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How much strehl difference is actually detectable?

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#1 starcruiser

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Posted 29 April 2021 - 09:51 PM

For example, suppose you have 2 scopes of identical aperture (and preferable same brand and model). One tested .94 while the other tested .98 (again, just an example)

 

What I want to know is:

 

1. How much magnification was needed to see the difference? For example, did it take 400x to even begin to see the difference or was it obvious at just 150x?

2. Any difference in Lunar/planetary or deep-sky contrast?

3. Are the differences detectable by the casual observer or did it take an experienced amateur to see it?

4. Anything else?


 

#2 Jim Waters

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Posted 29 April 2021 - 10:20 PM

On an 'average night...' I don't think you can tell the difference between 0.94 and 0.98.  Its more bragging rights than anything.  But, on a VERY GOOD night that another thing.


 

#3 RLK1

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Posted 29 April 2021 - 10:21 PM

For example, suppose you have 2 scopes of identical aperture (and preferable same brand and model). One tested .94 while the other tested .98 (again, just an example)

 

What I want to know is:

 

1. How much magnification was needed to see the difference? For example, did it take 400x to even begin to see the difference or was it obvious at just 150x?

2. Any difference in Lunar/planetary or deep-sky contrast?

3. Are the differences detectable by the casual observer or did it take an experienced amateur to see it?

4. Anything else?

I'll take a stab at it. My guess would be you would not see an appreciable difference. If the strehl ratio difference was a bit more significant than the spread in your example, then, under optimum conditions, you may, assuming all other variables being equal, see an actual difference in the eyepiece. This would likely require high power, similar to that in your example, and your target would be a faint object, such as either Umbriel and Ariel in close proximity to Uranus or splitting a close double star with one companion being appreciably fainter than the other. I did see such a difference between two identical reflectors, other than the strehl being different, on an observation of a moon of Uranus. Beyond those special circumstances, I doubt you'd see any differences unless other factors come into play, ie smoothness of an optical surface as an example...


 

#4 jimhoward999

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Posted 29 April 2021 - 11:39 PM

I think one could never tell the difference between strehl of 0.94 and perfection (strehl of 0.98 or 1.0) with a visual observation.   If the strehl is 0.94  6% more of the energy is in the rings instead of the central lobe as compared with a perfect system.  

 

The first image below shows a cross section a of the monochromatic point spread function (star image) produced by a perfect unobscured F/7 system.    The second image shows the PSF produced by the same system, but with 0.039 waves RMS of aberration (a mix of zernikes representative of fabrication and alignment errors) producing a Strehl ratio of 0.94.

 

I don't believe anybody could tell the difference between these PSFs and consequently these Strehl Ratios looking at stars, even double stars under clear conditions, with their eyes through a telescope.

 

perfect psf.jpg

 

94 percent.jpg

 

Now with an interferometer its a different story.   Here is a simulated interferogram of a system with a strehl of about 0.94.  You can tell the fringes aren't straight. 

 

interfergram.jpg

 

 

 

 

 


 

#5 DAG792

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Posted 30 April 2021 - 12:09 AM

For example, suppose you have 2 scopes of identical aperture (and preferable same brand and model). One tested .94 while the other tested .98 (again, just an example)

 

What I want to know is:

 

1. How much magnification was needed to see the difference? For example, did it take 400x to even begin to see the difference or was it obvious at just 150x?

2. Any difference in Lunar/planetary or deep-sky contrast?

3. Are the differences detectable by the casual observer or did it take an experienced amateur to see it?

4. Anything else?

I personally think that detecting a 0.05 difference in Strehl is nigh impossible using just 'visual' observation. I think 0.08-0.10 Strehl difference is probably nearer to the limit that experienced observers can detect. 

A casual observer? probably a difference of 0.15-0.2 Strehl would be required for threshold detectability.

Maybe, just maybe, a difference of 0.05 Strehl alters planetary contrast by enough to make it detectable. But I would still be highly skeptical of such a claim.


 

#6 lylver

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Posted 30 April 2021 - 02:39 AM

Strehl of what ? Range strehl or peak strehl ?

I can see difference between two achromats a 80-1000 Scopetech that passed 0.97 and the Maxi 80-1200 that reached 0.98 in D ray.

IMO peak strehl is useless.

Minimum range is e to D ray for Moon. Much more difficult for planetary efficient observation without filter. I think something like a 510/520 to 610/620 means more, depends on what you observe, focus / individual perception.


 

#7 dan_1984

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Posted 30 April 2021 - 07:59 AM

Don’t stress so much over numbers. Get a frac from a quality maker and use it. 
I have a CFF apo and I don’t know the Strehl. I do know the views are better than anything I had  before.


Edited by dan_1984, 30 April 2021 - 01:15 PM.

 

#8 coopman

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Posted 30 April 2021 - 08:31 AM

How good are your eyes at any given time?  


 

#9 SeattleScott

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Posted 30 April 2021 - 08:54 AM

Like many I have had a mass produced Apo and a Japanese Apo, and found the Japanese one noticeably (but not dramatically) sharper at high power planetary viewing. Now I don’t know what the strehl values are. I suspect the mass produced one probably wasn’t 94. Good chance the Japanese one isn’t quite 98.

In general it typically takes a 10% difference for people to be able to detect a difference. Like 10% brighter, etc. Maybe less if you get really detailed but for routine observing, more like 10%. If this holds for Strehl, then a 6% difference in airy disc would be very hard to detect. Might take more like a 90-92% to 98% jump to feel comfortable about being able to detect a difference. I have heard it said, once you get to 95% Strehl you aren’t really missing anything (with a corresponding P/V).

But ultimately Strehl is not a perfect measure. And testing is complicated. Single wavelength? What wavelength was used? Are the edges masked? Is this just the theoretical polychromatic Strehl? It is like the class on how to lie with charts. Just like you can manipulate charts to show the same data in different light, you can make an optic look good or bad depending on how the test is done. Or at least you can make a not so hot one look good. I don’t know if you can make a truly great optic look bad.

Scott
 

#10 barbie

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Posted 30 April 2021 - 12:00 PM

The proof of excellent optics is the quality of the final image at the eyepiece!! I don't know(or care) what the Strehl is of my two Takahashi refractors....just that they can both take ridiculously high magnification on the moon, planets and double stars well when my seeing permits, which is infrequent at my location.


Edited by barbie, 30 April 2021 - 12:12 PM.

 

#11 gnowellsct

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Posted 30 April 2021 - 01:24 PM

You can set up a .96 Strehl triplet apo and enjoy the view and saunter over to a .98 Strehl Wunderkind triplet and the difference will be so great it will sock you in the eye and knock you down.  When you get up again you'll say "take that .96 Strehl department scope away from me I never want to see it again."


 

#12 SeattleScott

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Posted 30 April 2021 - 01:51 PM

I think what it really comes down to, is if you already have a 94% Strehl refractor, that’s true 94% Strehl with the P/V and polish to match, as opposed to a biased vendor provided Strehl report (not saying all vendor supplied tests are biased), then it probably doesn’t make much sense to upgrade to 98% Strehl of the same aperture scope from the same brand. Now if you have two scopes from the same brand, one tests at 94 and the other tests at 98 and costs $200 more as a result, it might be worth paying up for the better optic. I mean it will actually deliver better views, even if it is hard to see the difference.

Now going from a budget Apo with a questionable test report showing 94 in red to a TEC at 98, there you would probably be able to detect a difference.

Scott
 

#13 jimhoward999

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Posted 30 April 2021 - 04:13 PM

People talk about impressive Strehl Ratios like 0.94 and 0.98, but I think the vast majority of telescopes, even very expensive high end ones,  are way worse than that.

 

In reflective systems, small polishing or alignment errors  in the mirrors increase the transmitted wavefront error fast.

 

And in refractors, you have residual chromatic aberration, even in an Apochromat, and this knocks the polychromatic Strehl down even if the monochromatic result is very high (which is what vendors report). 

 

I think if your telescope has a Strehl of 0.85, you are doing pretty well.


 

#14 DeanD

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Posted 30 April 2021 - 09:07 PM

You can set up a .96 Strehl triplet apo and enjoy the view and saunter over to a .98 Strehl Wunderkind triplet and the difference will be so great it will sock you in the eye and knock you down.  When you get up again you'll say "take that .96 Strehl department scope away from me I never want to see it again."

I would be interested if these two refractors were tested for their "Strehl" by the same method under the same conditions. If you can see such a difference, then I strongly suspect they weren't...


 

#15 teashea

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Posted 30 April 2021 - 09:23 PM

I think that generally one could not tell much, if any, difference.


 

#16 DeanD

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Posted 30 April 2021 - 09:24 PM

FWIW, I suggest people look at the older post about Strehl here: https://www.cloudyni...7-strehl-ratio/

 

In particular it is worth checking out the article by Zambuto that is referenced in the discussion, and is here: https://zambutomirro...oopticalca.html

 

One comment by Zambuto that stands out for me is this one:

"The Strehl ratio is a measurement of the large scale (geometric) surface. Regardless of the method of test being used, including the most advanced optical devices in existence, criteria such as medium scale surface roughness and turned edge are not taken into account." - this is when discussing a mirror that has tested at 0.99 Strehl, but has serious machine-induced ripples (see the discussion on "Criteria 4") that cause the image to be degraded quite badly when compared to a smooth figured optic. 

 

While this article is about mirrors, the same criteria apply for figuring of refractors: and from memory, Roland Crouch had very similar things to say about optical quality, and didn't therefore originally give measured Strehl for his scopes. I believe AP and SV and others do now because some amateur astronomers now expect it. 

 

Strehl ratio gives a useful indication of quality, but there is a lot more to it than simply saying "my scope is 0.98 and is better than your scope at 0.94"...

 

Like several comments above, I seriously doubt whether anyone can tell the difference (using the same diagonals and eyepieces) between 0.98 and 0.94 simply by looking through them if both scopes have smooth optics and have been tested in the same lab under the same conditions.


Edited by DeanD, 01 May 2021 - 12:06 AM.

 

#17 Jeff B

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Posted 30 April 2021 - 11:27 PM

Personally, I can easily see the difference between .50 and .95.


 

#18 Suavi

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Posted 01 May 2021 - 12:31 AM

I recall from high school science lessons that every measurement has associated uncertainty. Therefore I find those precise Strehl numbers quite vague, some even stated to three significant figures, because I am yet to see an optical report with Strehl for a clearly stated wavelength at which it was measured, which also shows uncertainty of the measurement.


 

#19 Rutilus

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Posted 01 May 2021 - 03:48 AM

For example, suppose you have 2 scopes of identical aperture (and preferable same brand and model). One tested .94 while the other tested .98 (again, just an example)

 

What I want to know is:

 

1. How much magnification was needed to see the difference? For example, did it take 400x to even begin to see the difference or was it obvious at just 150x?

2. Any difference in Lunar/planetary or deep-sky contrast?

3. Are the differences detectable by the casual observer or did it take an experienced amateur to see it?

4. Anything else?

Years ago I did dozens of side by side tests with my Takahashi TSA-102 and a 100mm f/13 Carton achromat

scope. With Lunar and planetary both scopes showed the same features. Sure you could tell that the Carton

was an achromat when you got over 150x, but it still resolved the same features. The TSA would take higher

powers, but the eyepiece view just got dimmer  without any extra detail being visible.

Can anyone see a difference in the high strehl numbers being talked about here, I doubt it.    


 

#20 DAG792

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Posted 01 May 2021 - 07:30 AM

Years ago I did dozens of side by side tests with my Takahashi TSA-102 and a 100mm f/13 Carton achromat

scope. With Lunar and planetary both scopes showed the same features. Sure you could tell that the Carton

was an achromat when you got over 150x, but it still resolved the same features. The TSA would take higher

powers, but the eyepiece view just got dimmer  without any extra detail being visible.

Can anyone see a difference in the high strehl numbers being talked about here, I doubt it.    

I'd bet that Carton achromat was a real nice one.


 

#21 Alan French

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Posted 01 May 2021 - 08:23 AM

If this came up before in this discussion, I missed it....

 

https://www.cloudyni...quality-optics/

 

Having participated in the comparison at Stellafane, I will say it was informal, rushed, and the seeing was not very good, so not an ideal situation. 

 

At Winter Star Party I've noticed that under really steady skies, with planets high in the sky, a good number of scopes provide fine views and aperture becomes very important.

 

I suspect most of us seeking fine lunar and planetary views are generally more significantly hampered by seeing than by the lack of quality optics.

 

Clear skies, Alan 


 

#22 LDW47

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Posted 01 May 2021 - 09:28 AM

I don’t get into any of this meaningless ........ but it has been pointed out, mentioned on these type of threads that some fanatical astronomers have literally sent the scopes that they bought back for a refund or exchanged because their strehl number was slightly less than someone else's, still above 0.95 but lower than a friends ! They didn’t care about the views, they thought they weren’t getting their advertised moneys worth so they literally went to the trouble of ......... ! Talk about brainwashed, talk about mindless unthinking, lol !


 

#23 Nippon

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Posted 01 May 2021 - 09:52 AM

Strehl anxiety and optical quality obsession seems to be solely a disease of the refractor lovers community. I know it's true of me when I use my refractor. But the mirror scope folks seem to spend more time enjoying the night sky than worrying about optics and this is also true of me when I use my mirror scope:)


 

#24 LDW47

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Posted 01 May 2021 - 09:58 AM

Strehl anxiety and optical quality obsession seems to be solely a disease of the refractor lovers community. I know it's true of me when I use my refractor. But the mirror scope folks seem to spend more time enjoying the night sky than worrying about optics and this is also true of me when I use my mirror scope:)

Especially when the consensus is that your eyes can’t tell the difference anyway between ....... and ..... , lol !    


 

#25 DAG792

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Posted 01 May 2021 - 09:59 AM

Strehl anxiety and optical quality obsession seems to be solely a disease of the refractor lovers community. I know it's true of me when I use my refractor. But the mirror scope folks seem to spend more time enjoying the night sky than worrying about optics and this is also true of me when I use my mirror scope:)

Shots fired lol.gif lol.gif


 


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