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just for FUN.....anyone care to speculate?

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#1 zawijava

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Posted 07 January 2013 - 10:47 AM

this is just for FUN...really. I may buy a new Celestron 8" Edge OTA[$1300]. I'm definitely NOT buying the brand new Questar 7 Astro that's currently listed on AM for $8500. But seeing the Questar ad, the question immediately crossed my mind as to what one might see if putting these two scopes side by side with equal magnification and FOV.....let's say a 24 Panoptic in the Questar and a 19 Panoptic in the Edge....so roughly 106X and .6 degree FOV in both scopes. All else being equal....thermal equilibrium, good to excellent seeing etc......would a seasoned observer see any substantial differences?

#2 KerryR

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Posted 07 January 2013 - 11:17 AM

It'd have to depend on the optical quality of both scopes. Questar, of course, has a reputation for perfect optics. Celestron and all 'low end' manufacturers have a reputation for variability, though this seems to be less of an issue over the past 5 or so years. So, get a poor sample of either, which is far more likely with the Celestron if reputation and cost are any indicator, and you'd see the difference under good seeing, probably easily. Get a good sample, and I doubt you'd see a difference at all,even under good seeing.

Most of us probably aren't going to see the difference, given most of us observe under less than ideal seeing.

One thing you can count on: the owner of the Questar is far more likely to see a difference...

#3 zawijava

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Posted 07 January 2013 - 01:05 PM

One thing you can count on: the owner of the Questar is far more likely to see a difference...


:funny:

#4 Rick Woods

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Posted 07 January 2013 - 01:14 PM

One thing you can count on: the owner of the Questar is far more likely to see a difference...


People who can see the difference are the ones who are willing to pay for the Questar! ;)

#5 Erik Bakker

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Posted 07 January 2013 - 02:07 PM

If you increase the magnification to about 220x, the difference will become increasingly obvious :)

#6 Eddgie

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Posted 07 January 2013 - 03:55 PM

My EdgeHD 8" has optics that have about the same quality as the two Eastern Eurpoean Mak Newts I have owned. I don't know if this is still typical, but if it is, expect a very good showing from the EdgeHD.

Equal? Maybe, maybe not.. But close. Very close if the optical quality is as high as my sample on contrast, and perhaps a bit brighter on deep sky.

#7 Darren Drake

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Posted 07 January 2013 - 04:17 PM

I asked this very quesion here a while back.

http://www.cloudynig...5518500/page...

#8 Rick Woods

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Posted 07 January 2013 - 04:51 PM

Equal? Maybe, maybe not.. But close. Very close if the optical quality is as high as my sample on contrast, and perhaps a bit brighter on deep sky.


I suspect it's like the UO ortho vs. ZAO II comparison: it gets *really* expensive to get that last bit of quality; but if you just have to have it, you pay the money and don't look back.

#9 maknewtnut

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Posted 07 January 2013 - 06:17 PM

Not to stir the pot, and having bit my tongue for years on the subject...

Having attended many star parties, and subsequently observing through a big handful of the newer breed of SCT's...I really didn't see the huge improvement many seem to report their newer SCT's provide to them.

I recall one year at OSP where the folks parked next to my booth had 8", 9.25", and 11" models one right after the next (but not in that order). That provided an excellent opportunity for more than just a quick peek. To be honest, I still saw the same oversized star images I saw in orange and black tubed models before them. Edge correction was a bit improved, but nowhere near the 'sharp to the edge of the field' many others seem to report.

Could every newer SCT I happened to observe with be a poor sample? Possible, but not likely. Is the Questar still very expensive for it's aperture? Sure. However, just as some believe the Questar owner is more likely to detect a difference; it's just as likely (as it has always been) that the mass produced SCT owner will claim they are equal regarding image presentation (although the latter often never used 'the other one', whereas the former almost always has). No need to speculate at all.

Until one does side by side comparisons for themselves, it's all empty talk.

YMMV

#10 Eddgie

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Posted 07 January 2013 - 06:38 PM

The EdgeHD will not make any improvement at the center of the field. All of the benefit is at the outside of the field, and I don't know what eyepeices where in the scopes you viewed though, but the difference is obvious when using high quality wide field eyepeices. Everyone that has looked though my EdgeHD and compared it to my C14 always agree that the stars are far sharper at the edge of the field in the EdgeHD.

The reason I think the EdgeHDs are performing well is that the optical quality appears to be consistently very high from the reports I have seen. Rohr (Astro-foren) tested an EdgeHD 11 and the Strehl was .95, which is in the excellent catagory. You can make a scope better, but it is difficult to see the difference once the quality gets this good.

My own EdgeHD appears to be on par with the MNs that I have owned. Excellent optics.

Most of my previous SCTs have not been this good. The C9 I owned was barely passable, and the C11 was OK, but not great.

My C14 is very good, but I think just shy of excellent. It has a zone which is a very typical feature in larger Celestron SCTs. Only my previous C8 (second of three) had optics as good as my current EdgeHD 8".

But the EdgeHD is near perfect. And all accounts I have heard are that the quality is very high. Also, recent standard SCTs are getting consistently good commments on optical quality.

This is what has hurt the SCT in the past. The quality varied quite a bit.

IF, and only IF the quality has improved to be consistently in the .95 Strehl then todays standard SCT or EdgeHD will be a better scope.

And this. If you say "oversized" star images, blame seeing.

Last night, was out with my C14 under good seeing conditions. I was easily seeing a clean diffractoin pattern at 300x in my C14. It was forming and breaking because of seeing, but patience always showed it.

And it has been the case with all of my other SCTs. When seeing is very good, they will present a clean Airy Disk. When it is not, they won't.

And the Airy disk won't be any bigger than it would be in a similar size refractor...

#11 maknewtnut

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Posted 07 January 2013 - 07:06 PM

Seeing can be ruled out as an affect on star images when other scopes of comparable and larger aperture do not present the same.

#12 GlennLeDrew

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Posted 07 January 2013 - 07:14 PM

At larger exit pupils, on axis the image quality will probably be the same; the aberrations are not magnified fir the eye to resolve. At some point, as exit pupil decreases, differences probably will be detected. It might require to get the exit pupil to below ~2mm, or even less.

Differences in scope and eyepiece field curvature, and how well they mate, as well as off-axis aberrations, and efficiency of baffling/quality of coatings as regards veiling glare, are other matters yet.

#13 Dwight J

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Posted 07 January 2013 - 08:06 PM

I had the opportunity to look through a Questsr 7 at the Alberta Star Party in 1988. Prior to looking I observed M13 in my 1983 SC8 and I went back after and re-observed it after looking at M13 in the Q7. My C8 had better than average optics (tested by Peter Cerevolo). I used the silver top 17mm Celestron eyepiece but I forgot what eyepiece was in the Q7 - probably a Brandon which gave about the same magnification. Although this was not a controlled test and clouded by the passage of time, the C8 gave a much brighter image but the Q7's stars were finer and the globular seemed better resolved. We also looked at M57 in the Questar and the view was nice but not as bright as I thought it should be. To be sure it was a beautiful instrument to behold. I took a photo of it during the day and if I can find it I will post. One thing I didn't like was the F 15 focal ratio which didn't suit my style of observing at the time.

#14 Rick Woods

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Posted 07 January 2013 - 11:14 PM

Rohr (Astro-foren) tested an EdgeHD 11 and the Strehl was .95, which is in the excellent catagory.


OK, I'd like some clarification. It seems to me that I've recently read, on this forum, that an SCT with a 30% CO can't possibly do better than a Strehl of .80. (IIRC, this was from Jon Isaacs; but I may be wrong about that.) I know I've seen the reference repeatedly as to the impossibility of having a Strehl as high as .95 on an SCT, due to design constraints.

Now you're saying a C11 tested out to Strehl of .95; which of these contentions is correct, and where's the proof of one or the other?

#15 Jared

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Posted 08 January 2013 - 12:40 AM

Most people when quoting Strehl numbers quote the measured values without the effects of central obstruction. This allows comparisons based purely on optical quality rather than introducing the complications of design choices. It allows one to determine how close to perfection for the particular design the manufacturer has come.

#16 GlennLeDrew

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Posted 08 January 2013 - 02:20 AM

Indeed, as Jared relates, the Strehl ratio is usually supplied as it relates to the potentially perfect system as designed. And this is as it should be. A scope having a central obstruction, if perfectly executed, will have a Strehl ratio of 1.00. The diffraction induced by the obstruction, while impacting the image, should not be automatically included as a measure of performance. Why? Because the effect of the diffraction is completely and accurately predictable. And so far better to not incorporate extra diffraction from obstructions, relying instead upon expressing *wavefront* deviations only, to which the additional losses imparted by diffraction--which are completely decoupled from optically induced aberrations--are added.

The danger is this. Unless explicitly stated, the expressing of a Strehl ratio which already incorporates the effects of diffraction can be all too easily taken as an artificially poorer quality optic.

#17 freestar8n

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Posted 08 January 2013 - 03:53 AM

Strehl ratio should never include the effects of central obstruction because then it would lose its direct connection with wavefront error (RMS). A central obstruction contributes nothing to wavefront error. If you include central obstruction, you might as well compare 'scopes with different aperture - in which case the "relative Strehl" could be 100 or 0.01 or anything - since it just refers to the maximum intensity of the central peak of the Airy pattern.

It's very convenient and useful that the peak intensity is directly related to the rms wavefront error - regardless of the nature of the aberrations involved (as long as they are small). If anyone includes the effect of CO in a "Strehl" value - they are missing this point.

Frank

#18 KerryR

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Posted 08 January 2013 - 09:52 AM

I had the opportunity to look through a Questsr 7 at the Alberta Star Party in 1988. Prior to looking I observed M13 in my 1983 SC8 and I went back after and re-observed it after looking at M13 in the Q7. My C8 had better than average optics (tested by Peter Cerevolo). I used the silver top 17mm Celestron eyepiece but I forgot what eyepiece was in the Q7 - probably a Brandon which gave about the same magnification. Although this was not a controlled test and clouded by the passage of time, the C8 gave a much brighter image but the Q7's stars were finer and the globular seemed better resolved. We also looked at M57 in the Questar and the view was nice but not as bright as I thought it should be. To be sure it was a beautiful instrument to behold. I took a photo of it during the day and if I can find it I will post. One thing I didn't like was the F 15 focal ratio which didn't suit my style of observing at the time.


This was a good post, even if a bit dated.

#19 TG

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Posted 08 January 2013 - 05:10 PM

Rohr (Astro-foren) tested an EdgeHD 11 and the Strehl was .95, which is in the excellent catagory.


OK, I'd like some clarification. It seems to me that I've recently read, on this forum, that an SCT with a 30% CO can't possibly do better than a Strehl of .80. (IIRC, this was from Jon Isaacs; but I may be wrong about that.) I know I've seen the reference repeatedly as to the impossibility of having a Strehl as high as .95 on an SCT, due to design constraints.

Now you're saying a C11 tested out to Strehl of .95; which of these contentions is correct, and where's the proof of one or the other?


Yes, I remember Jon Isaacs' arguments and they are wrong for the simple matter that no manufacturer measures Strehl from measurements of the diffracted PSF, as far as I know. It's instead computed using the formula S = exp(-(2*pi*RMS)^2), where RMS is the RMS deviation, measured using an interferometer, from the reference wavefront in terms of fractions of the wavelength used. From this perspective, it's a great way to quantify, using a single number, the entire wavefront quality. E.g., Astro-Physics guarantees 1/50 RMS. Then the Strehl ratio for the wavelength used would be exp (-(2*3.14*0.02)^2) = exp(-0.0158) = 0.984. This formula is only an approximation and Vladimir Sacek's site details the subtleties involved at http://www.telescope...net/Strehl.htm.

I own a C11HD with virtually identical inside/outside diffraction patterns and it wouldn't surprise me if it has better than 1/30 RMS optics (= 0.96 Strehl), which is what Roddier testing my erstwhile C9.25 showed me. It's "real Strehl" may be 0.80 but then it's an 11" scope and where do I find an unobstructed 11" scope in a compact package? My current unobstructed 7" is a beast to mount and often requires me to humble myself on my knees when viewing the heavens. :grin:

Tanveer.

#20 Dwight J

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Posted 08 January 2013 - 08:58 PM

I found the picture I took of the Questar 7 on it's wedge and tripod. A rather unique wedge system.

Attached Files



#21 yonkrz

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Posted 08 January 2013 - 10:24 PM

Pretty awsome Dwight,a lot bigger than i was expecting,thanks for the pic.

#22 freestar8n

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Posted 09 January 2013 - 02:34 AM

I own a C11HD with virtually identical inside/outside diffraction patterns and it wouldn't surprise me if it has better than 1/30 RMS optics (= 0.96 Strehl), which is what Roddier testing my erstwhile C9.25 showed me. It's "real Strehl" may be 0.80 but then it's an 11" scope and where do I find an unobstructed 11" scope in a compact package?



The "real" Strehl is what it is and somehow factoring in CO would not be a real Strehl. Strehl has two magic properties:

1) For a given optical system (with or without CO) the peak irradiance only depends on the RMS wavefront error (including defocus) - for small amounts of aberration. The nature and profile of the wavefront error doesn't matter.

2) In addition, as you increase the error, the energy will shift from the central peak to the rings in the same way - regardless of the type of aberration. This means that not only is the height of the central peak determined by RMS alone, but the entire diffraction pattern and MTF. This is all an approximation assuming small RMS.

In contrast, when you change CO, you lose light in the central peak, and the central peak narrows as energy goes into the rings - which themselves shift. The MTF and PSF are now different from the effect of aberration alone, and you can't compare them directly.

Frank

#23 Asbytec

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Posted 09 January 2013 - 03:41 AM

Frank, you are correct and really what I think Tanveer was driving at using the term "real Strehl." Maybe it's better said to be nominal or system Strehl. In any case the lower peak intensity is a Strehl-like number defining peak intensity.

It can be approximated using Strehl(nominal) = I * Strehl (aberrant.) Where intensity is approximated using I = (1 - o^2)^2. So a scope with 0.95 aberrant Strehl can have a "diffraction limited" peak intensity of 80%, which I am sure Jon was driving at earlier. It's not as good as a refractor where it's peak intensity equals it's aberrant Strehl.

Anyway, in accord with the MTF (such as it is) the main difference between an aberration and CO is the size of the Airy disc: 1.22 Lambda/D in an unobstructed scope and 1.11 Lambda/D with a 0.3D obstruction. This accounts for the large shift of the curve to the right near maximum spacial frequencies (which are correspondingly higher in an obstructed scope, more so than an unobstructed same aperture.) The rings are correspondingly brighter with an obstruction, but the central disc is smaller allowing better resolution smaller that Raleigh's and Dawes limits (defined using a refractor.) The rings do not shift significantly in radius, unless one takes into account human vision. But, on the focal plane, the radii should be very similar, save for the first minimum.

#24 freestar8n

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Posted 09 January 2013 - 04:25 AM

Maybe it's better said to be nominal or system Strehl. In any case the lower peak intensity is a Strehl-like number defining peak intensity.



Ugh - no. I would not associate it with Strehl at all - the poor guy. You could compare two clouds based on their heights and call it a Strehl - or refer to the Strehl of the stock market also based on the peaks achieved on different days.

When you have a small amount of aberration - both Strehl and RMS characterize the entire effect of that aberration - magically - with a single number. When you compare one CO to another, there is no such magic single number that describes the impact of the change on the optical system.

So - I would avoid using "Strehl" at all.

Also - when you only have aberration involved, there is no change in throughput to deal with and you don't have to worry about re-normalizing the peak intensity due to the change in area. When you add a CO, you change the throughput - so not only has the entire MTF and PSF changed in unusual ways, but the total energy is changed also.

I guess you could say one system has a peak irradiance of X watts/cm^2 and the other has Y watts/cm^2. And then describe the peak width and the heights of all the rings. That's a lot more stuff than the single Strehl or RMS value that works for a single system with different but small amount of arbitrary aberration.

Frank

#25 Asbytec

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Posted 09 January 2013 - 06:12 AM

Yea, it's a good point. Normally, I just refer to the final diffraction product as peak intensity since that incorporates both the CO effects (aberration and obscuration) and wavefront. Using Strehl loosely creates some confusion.

So, what was the original question? Care to speculate on a C8 and Q7? Well, let say the Q7 has a Strehl of 0.98 and a CO of 33%, while the C8 has a Strehl of 0.95 and a CO of 37%. I don't know their actual figures, but then again we're just speculating here.

Q7: I = (1 - 0.37^2)^2 ~ 0.79. (for both obstruction and added diffraction). Maximum intensity with aberration (approximated) 0.79 * 0.98 ~ 78% peak luminance, or something similar. All approximations.

C8: I = (1 - 0.37^2)^2 ~ 0.75. (for both obstruction and added diffraction). Maximum intensity with aberration (approximated) 0.75 * 0.95 ~ 71%.

Between the two and based solely on these conditions - and for fun - so far the Q wins. Now, add an inch of aperture. Can you make that call with approximately 8% difference in final peak intensity (and marginal reduction in ring intensity?) On star images, maybe. On planets, I'd speculate it's too close to call in one sitting.

Sometimes I think these comparisons come about because they are so close. Someone probably sits down and crunches the numbers to find a very closely matched pair then asks which one is best in this forum. Truth is, they are very close with the Q offering a slight improvement in mid contrast (more refractor like with a smaller CO and higher Strehl) and the C8's improvement is at higher resolution and the inch of added aperture might make up some mid range contrast advantage of the Q7. Anyway, its the same old argument wrapped in a different condition.

Next time, make it tricky using a Newt with a spherical wavefront and spider vanes. (:lol:)






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