Optical quality vs seeing

[jetstream]

Heres a question that is not meant to be controversial.

Over the years I have heard others, respected observers having had many scopes and having high end glass report that the better the optical quality of the scope the more resistant it is to poor seeing. The reports Ive seen refer to refractors basically .

I have zero interest in "bias" and dont want it brought up here. I do have a great interest in this idea of optical quality vs seeing and would appreciate anyones thoughts or experiences with this. Myself, I dont know and am going to test it out coming up. Ive never really tested it out because as seeing deteriorates I go back in the house.

Gerry

[neil phillips]

Just now, jetstream said:

Heres a question that is not meant to be controversial.

Over the years I have heard others, respected observers having had many scopes and having high end glass report that the better the optical quality of the scope the more resistant it is to poor seeing. The reports Ive seen refer to refractors basically .

I have zero interest in "bias" and dont want it brought up here. I do have a great interest in this idea of optical quality vs seeing and would appreciate anyones thoughts or experiences with this. Myself, I dont know and am going to test it out coming up. Ive never really tested it out because as seeing deteriorates I go back in the house.

Gerry

I understand the question. In my experience imaging with different scopes for 23 years, One thing i am certain of. The better the optical quality. the better it will do from poor seeing to good. it just performs better period. Across the board of differing seeing conditions. The better optics don't affect the seeing. They just do a better job, with what they are presented with. That's been my experience. 

[Astrobits]

It's quite simple really. Poor seeing is caused by a distorted wavefront and poor optics simply adds another layer of distortion to that. Hence the better the optics the less noise they add to the image.

Nigel

[jetstream]

Excellent, thanks guys. Ive wondered this for a while but never did know if there was anything to it.

[michael.h.f.wilkinson]

Another way to look at it, is that the perfect wavefront is perfectly flat, Likewise, the average wavefront in poor seeing is flat, so optics that correct superbly for a flat wavefront are best on average. It is possible, but extremely unlikely that a wavefront distorted by seeing happens to correct for any imperfection in the optics, but a little maths will show the chance of this is vanishingly small, or at least far smaller than the chances of having fleeting moments of near flat wavefronts in situations of poor seeing.

[davidc135]

Whilst all scopes benefit from better seeing it's also true, IMO, that poorer standard optics will benefit more or, put another way, better quality optics should better withstand a dip in seeing.

The different causes of poor performance such as seeing, Spherical aberration and roughness, thermals and obstruction each cause a loss in Strehl and are additive as the sq. root of the sum of the squares of each indivifual contribution.

E.g Take two causes; SA and seeing, and in one scope with a perfect figure there is only poor seeing equivalent to 1/4 wave SA. but still performs well. The mediocre scope suffers 1/4 wave SA plus, say,1/4 wave due to seeing giving a total 0.35 waves and performs noticeably worse. There's a bigger difference noticed between .35 and .25 than between .25 and zero waves SA, so that's the theory.

David

PS Posts crossed

Edited April 3, 2023 by davidc135

[davidc135]

2 minutes ago, davidc135 said:

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Edited April 3, 2023 by davidc135

[jetstream]

5 minutes ago, michael.h.f.wilkinson said:

Another way to look at it, is that the perfect wavefront is perfectly flat, Likewise, the average wavefront in poor seeing is flat, so optics that correct superbly for a flat wavefront are best on average. It is possible, but extremely unlikely that a wavefront distorted by seeing happens to correct for any imperfection in the optics, but a little maths will show the chance of this is vanishingly small, or at least far smaller than the chances of having fleeting moments of near flat wavefronts in situations of poor seeing.

Thanks Michael, I dont really understand how you put this- ie that the wavefront in poor seeing is flat- can you help me understand please?

[jetstream]

7 minutes ago, davidc135 said:

There's a bigger difference noticed between .35 and .25 than between .25 and zero waves SA, so that's the theory.

I do understand this, thanks David. In my own tests I have purposely decollimated my scopes to simulate poor optics and have noticed at a certain point the image degrades very quickly, I did this years ago. Anything .25 wave and above gives great views IMHO.

[jetstream]

21 minutes ago, davidc135 said:

The different causes of poor performance such as seeing, Spherical aberration and roughness, thermals and obstruction each cause a loss in Strehl and are additive as the sq. root of the sum of the squares of each indivifual contribution.

Lets say seeing is "fixed" at "very good" . I use 2 different scopes of the same aperture and f ratio, one goes to "X" magnification and one goes to "X +" mag.  Even though both scopes are above diffraction limited the fact that one takes more mag could be an indication of differing optical specs seeing considered? If so then under prefect seeing the views should be the same and magnification very similar?

I have sat for years at the eyepiece shaking my head trying to understand how a diffraction limited scope takes a certain mag while another scope can take more for given seeing.

[michael.h.f.wilkinson]

47 minutes ago, jetstream said:

Thanks Michael, I dont really understand how you put this- ie that the wavefront in poor seeing is flat- can you help me understand please?

On average the wavefront is flat, is what I claimed. The idea is that there is no systematic deviation from flat, it is just a series of random wobbles, equally likely to go up or down at any point along the front. During really bad seeing there may not a single moment of truly flat wavefronts, but adding all the distortions up taking the average should result in something approaching flat. Put differently, it does not make sense to alter the optical design to correct for e.g. concave spherical wavefronts due to seeing, because the perturbations are random

[John]

Perhaps not quite the same question but I have noticed that with eyepieces and scopes, good to excellent seeing conditions helps the better optical quality to stand out a little more.

During more ordinary or lesser quality seeing the performance gap seems (to me) to be narrower or even non-existent.

I have also noticed that smaller aperture scopes (refractors in my case) seem to make more of whatever the seeing conditions are than larger aperture ones although the larger apertures show their additional potential under good to excellent seeing. Likewise when observing targets that are not far above the horizon.

Not sure how those observations help, but that is what I have experienced 🙄

 

Edited April 3, 2023 by John

[Mr Spock]

I'm not sure of the technical aspects involved. All I know is both my scopes were hit equally last night by the appearance of the jetstream overhead and the consequent waves of poor seeing. Both scopes were rendered useless and I had to call it a night.

[jetstream]

19 minutes ago, michael.h.f.wilkinson said:

On average the wavefront is flat, is what I claimed. The idea is that there is no systematic deviation from flat, it is just a series of random wobbles, equally likely to go up or down at any point along the front. During really bad seeing there may not a single moment of truly flat wavefronts, but adding all the distortions up taking the average should result in something approaching flat. Put differently, it does not make sense to alter the optical design to correct for e.g. concave spherical wavefronts due to seeing, because the perturbations are random

Thank you Michael, I understand this now.

[jetstream]

17 minutes ago, John said:

Perhaps not quite the same question but I have noticed that with eyepieces and scopes, good to excellent seeing conditions helps the better optical quality to stand out a little more.

It is all very related John, great observations.

Assuming we have very good seeing and for this consider it "fixed" the better optic will pull away as the lesser (but good) one is still degraded to a level that differences appear by the seeing, as related to what @davidc135 mentions. I think its related to Suiters wobbly stack idea.

Eventhough I can only speak for myself I think we might assume that excellent seeing is "perfect seeing" with respect to optical performance if you know what I mean.

I'm still very curious how much difference in the views there would be under perfect seeing between a diffraction limited scope and one much better optically. ie lets take seeing out of the picture. My feeling is they would be very close, but dont know.

[jetstream]

23 minutes ago, Mr Spock said:

I'm not sure of the technical aspects involved. All I know is both my scopes were hit equally last night by the appearance of the jetstream overhead and the consequent waves of poor seeing. Both scopes were rendered useless and I had to call it a night.

I agree, when seeing gets so bad that it degrades optical quality below diffraction limited, any scope is going to perform poorly and its time to pack up.

[davidc135]

44 minutes ago, jetstream said:

Lets say seeing is "fixed" at "very good" . I use 2 different scopes of the same aperture and f ratio, one goes to "X" magnification and one goes to "X +" mag.  Even though both scopes are above diffraction limited the fact that one takes more mag could be an indication of differing optical specs seeing considered? If so then under prefect seeing the views should be the same and magnification very similar?

I have sat for years at the eyepiece shaking my head trying to understand how a diffraction limited scope takes a certain mag while another scope can take more for given seeing.

I should say that the term 'diffraction limited' is a bit loose; a smooth, 1/4 wave pv of SA or a Strehl of 0.80... and I can well imagine a scope with a Strehl of 0.95 taking a higher magnification than one with 0.80 although both are diffraction limited. In perfect seeing there would still be some difference. Performance decline would be gradual down to 0.80 and then increasingly rapid.

David

[neil phillips]

Just now, Astrobits said:

It's quite simple really. Poor seeing is caused by a distorted wavefront and poor optics simply adds another layer of distortion to that. Hence the better the optics the less noise they add to the image.

Nigel

Exactly Nigel which tally's perfectly with my experiences

[jetstream]

2 minutes ago, davidc135 said:

I should say that the term 'diffraction limited' is a bit loose; a smooth, 1/4 wave pv of SA or a Strehl of 0.80... and I can well imagine a scope with a Strehl of 0.95 taking a higher magnification than one with 0.80 although both are diffraction limited. In perfect seeing there would still be some difference. Performance decline would be gradual down to 0.80 and then increasingly rapid.

David

Excellent, thanks for helping me wrap my head around this.  A lot of my puzzlement involves the H130 and the mag it takes. I believe it to be diffraction limited or a little better. The 2.4 HR in this scope provides stupidly good views eventhough the structure is lacking at this level. The 2.4mm gives 270x.

The next question or thought would be assuming perfect seeing how much difference would be between our idea of diffraction limited- .80 strehl, .25 wave SA and one much better? For lunar/planetary I think the MTF graph could be of help eventhough this introduces the effects of aperture in there.

So if we assume that optical quality does affect performance with regards to seeing and that the sag in the MTF graph illustrates this optical quality - then the details observed in a scope at the spacial frequency where the sag occurs would be more affected than those more closer to the perfect MTF graph?

[jetstream]

9 minutes ago, neil phillips said:

Exactly Nigel which tally's perfectly with my experiences

I truly appreciate real world observations of things and this is much appreciated Neil, thank you.

[John]

My understanding of the term "diffraction limited" is that, seeing conditions allowing, the optical quality is sufficient for the instrument to perform to the theoretical limits of the aperture. 

There seems to be a view that seeing conditions, much of the time, determine the effective limit above a certain scope aperture but there is much less agreement, probably understandably, on what that aperture limit is. My guess is that it varies widely.

The actual optical accuracy tolerances to achieve "diffraction limited" in a scope seem relatively relaxed compared to many of the strehl / PV / RMS statistics that we often discuss these days but I don't know if an optical system can actually exceed the theoretical limits for it's aperture and therefore be "diffraction exceeding" ? It sounds doubtful to me but you never know 🙂

I suspect it's more likely that the better optical quality shows itself in being able to "see" to the theoretical limits a little more easily and under a wider range of seeing conditions ?

 

[Mr Spock]

I know the type of telescope involved affects the outcome. I had a C9.25 which was extremely susceptible to seeing conditions. On its day it was a fine scope - finding a 'day' was another matter.
I had more excellent seeing (instrument wise) in the first few months of owning the 12" Dob than I had in ten years of the SCT!

[jetstream]

4 minutes ago, John said:

My understanding of the term "diffraction limited" is that, seeing conditions allowing, the optical quality is sufficient for the instrument to perform to the theoretical limits of the aperture. 

I think but dont know that the term diffraction limited is a description of an optics ability with regard to angular resolution which may not describe everything we see on the moon and planets.

6 minutes ago, John said:

I suspect it's more likely that the better optical quality shows itself in being able to "see" to the theoretical limits a little more easily and under a wider range of seeing conditions ?

I believe this to be the case but Ive noticed the same sharp definition on certain lunar features with scopes of the same aperture and of differing optical quality. Strangely I have also noticed that on certain features the scope with the better optical quality will show them, whereas the other either does not or "blends together" so to speak.

It seems that certain features can be more prone to seeing in differing optical spec scopes?

[jetstream]

2 minutes ago, Mr Spock said:

I know the type of telescope involved affects the outcome. I had a C9.25 which was extremely susceptible to seeing conditions. On its day it was a fine scope - finding a 'day' was another matter.
I had more excellent seeing (instrument wise) in the first few months of owning the 12" Dob than I had in ten years of the SCT!

Excellent observation Michael. Most people attribute this to cooling, thermal issues which is a factor. Another factor, possibly overlooked is the average optical quality of SCT's. I know some who have tested them and most are just below diffraction limited and a few at or just above. Combine this with the cooling factor and the seeing issues being discussed it does not surprise me at all that your dob shows better and more consistently I presume.

I'm not trying to disrespect SCTs or their owners by my comments, but many have reported the same thing as you.

[John]

13 minutes ago, jetstream said:

I think but dont know that the term diffraction limited is a description of an optics ability with regard to angular resolution which may not describe everything we see on the moon and planets....

 

That is a good point. Many features are more about contrast differences rather than angular resolution. I think this is where Modulation Transfer Function (MTF) comes into play but it is also where my knowledge peters out so I can't "shed any light" on MTF really 😄

Edited April 3, 2023 by John