be on-topic. You'll also find informed discussions of various methods of measuring optical quality in Texereau, Suiter and the '92 S&T article by Ceravolo, et al. The Rayleigh criterion of a total wavefront error of 1/4-wavelength (in yellow-green light) translates to a total wavefront error of 1/14-wavelength RMS. Measured on the mirror surface, this is cut in half to 1/27-wavelength RMS. And if you measure the RMS surface error using a helium-neon laser, the standard is 1/43-wavelength. 1/8-wave RMS quality (even if measured as a total wavefront error) should be readily identifiable as inferior to a well-crafted scope having a total wavefront error of 1/4-wavelength.

The test you mention was not I believe on RMS mirrors. I have the article at home and will look when I get there in a couple of days.


As stated in the original post, the Ceravolo-produced scopes had total wavefront errors of 1-, 1/2-, 1/4- and 1/10-wave. The 1/4- and 1/10-wave instruments delivered excellent views that compared favorably to similar aperture refractors of unquestionable quality. I'm sure you didn't intend to question the accuracy of Ceravolo's measurements with your above statement.

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#9 BillFerris

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Posted 09 July 2011 - 02:58 AM

Being able to distinguish between optics of different quality doesn't mean a person won't enjoy observing with the lesser instrument. It doesn't even mean that the lesser instrument is bad. A scope with a final wavefront error of 1/4-wavelength (assuming a smooth figure and good edge) is capable of delivering excellent in-focus images. A lot of observers are satisfied with the views produced by 1/3- or even 1/2-wave instruments. Speaking personally, I thoroughly enjoyed the views delivered by the entry-level scopes (and their mediocre--at best--optics) that launched my interest in visual observing.

The point of my post was merely to demonstrate the fact that observers are able to distinguish between very high quality optics. The star test makes this fairly easy and a discerning eye can make the distinction when comparing planetary or deep-sky views.

Bill in Flag

Sigh, another discussion on optics. :foreheadslap: I think any person with a bit of survey can quite understand how "quality optics" play their part in the seeing. Problem is, how much would one average astronomer is willing to spend to get that bit of an extra oomph between an average mirror to a quality one.

Me, if I had the resources and availability, most probably opted for Zambuto or similar.



#10 BillFerris

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Posted 09 July 2011 - 03:04 AM

At the eyepiece, it is possible to distinguish between very good and outstanding optics. The star test makes this a fairly trivial matter.



During nights of good seeing. :grin:


Good atmospheric seeing certainly helps when making such comparisons. That said, it's interesting to note that the sidebar article in the Ceravolo piece describes the seeing during the Stellafane shootout as "generally average to below average."

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#11 Astrojensen

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Posted 09 July 2011 - 03:48 AM

Good atmospheric seeing certainly helps when making such comparisons. That said, it's interesting to note that the sidebar article in the Ceravolo piece describes the seeing during the Stellafane shootout as "generally average to below average."


It is my experience, using both reflectors and refractors of varying apertures, that high quality optics are easily distinguishable from mediocre ones also in average or below average seeing. This seems to be true especially for contrast. High end optics deliver contrasty views even in poor seeing, while average optics do less well.

In very good seeing, even so-so optics can do incredibly well.


Clear skies!
Thomas Jensen

#12 idealistic

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Posted 09 July 2011 - 10:58 AM

The mirror under the stars is as far as I know an unknown, as far as quality, what is known is that it was pinched, in an unflocked and in baffled tube. Had Chinese coatings as opposed to a new mirror with high quality coatings.

As I said in the other thread, Im not 100% convinced that the star test didnt show astigmatism rather than a pinched optic. But either way, the comparison was between a stock 10" scope (inferior coating, possible bad figure, warts and all) vs. a Zambuto mirror under the same conditions. The only variable was the flocking, which is a non issue due to the lack of stray light at the test site. The thing to take away from it all is the fact that there was a difference at the eyepiece, stock vs. premium. What caused the difference is not really what we were out to find out, that we saw a difference is the point. And Im quite sure the stock mirror was an average representitive of, well... a stock chinese mirror.

What we did learn is that a $1400 scope under 3 arc second conditions cannot do what a 60mm refractory can do under under great conditions.

I dont remember anyone saying anything like that. Could you explain? Maybe Im wrong, but I dont think any part of thet statement is accurate.

#13 Relativist

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Posted 09 July 2011 - 11:58 AM

Any aberration of the light on its way from space will effect the views, and the effects are cumulative - it is highly unlikely that the aberration in a mirror will correct the light coming in (and no one is claiming such). Having a better corrected mirror would eliminate a source of abberations.

All that said, under practical use there is the human factors to consider. Different people have distinct ways of using their equipment, and some are more successful at coaxing the best out of their equipment than others.

Much like buying a high quality eyepiece or a parracor, a well corrected primary will clean up the views noticeably, but it also will not correct aberrations from other interactions the wavefront has gone through on the way to your retina.

My advice for those considering an upgrade in optics is to get some testing done of your existing optics if you can and also try and experience the difference for yourself, again if it's available.

#14 wh48gs

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Posted 09 July 2011 - 12:43 PM

In another thread, a statement was made that it is not possible under real world conditions to distinguish between a 1/8-wave peak-to-valley mirro and perfection at the eyepiece. In fact, it has been demonstrated that this is possible; not even all that difficult.

The best example that comes to mind is a March 1992 article published in S&T. Peter Ceravolo, Terence Dickinson and Douglas George collaborated on the article.



That statement pretty much agrees with what Suiter says in his book, using this same experiment as an example (p267, 1st ed.). Then he goes on to analyze why, and concludes that the main reason was reported seeing - which he estimated to be at 0.1 wave RMS level.

That is pretty much a "real life" scenario, and so is any with the seeing between sub arcsecond and several arc seconds, and with apertures from below 4 to over 20 inches. How much of a difference - if any - will be noticeable between near-perfect and 0.80 Strehl system depends on the actual magnitude of the seeing error.

For instance, in the above seeing a 12-inch aperture would have as much as 0.18 wave RMS seeing error (comparable to 0.6 wave p-v of primary spherical aberration), reducing the effective Strehls of the two systems to below 0.3. Needless to say, there would be no appreciable difference in their performance level.

In terms of c. obstruction size, the effect of 1/10 and 1/4 wave is comparable to 0.13D and 0.32D c. obstruction diameter, respectively (D=aperture diameter). In good seeing, the difference can be noticed in smaller apertures, diminishing toward larger apertures, in part due to all other error sources, not only seeing, tending to increase with the aperture.

So, the short answer to the question whether this much of difference in optical quality can be detected seems to be: All depends...

Vla

#15 Jarad

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Posted 09 July 2011 - 06:26 PM

Please take the time to get the quote right. It was 1/8 wave RMS. Please Google what the RMS means..

The Ceravolo scopes for that test article were each made to have 1/X wave of Peak-to-Valley smooth spherical error (undercorrection compared to a parabola). They were otherwise well-figured (not rough, not astigmatic).

The RMS numbers for those scopes would be significantly lower than the P-V numbers (i.e. X would be higher in the 1/X measure).

Jarad

#16 gatorengineer

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Posted 09 July 2011 - 07:00 PM

Guys one more time....

What I said is pv RMS...... Back in the old days, before the advent of computerized optical tests a few points were used to give a pv error... I absolutely guarantee you I can take any Chinese optic ever shipped and rate it 1/10th wave, and do it based on a couple of surface points, and it would be an honest test.

To fully demonstrate the quality of their optics opticians went to the Rms method which tests hundreds of points on the surface of their mirrors. A full discussion of this is on the omi website....... I am sure ceragoli uses this today, along with all of the other name makers....

Getting back to your comments on th sky and Telly test. Do you think it's possible for an optician to intentionally make an exactly quarter wave optic? Answer is no....... To do so well would be well near impossible. and far harder Than making a .99 strehl optic...

The optics for the test were most likely made and the final figuring stopped when the optic was about a quarter wave, how and when in the the process I dont know... If the optics were rmsed, you would know with certainty... We don't.... If the optics are still around it would be very interesting to see what a modern test would say...

Roger is around sometimes and perhaps he will chime in....

#17 gatorengineer

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Posted 09 July 2011 - 07:24 PM

On to the subject of dark skies.. Bolton is about 35 miles crow flight from Logan and 12 or so from Worcester. It's an orange zone... Flocking will help. Observe from a blue black zone where jupiter is so bright it is annoying..

One scope has a three quarter inch focuser offset, believe me that cuts the stray light coming directly into the tube tremendously... The old timers used to exited the tube one diameter above the focuser for a good reason. Don't believe me... A dollar store piece of black poster paper and extend you tube.... You will see it or I will send you your buck back.... The Chinese don't because it's weight and dimensions to ship....

#18 Pinbout

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Posted 09 July 2011 - 11:18 PM

If the optics were rmsed, you would know with certainty...



I'm really scratching my head on that one. :lol:

The optics for the test were most likely made and the final figuring stopped when the optic was about a quarter wave,



yeah, what would a very slightly parabolized f5 give in a star test. oh yeah I have one, let me go see. :grin:

#19 David Knisely

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Posted 10 July 2011 - 04:10 AM

gatorengineer wrote:

What I said is pv RMS...... Back in the old days, before the advent of computerized optical tests a few points were used to give a pv error... I absolutely guarantee you I can take any Chinese optic ever shipped and rate it 1/10th wave, and do it based on a couple of surface points, and it would be an honest test.

To fully demonstrate the quality of their optics opticians went to the Rms method which tests hundreds of points on the surface of their mirrors. A full discussion of this is on the omi website....... I am sure ceragoli uses this today, along with all of the other name makers....


Well, here is the 7-zone mean plot of the Chinese mirror I got with my XX14i. It has a P-V wavefront error that is nowhere near 1/10th wave (1/3.1 wave p-v), and even the RMS wavefront error is 1/12.8 wave, which isn't even as good as the Marechal 1/14th wave "diffraction limited" criteria. Even Orion's own interferometer readings indicate an p-v wavefront error of greater than 1/4 wave. Indeed, my first indication of trouble with this mirror was with a visual observation of Jupiter and its moons, so the problem was easily detectable well before extensive testing was done. I consider the 1/4 wave p-v wavefront error and the 1/14th wave RMS wavefront error limits to define the edge of acceptable quality. Sorry, but this mirror wasn't a decent primary by a long shot (although it is now, thanks to a proper refiguring job by Mike Lockwood):

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#20 wh48gs

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Posted 10 July 2011 - 05:25 AM

The Ceravolo scopes for that test article were each made to have 1/X wave of Peak-to-Valley smooth spherical error (undercorrection compared to a parabola). They were otherwise well-figured (not rough, not astigmatic).

The RMS numbers for those scopes would be significantly lower than the P-V numbers (i.e. X would be higher in the 1/X measure).



For a smooth conic error, the PV-to-RMS ratio is constant at 3.354. So 1/4 wave PV corresponds to 1/13.4 wave RMS.

Vla

#21 wh48gs

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Posted 10 July 2011 - 05:31 AM

I absolutely guarantee you I can take any Chinese optic ever shipped and rate it 1/10th wave, and do it based on a couple of surface points, and it would be an honest test.



Would you care to elaborate on this assertion?

Do you think it's possible for an optician to intentionally make an exactly quarter wave optic? Answer is no...



No, the answer is "yes". There is more than one way to null a surface for calibrated error, and I'm sure that Ceravolo knows how to do that.

Vla

#22 wh48gs

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Posted 10 July 2011 - 06:13 AM

I consider the 1/4 wave p-v wavefront error and the 1/14th wave RMS wavefront error limits to define the edge of acceptable quality. Sorry, but this mirror wasn't a decent primary by a long shot



But 0.79 Strehl should make for a decent mirror. Since the wavefront plotted obviously doesn't come from the actual surface, rather from the averaged zonal measurements, I suspect that the actual surface had some roughness and, possibly, some astigmatism, with the actual Strehl closer to 0.7.

Vla

#23 CHASLX200

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Posted 10 July 2011 - 06:43 AM

I can tell great optic and a so so optic on any good nite of 8+ seeing. With all my Zambuto's and OMI optics, Jupiter and Saturn would show unreal detail at power from 800x to 1100x on my 9+ seeing nites. Also deepsky viewing takes on a whole new meaning as images snap into focus with much better contrast and a darker sky background.

Not even close to the cheap stuff being made these days, as images mush into focus with lesser optics.

Ya get what ya pay for.

Chas

#24 gatorengineer

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Posted 10 July 2011 - 07:22 AM

The graph Dave throws up is a good talking point. In 1992 was that graph available, at the amateur optics level? Did coulter or Galaxy as examples produce one.... I never saw one.... Daves mirror in zones 4, 5, and 6 is a 10th wave mirror... There were some optics firms back in that vinatage, that were doing quicky incorrect tests (Coulter) see other discussions here, that would have passed this mirror as a much higher quality optic than it was likely based on having a couple of decent zones.

I guess Daves mirror in its unrefigured state also blows out of the water any arguement about a constant relationship between P-V and P-V RMS, for a real world optic, that contains real world errors...

Also another interesting point, is that from a strehl ratio standpoint the mirror is 0.79 (diffraction limited is 0.80), while the P-V and P-V RMS are not close to diffraction limited..... These two sets of numbers strehl and P-V's do not match, and its impossible to say which one is right, but they are contradictory.....

#25 Daniel Mounsey

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Posted 10 July 2011 - 09:46 AM

In another thread, a statement was made that it is not possible under real world conditions to distinguish between a 1/8-wave peak-to-valley mirro and perfection at the eyepiece. In fact, it has been demonstrated that this is possible; not even all that difficult.


Bill,
I'm sure you were referring to my thread. Well I personally don't dissagree with you but with the general public I do dissagree. This article is a great one and was published in the 1st edition of Backyard Astromomers Guide, the actual guide I still refer to today. I guarantee you there's a lot of SCT owners out there happy with the design who don't complain about the optics in fact they even believe they're just as good as Newts and refractors. I myself just prefer Newtonians and refractors because I observe in areas that are condusive to see that there really is a difference in the quality of the stars and planets.

So if the average consumer obviously doesn't appear to see, appreciate or care about any differences, then what's there to debate over? My argument is that the "average consumer" doesn't really see much difference. For example I went to the observatories forum and looked at some of the most beautiful and expensive observatories you can imagine only to see that an SCT was their scope of choice and I'm thinking, something just isn't clicking here for me.


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