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Yuri at TEC recent comment regarding APO vs. Mak

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#301 SteveC

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Posted 14 August 2014 - 08:52 PM

 

 

very few people here have actually made the visual comparison between a MC and an APO, and so far as i can tell, their testimony is "meh ... not make a difference."

I have - TEC140 & Intes Micro 715 Deluxe

 

6206698-2273217-Mak-TEC.jpg

Edited by SteveC, 14 August 2014 - 09:07 PM.


#302 dedo

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Posted 15 August 2014 - 01:54 AM

In my experience you simply cannot resolve what contrast loss hides you. Sure enough a bigger aperture supplies to contrast loss but will likely  never operate, visually, at his full resolution possibility. It really depends not only by seeing, currents etc. but on the subject too.

As an avid jovian observer I can fairly easy tell the differences between two premium eyepieces  in a >120mm apo and cannot imagine to not being able to tell those same differences between a co free and a 25% co scope, and even less, provided they both are of premium quality and in excellent seeing



#303 Cotts

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Posted 15 August 2014 - 07:11 AM

In my experience you simply cannot resolve what contrast loss hides you. Sure enough a bigger aperture supplies to contrast loss but will likely  never operate, visually, at his full resolution possibility. It really depends not only by seeing, currents etc. but on the subject too.

As an avid jovian observer I can fairly easy tell the differences between two premium eyepieces  in a >120mm apo and cannot imagine to not being able to tell those same differences between a co free and a 25% co scope, and even less, provided they both are of premium quality and in excellent seeing

 

 

Contrast loss is not total, but gradual.  Look at the MTF graph below from Damian Peach's site (and he's a guy who knows a thing or two about contrast...).

 

Imagine two perfectly black lines that are close together, around 1" separation for the 9.25"  scope in question. About the size of a small 'barge' in the Jovian cloud belts, e.g.) That corresponds to the middle of the curves, around spatial frequency 0.4.  The unobstructed scope does not show them as black lines but 50%  gray.  The 35% (!) obstructed, 1/4 wave (!!) scope shows those same black lines as 35% gray, not invisible.  They are more gray but not so gray as to disappear.   There is virtually no loss of information.  You and I could notice the contrast difference, sure, but nothing disappears, the lines are resolved in both telescopes.....

 

Now, as we move toward the right, indicating lines that are  closer together and thinner, too, the obstructed scope catches up with the unobstructed and passes it slightly (too slightly to notice I'd say...).   This means for the very tiniest features, at the utmost limit of detection, with very low contrasts such as albedo features on Ganymede, for example,  the telescopes are identical in their ability to deliver contrast and information.   

 

To sum up, the obstructed scope resolves everything that the unobstructed one does, especially with planetary features that are the very smallest.

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#304 Neptune

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Posted 17 August 2014 - 11:47 AM

My observations with a TEC140APO and subsequent AP 175EDF VS a nice sample of a C-11 were as follows:

The sky conditions were the determining factor by a long shot.  The 140 was almost always the clear winner for contrast and sharpness of details ( in Phoenix, AZ). Yes, the images were a little brighter in the C-11 but, I could see no more detail than in either of the APO's.

 

Additionally, don't you think that looking through a $20 grand plus scope (AP175) has some kind of psychological factor that HAS to be factored in? It seems like a kind of the 'Emperors New Clothes' effect.

 

I have read most of the 'books' on telescope design and evaluation. The latest one, 'Telescopes, Eyepieces, Astrographs' by Smith, Ceragioli and Berry proved to be very good at divulging the secrets of the current crop of optics for the armature.

 

David



#305 Neptune

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Posted 17 August 2014 - 09:11 PM

"Good reading on system-level Strehl:  http://whichtelescop...benchmarks.htm"

 

WIsh I would have seen this website years ago... very informative.

 

Thanks for posting it!

 

David



#306 PowellAstro

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Posted 17 August 2014 - 09:22 PM

The link does not work for me. It may be my tablet but it gives an error.



#307 Neptune

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Posted 17 August 2014 - 09:35 PM

try this:

http://whichtelescop.../benchmarks.htm

David



#308 PowellAstro

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Posted 17 August 2014 - 09:52 PM

This one works, thank you.



#309 TG

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Posted 20 August 2014 - 09:56 AM

 

The author of this page seems to have studied for his PhD in optics exclusively using CN forums. 

 

Tanveer 



#310 BillP

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Posted 20 August 2014 - 07:09 PM

 

 

Great!!  ...some folks will be bothered by this though :lol:



#311 Asbytec

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Posted 20 August 2014 - 07:54 PM

 

 

The author of this page seems to have studied for his PhD in optics exclusively using CN forums. 

 

Tanveer 

 

 

And skipped a few classes. It's a little more complicated than the explanation in the link above.

 

My guess is you're referring to the idea the central obstruction affects the system Strehl. It does not. Strehl is strictly, by mathematical definition, determined by the aberration only. There is a final Strehl-like property of the Airy pattern, called peak intensity and sometimes nominal Strehl, influenced by the central obstruction and a more complete discussion of obstruction effects can be found here. 

 

http://www.telescope...obstruction.htm

 

From the link above, "Since the negative effect of CO is so similar to that of wavefront aberrations, the question of what is its maximum acceptable size can be answered in terms of the conventional aberration limit of 0.80 Strehl."

 

"For an actual optical set of the Strehl ratio S higher than 0.80, the minimum acceptable obstruction size for the combined ~0.80 Strehl level for low-to-mid MTF frequencies would be obtained from SI=0.80, with the peak intensity factor for the obstruction, as mentioned, I = (1-ο^2)^2 = 1-2ο^2+ο^4."

 

A better approximation for determining the maximum CO allowed in order to achieve a desires peak intensity (nominal Strehl) is οmax~[0.6-(0.6SN/S)]1/2, where S is the system Strehl and SN is the desired peak intensity (nominal Strehl) on the focal plane.. 

 

"In general, central obstruction will reduce aberrations causing significant wavefront deformation over the inner pupil portion. And vice versa, it will worsen those causing only insignificant wavefront deviations over the inner pupil area."

 

A scope with a 0.95 Strehl can have an acceptable CO in the range from 0.28D to 0.31D and still be considered, the latter being adjusted for better contrast transfer in the mid to low frequencies (larger than the Airy disc). A CO of 0.45D is adjusted for the entire MTF, low to mid frequencies as well as high frequencies.

 

The obstruction has the effect of dimming the image by it's surface area and adding diffraction (not aberration) to the system. This diffraction has an affect on both the Airy disc diameter and the light distribution to the rings. While light is diverted to the rings, the Airy disc is physically (and actually, not just visually) smaller by a factor of 1 - co^2 giving a slight boost to resolution (high frequency MTF response) to better than perfect unobstructed apertures in very good obstructed scopes. Since the PSF peak is smaller at it's base, it is also a tiny bit brighter since it's energy is encompassed in the area under the peak. The Airy disc is a little smaller and brighter (that it would otherwise be) in an obstructed aperture.

 

The article points out, "Lord Rayleigh calculated that the effect of diffraction puts a limit on the smallest detail (angular resolution) that we can see in any (unobstructed) telescope , however perfect. He calculated the limit at 138/D arc seconds, where D is aperture of telescope in mm. This is called the "Rayleigh Limit."

 

Note, the Raleigh limit is determined for unobstructed apertures to be 138.4/Dmm. For an obstructed scope, this limit is a little smaller depending on the percentage of the obstruction diameter relative to the aperture approximated by (1 - co^2). Wilfried is doing some empirical double star (see double star forum) research with varying obstructions and has some visual results reporting not much gain over about 0.4D. One might imagine this increased resolution is generally applicable when one can actually "see" the Airy disc in modest apertures, in good seeing and maybe in images.

 

But, the statement in the article, "No telescope can have resolution better than the Rayleigh/Dawes limit, so this has been used as a criterion for the optical quality of telescopes" is not entirely true. If we modify the Raleigh and Dawes limits due to the obstruction, an obstructed scope can push resolution below these limits. Questar used to boast their scopes can best the Dawes limit and that claim should be true. I've resolved doubles and extended objects to better than Dawes approaching Lambda/Dmm, so I believe it.



#312 DesertRat

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Posted 20 August 2014 - 08:20 PM

Tanveer,
Thats funny!  It reminds me of the quote from Wolgang Pauli:
"This isn't right. It's not even wrong."

 

 

Norme,

 

I agree the pupil geometry has nothing to do with the formal Strehl.

 

However an obstruction does make a scope more seeing sensitive.  We all know larger apertures are already more sensitive, but the obstruction diverts some light to the outer rings.  The rings are more prone to be perturbed by seeing than the central spot.  Hence contrast suffers for extended objects, thought of as a continuum of point sources.

 

Also the argument an obstruction improves resolution should be qualified.  Yes it can help separate equal magnitude stars, but that effect is only worthwhile for fairly massive obstructions, like 50% or more.   At that level you do not have a visual friendly system.

 

Separating point objects like double stars is not a good test, especially equal magnitude systems.  Even for faint companions Dawes found by tilting the object glass, moving the first ring out of the way, sometimes was beneficial.  What happens then to the Strehl?  Also a scope can have somewhat large levels of spherical aberration  and separate doubles.

 

Glenn


Edited by DesertRat, 20 August 2014 - 08:21 PM.


#313 schang

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Posted 20 August 2014 - 09:27 PM

Let me ask a question on the link for system Strehl ratio as shown below:

 

"SR(telescope system) = SR(main mirror) x SR(secondary mirror) x SR(central obstruction)

= 0.96 x 0.96 x 0.90

= 0.83  "

 

While it is intuitive to take this formula for granted, has anyone actually measure or prove that this formula is correct for system Strehl calculation? 



#314 DesertRat

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Posted 21 August 2014 - 03:19 AM

Let me ask a question on the link for system Strehl ratio as shown below:

 

"SR(telescope system) = SR(main mirror) x SR(secondary mirror) x SR(central obstruction)

= 0.96 x 0.96 x 0.90

= 0.83  "

 

While it is intuitive to take this formula for granted, has anyone actually measure or prove that this formula is correct for system Strehl calculation? 

 

Shien,

 

The formula is invalid for two reasons.

 

First an obstruction does not aberrate a wavefront.  Simply put it does not add any errors to the wavefront.  It does some other things but not that.  A better way to incorporate an obstruction and aberrations together is using the concept of encircled energy.

 

Second, you cannot simply multiply Strehl numbers.  For example a secondary might have corrections that null out errors from the primary.  You could have two mirrors with Strehls of 0.8 whose combinatiion yields Strehl 1.0.  So the whole formula to me is bogus.

 

If it came from a text I would like to know which one.   Its possible it was taken out of context or misunderstood or who knows what.

 

Glenn


Edited by DesertRat, 21 August 2014 - 03:25 AM.


#315 schang

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Posted 21 August 2014 - 06:51 AM

Glenn:

 

Thanks for the answer.  Another valuable lesson learned.



#316 Asbytec

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Posted 21 August 2014 - 09:44 AM

For example a secondary might have corrections that null out errors from the primary.  You could have two mirrors with Strehls of 0.8 whose combinatiion yields Strehl 1.0.  So the whole formula to me is bogus.

 

 

Glenn, thanks for pointing that out. It dawned on me later, as well. I think you're spot on.

 

Also the argument an obstruction improves resolution should be qualified.  Yes it can help separate equal magnitude stars, but that effect is only worthwhile for fairly massive obstructions, like 50% or more.   At that level you do not have a visual friendly system.

 

 

Glenn, Wilfried has some empirical results that much over 40% is actually detrimental to resolution and certainly for contrast. He's observing at a minimum magnification required for a split and at large CO's he's less successful. I suspect it's because the first ring is interfering with his relatively low power observation, as I understand his explanation, giving a mere notched split, as he describes it and IIRC. In any case, he's finding that much above 30%, resolution is not as clean. But, I think it's in his method of minimal power observing according to his acuity.

 

In my own experience, I've bested Dawes on a couple of occasions and attribute that to a 30% CO as the lab math seems to work in the real world. Now, there are errors in measurements and calculations, but something is happening allowing nearly equal doubles (STT 507 granted 7th magnitude pair) to be resolved. Not so much a test of Strehl, but attesting to CO effects of higher resolution in accord with 1 - co^2. I dunno how to explain it otherwise, unless one magnitude fainter made the tight split possible.

 

A real stunner was observing a crater form (unbelievable, I know) at a mere 1.4 km in diameter near lunar apogee. The math suggests this small crater subtended a mere ~0.71" arc (give or take due to errors in the calculation.) Using the small angle formula as an approximation, 0.71" arc = 206,265 * 1.4/400,000 (kilometers). It turns out, more accurate measurements of that crater are slightly smaller but I'd have to find the thread where David Knisely gave a better measure along with the known distance to the moon that night. The point being, that crater should not have been observable, but it was...plain as day and exactly three times on a superb night. This is what sent me on a rampage trying to understand how in the heck...

 

Also, other close equal binaries such as 31 and 7 Tau, both very close to Dawes pairs, were easily split. Even 7 Tau, the closer of the two and advertised at less than Dawes, had a clear grey space diving it suggesting a slightly tighter split is possible with less contrast between them. But, what are we talking about here, mere hundredths of an arc second. Very tiny and prone to errors of measurement. But, for some reason, resolution below the standard Dawes separation is occurring on several objects.

 

I dunno, Glenn, I am uneasy about disagreeing with you as I have so much respect for your views. Unless I am missing something, my own experience tells me the obstruction is making a difference albeit a very tiny but exiting difference exceeding the limits. Oh, and elongation of Chi Aql at 0.42" arc (again, given errors in measurement) or about Raleigh/2 is another example of another observation that should not be possible, but it was. Not easy, but with some patients over two or three nights, the (very nearly) correct PA could be determined. Oh, and the "apparent" elongation of Io and albedo on Ganymede /might/ be additional anecdotal evidence.

 

Something is allowing better resolution than I could have ever expected. It's not my aging eyes, it might be the seeing. I doubt it's the performance of the scope being simply diffraction limited (best I can tell) and not premium quality surely. It has to be the CO. (Well, it does not have to be, but it seems to be.)

 

If it came from a text I would like to know which one.

 

 

See Neptune's post above.

 

Great!!  ...some folks will be bothered by this though

 

 

Had I read that before I posted, I would not have bothered. But, in fact, I'm excited, having the observing experience of a lifetime. :)



#317 DesertRat

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Posted 21 August 2014 - 10:10 AM

Hi Norme,
No problem on disagreement.

 

The website link above does not mention a reference or text I could find.  There is a lot of stuff out there, and I don't mean space junk.

 

Back to obstructions - on a Refractor forum!

 

One can view an obstruction as a high pass spatial filter.  That is it takes some low frequencies out.  Whats left is a response biased for the higher frequencies.  So 'resolution' of finer details may be improved.  The effect is small and nothing to champion.

 

I choose not to get too religious about obstructions.  Half my scopes have them and I view the obstruction not as a malady but just a feature of the design.

 

A 30% obstruction does result in an ever so smaller central spot.  But its a small effect.  At 50% things get more interesting.  I read in Argyles book someone going all the way to 75% and perhaps beyond for effect.  Of course contrast suffers, but with double stars you have the ultimate in object contrast to begin with.  So losing contrast as you mentioned at 40% is not a deal breaker for doubles, since it is a low freq loss.

 

Resolution is a fuzzy subject.  And as I have mentioned before it is more of an engineering subject where signal to noise plays a key role.  Even visually it does.  A bigger obstruction may result in a light loss making these things difficult to present.

 

Dawes criterion is only an empirical value for a specific situation.  Burnham exceeded it many times with smaller scopes.  As he went to larger scopes it fell off quickly.  When he got to > 9" aperture Dawes was very tough.  That's seeing and also a kind of signal to noise situation. 

 

Glenn



#318 DesertRat

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Posted 21 August 2014 - 10:47 AM

Norme, me again.  :)

 

I should have added that I do not doubt Wilfried's findings.  However other observers have seen contrary evidence.  Rectifying the difference suggests something else is at work.  Naturally a large obstruction like 75% would only be of benefit for near equal components.  A fainter companion would be obliterated by the huge increase in energy of the surrounding rings.

 

Observing small features on an extended object, like a lunar crater, has little to do with the Rayleigh or Dawes criterions.  An isolated spot or a linear feature of sufficient contrast difference with the surrounding area can be detected well below those benchmarks.  But separating features like adjacent lines is more in line with what we normally think of as resolution.  In addition a crater has an associated shape, which requires a certain relationship of adjacent points, so observing a spot does not qualify as resolution of the crater.  Sorry. :flowerred:

 

Glenn


Edited by DesertRat, 21 August 2014 - 10:48 AM.


#319 Asbytec

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Posted 21 August 2014 - 11:44 AM

Hi Glenn. Thanks for replying. Yea I agree observing a crater is not like observing a spot. Some use Dawes or Raleigh as limits to extended object resolution. I'm not sure they apply, but something interestng appears to be occuring. It was interpreting the interplay between the adjacent points that is intetesting and mysterious.

 

Each of those examples are high contrast, so...

 

Lemme absorb your comments more fully. I could not find anything to disagree with. Lol

 

Okay, if Burnham regularly observed at higher spacial frequencies, then nothing to crow about. Except that it's fun. :)

 

Cheers, Glenn.



#320 BillP

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Posted 21 August 2014 - 01:13 PM

In my own experience, I've bested Dawes on a couple of occasions and attribute that to a 30% CO as the lab math seems to work in the real world. Now, there are errors in measurements and calculations, but something is happening allowing nearly equal doubles (STT 507 granted 7th magnitude pair) to be resolved. Not so much a test of Strehl, but attesting to CO effects of higher resolution in accord with 1 - co^2. I dunno how to explain it otherwise, unless one magnitude fainter made the tight split possible.

 

 

Don't you think it is a little risky making the assumption that it was the CO of your instrument doing this?  What would be interesting to confirm would be for you to hook up with someone with a 150mm APO, observe the targets you mentioned, then add a mask with varying size COs to the APO to see how this impacts things and if you can replicate the besting of the limit.  Alternatively you can create a 40% and 50% CO mask to put on your current instrument with a 31% CO to see how the observsations differ.  That would be a real interesting thread reading your results on that.


Edited by BillP, 21 August 2014 - 01:41 PM.


#321 Asbytec

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Posted 21 August 2014 - 07:55 PM

Hi Bill. Risky, yea maybe. I'd love to know how such observations stack up with a refractor, too. There is a thread in the double star forum on challenging doubles for a 6" refractor, I pointed one out to see if he could observe the same for that very reason. Inquiring minds want to know, well would like to know if an obstructed scope can enjoy it's high frequency resolution. It is the realm where they are supposed to dominate, just as refractors dominate in their realm at mid frequencies. Theory says they should. So, do they?

 

The only thing I Might know at this point is what a Dawes split looks like and what it might look like if Dawes separation is more easily split, say more than 5% contrast between the discs - an obvious dark space. And that a tighter dark space (not black space) split was made. When I crunch the numbers, it's consistent with the math. So...If Dawes empirical result should be 0.77" with a faint dark space and I managed 0.70" separation (as reported and dated with the double apparently widening very gradually in the three years since last report), there is a reason for it.

 

If such observations are consistent with the mathematical approximation, the PSF, and MTF, maybe there is something to it. Maybe not. But there /should be/ a reason for it and that reason /should be/ the obstruction effect. It could be excellent seeing of 8/10 or better, I doubt my acuity is super human and can see what's not on the focal plane. I also doubt my scope has a premium optic doing super human things (if they are super human observations at all.) Or it could be something else, like Dawes is wrong, the separation data is off significantly, or some other less likely cause. If a 6" APO could repeat that observation, though, then we'd be more certain it's not the obstruction.


Edited by Asbytec, 21 August 2014 - 08:04 PM.







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