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How exactly does one go about determining which of two identically scopes is optically superior?

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#1 thecelloronin

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Posted 26 January 2022 - 02:41 AM

Let’s say I have two small maks, identical in model, condition, collimation, and equilibration. How would one go about determining which is the superior scope using relatively simple tests at home?

Please don’t say:

A) It doesn’t matter for visual
B) Go to a lab/perform some test that can only be done in a lab with expensive equipment
C) Tell me to read Suiter’s book in full

I actually do have Suiter’s book, but don’t have time to take the whole thing in at the moment. Feel free to recommend specific chapters or pages, though.

Any guidance would be appreciated!
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#2 gpaunescu

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Posted 26 January 2022 - 02:51 AM

Is difficult to check, also if you are not optical expert (like me).

But what I would do:

   - check mechanical parts, like focuser (to have to be smooth feeling).

   - inspect visual lens/mirrors

  - with the same eyepiece, same conditions check in day/night the same targets

  - check some reviews about reliability of each brand


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#3 markb

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Posted 26 January 2022 - 03:47 AM

Surprisingly simple things have worked quite well for me, particularly in daytime. That certainly was not what I once expected.

 

Daytime snap to focus is a remarkably telling and tough test that can separate even good optics, daytime distant tree branches and leaves can be tough targets too. CA becomes obvious, particularly with uninsulated power lines or support cabling in the sun.

 

The focus snap alone easily differentiated my perfect etx90 from the two that were merely excellent, confirmed by other tests later on. A very good but not excellent etx125 quickly revealed itself on a snap test and distant leaves, later confirmed by more conventional tests.

 

All maks, so I'd think that was on point.

 

 

Fine print at a distance is also rough on less than ideal optics and is great for side by side comparisions. Print is a target that is far easier for our brains to interpret differences than plain images.

 

Threads on distant sign bolts also is an okay test but print is better I think.

 

My neighbors roof AC label and brand plate easily separates awesome binoculars from just excellent ones, and warning labels can challenge optics too.

 

I still need to make hi res printouts of the old military resolution/contrast test targets for more conventional daytime comparision use.

 

 

A Suiter based quick evaluation is to check pattern just on either side of focus, identical is obviously better than different patterns.

 

I think few folks can legitimately validly and consistently identify and explain defects (wish I could!), but identical patterns are relatively easy to spot. 

 

Ronchi testing is also relatively good for spotting turned edges, zonal defects and SA, at least to see it and compare two scopes, quantifying is said to be a weak point but spotting many errors is straightforward. DPAC is wonderful but complex, and a Ronchi test is 'only' 1/2 as sensitive but very simple to set up. 

 

Ronchi screens are currently hard to find but PDFs are around that can be printed at FedEx or staples. The Gerd Neumann metal on glass can be bought from TS in Germany, my first order there had reasonably shipping and came quicker than expected. But printed ones on acetate work well for most testing.

 

But the daytime snap and distant branches and fine print at a distance have been excellent fast tests that matched later, more accepted treating.


Edited by markb, 26 January 2022 - 03:59 AM.

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#4 Asbytec

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Posted 26 January 2022 - 05:45 AM

Be careful star testing MCTs. They are complex designs that do not form a spherical wavefront like parabolas. By design. Of course, parabolas often don't either, but the good ones get pretty close to spherical with similar inside and outside defocused images. The MCT wavefront only approximates a sphere by design. So it can and likely will test differently on both sides of focus, including the shadow break out test, but they can still be very good. 

 

+! for the snap to focus test. Better optics, cooled and collimated in good seeing, have less caustic focus between marginal and paraxial zones. So focus is more precise and sure than scopes with more primary spherical aberration. With increased primary spherical aberration and more caustic focus, lesser scopes tend to mush through focus. Focus is less sure. You may have to scroll through focus a few times to get it. That is one way to tell. 


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#5 Chucky

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Posted 26 January 2022 - 08:58 AM

Just look at Jupiter with some power under steady skies.


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#6 spereira

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Posted 26 January 2022 - 09:03 AM

Moving to Equipment.

 

smp



#7 csrlice12

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Posted 26 January 2022 - 11:44 AM

Just look at Jupiter with some power under steady skies.

Yep.....get some eyepieces put them in the scopes and look thru them......


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#8 markb

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Posted 26 January 2022 - 12:35 PM

Interesting comments by Asbytec, that's the first simple mak explanation I've come across, thanks.

 

I would think that ties into why Maks should only be final collimated at focus. All scopes should, but the warnings on mak collimation have been particularly pointed. When I had to tweak two ETXs, I started with centering Poisson spot diffraction rings near focus and then iteratively 'walked' my way to focus. An artificial star simplified things. Maks rarely get far out of collimation and that helped the starting point.

 

As to using planetary images, that is so nighttime seeing dependent (and my general seeing conditions so marginal due to location) and time of year dependent that I was happy to find daytime methods. Daytime atmosheric effects can be observed, worked around and often limited by target choice. They are immediately obvious with Poisson spot diffraction patterns on point reflections, and in the eyepiece as well in daylight testing.

 

For years I though I had bad luck on scope buys, based on planetary backyard views, when daytime testing, Ronchi and Suiter tests showed how truly excellent some were. My problem was dense suburban atmospherics.

 

I forgot to mention I use the Hubble Optics artificial star device, and, more recently, chromed ball bearings (Amazon) in the Sun or lit by a GLP, on an inverted garden hose shutoff valve 'stand' or stuck to a magnet, as an (day or night) artificial star.

 

But snap to focus, and trees, tiny print (used yesterday to clearly differentiate two alpha-level bino images) , and bare power lines always come first.



#9 csrlice12

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Posted 26 January 2022 - 12:46 PM

And that will tell you which is the best terrestrial scope.....nighttime might be a different animal as to which gives a "brighter" view, which shows detail better, etc....



#10 markb

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Posted 26 January 2022 - 01:44 PM

No, the daytime testing directly translates to nighttime, insofar as resolution and, less so, contrast.

 

That was very unexpected, indeed.

 

Transmission/brightness, not a chance in daylight, but comparing similar scopes rarely has brightness differences as a concern. Overall contrast can best be judged at night but daytime give some indications.

 

I'll take a sharp, contrasty scope every time over one with a few percent more transmission.

 

But, this is the ultimate "personal opinion rules" hobby I've been in, and all opinions are valid, I believe.

 

Since the OP was comparing two scopes, resolution seems to be the primary first stop, to me.

 

Differentiating scope sharpness in daytime has turned out to be a solid choice for me, starting when I compared multiple scopes of identical design, later verified with traditional tests. And, of course, at night. Every one that excelled in daytime excelled at night insofar as resolution, and, so far, overall performance.

 

Initial collimation of refractors, maks and SCTs are much easier in daytime with artificial stars, too. Nighttime tweaks are usually minimal but essential.

 

Eyepiece performance comparisions, I absolutely agree, show up almost entirely at night, in subtle details and contrast, and overall transmission.

 

But daytime testing, if not stated outright, assumes using a known top resolution eyepiece compatible to the scope design, absolutely.

 

It's a bit off, but I found my best performing nighttime and daytime binoculars by starting with birder critical reviews. The transmission charts give indication of the night brightness. I use a BT at night rather than handhelds, but that's my happy spot.


Edited by markb, 26 January 2022 - 01:45 PM.


#11 thecelloronin

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Posted 26 January 2022 - 01:51 PM

Cheers guys. I'll set aside some time in the coming days to put all suggestions into effect, and will return with any followup questions, concerns, or comments.


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#12 Asbytec

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Posted 26 January 2022 - 05:27 PM

Interesting comments by Asbytec, that's the first simple mak explanation I've come across, thanks.

 

Thank you. Here are some graphics posted on CN years ago showing the higher order wavefront from the meniscus balanced with the lower order wavefront from the spherical primary resulting in the wavefront at the bottom. Then balanced with defocus (from paraxial focus to best focus). MCTs are pretty clever designs. Yes, in focus collimation is best. Spherical mirrors are interesting things. Read here. It's about SCTs, specifically, but MCTs are similar. Also google Roland Christen Star testing Complex Designs

 

Balanced HiLow Graphic.jpg

 

High Order Spherical Illustrated.png


Edited by Asbytec, 26 January 2022 - 06:37 PM.

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#13 dnrmilspec

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Posted 26 January 2022 - 05:54 PM

Well the challenge is specifically, "all things being equal".    Some thoughts.

 

First of all if you are using these visually, the best test is to put them under the stars and try them out.  Given that you specify that they are perfectly collimated, there really should be no discernible difference between the two.  The suggestion to look at Jupiter if you are at the right time of the year is a good one. Also check out some open clusters and look at the star images.  Use the same diagonal and eps in both scopes.   But:

 

If you see a noticeable difference between the two scopes when looking at Jupiter then one of the scopes is broken.  Maks from the same manufacturer rarely show that much variation that it would be obvious. 

 

If you come to the conclusion that one is just a wee bit better than the other but you cannot point to specifics then keep that one and your own council.  It will make you feel better.

 

If you want guidance, I would tell you that unless you have both scopes handy already, do not try to do this.  It is most likely be a waste of time and will make someone unhappy. 

 

If you want expectations I do not believe that you will see any difference at all and that any you do see will likely be just fooling yourself.

 

Interesting you chose Maks though.  Their optical excellence is frequently mentioned here.  The two I have are both excellent though off-the-shelf imports.  The only one I ever found lacking was an old 7" Meade and it was a surprise compared to the other Meade Maks that I had tried.  I was suspicious that its owner had tampered with it.   For that for which they are best suitable, high powered looks at moon and planets,  the Maks that I have seen are almost always possessed with very good optics. 


Edited by dnrmilspec, 26 January 2022 - 05:56 PM.

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#14 luxo II

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Posted 26 January 2022 - 06:27 PM

It does matter, for visual.

 

Pull out a double star catalog. Choose a night with good seeing, line them up side-by-side with a selection of high power eyepieces and test them on close double stars - and at the same magnification...

 

What I'm looking for are:

 

- which one can or cannot resolve a double, while the other does;

- how they compare regarding scattered light (glare);

- how they fare when the object is moved off-axis with the eyepieces you prefer to use.

 

It's also important to swap the same eyepieces across the two scopes to make sure what you're seeing is not due to the eyepiece.


Edited by luxo II, 26 January 2022 - 06:34 PM.

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#15 Asbytec

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Posted 26 January 2022 - 06:37 PM

...test them on close double stars to see which one can or cannot resolve a double.

Close unequal double as an indicator of aberrant light in the rings? Most every scope can form an Airy disc. 



#16 luxo II

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Posted 26 January 2022 - 07:27 PM

Ah. I should have added:

 

- the stars chosen should be bright and use magnification at least 60X - 75X per inch aperture. Then look at the Airy disks closely to see if there are visible defects in the first ring - and which are not due to thermal effects. The point being that most defects imply energy is being scattered from the central spot into the diffraction rings.

 

This assumes the scopes do produce a reasonably clear diffraction pattern to start with.

A scope that doesn't I would walk away from pretty quickly - and I have seen (and owned) a few.

 

If the darned la-Nina here ever breaks I am poised to make a video showing what "perfect" looks like using my 10"; it seems summer is going to be a total white-out.


Edited by luxo II, 26 January 2022 - 07:33 PM.


#17 thecelloronin

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Posted 26 January 2022 - 08:49 PM

A little more information:

 

I do have both scopes to hand, and they are modern Synta C90s. There have been enough folks in the massive “Something for Nothing…” CN thread sending their lemons back to Celestron that I reckon there might be reason to doubt the two being on equal standing.


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#18 carolinaskies

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Posted 26 January 2022 - 09:31 PM

Daytime tests and close-range terrestrial testing can report erroneous results due to the mirror relationships being much different than night time near-infinity focus if usage will be for night observing.  Daytime observing the worse instrument may pass with an acceptable performance due to the abundance of light, but at night this is where the quality comes through.  Considering the amount of light available at night is miniscule, this is when the better optics will shine.  

C90's are definitely a mixed bag since they are marketed often as spotting scopes for daytime use, so the 'acceptable' quality standard is lower.  This is why I think many of them bought for astronomy turn out to be underperformers.  



#19 luxo II

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Posted 26 January 2022 - 10:08 PM

Daytime tests and close-range terrestrial testing ..

... are not helpful.

 

In daytime atmospheric turbulence limits the magnification to about 50X - 100X at most, which is inadequate to sort the sheep from the goats - not even for a C90 - even if you had an artificial star (eg sun glinting off a shiny object at several hundred yards) it will be a blur.

 

The second issue is that unless you can see the Airy disk and rings, terrestrial objects tell you nothing useful about the optics.


Edited by luxo II, 26 January 2022 - 10:16 PM.


#20 Jon Isaacs

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Posted 27 January 2022 - 08:11 AM

Regarding star testing complex optical systems, this is what Roland Christen wrote:

 

http://www.csun.edu/.../startest2.html

 

He discusses Maks extensively.

 

I am one who believes in focus, high magnification comparisons of the planets and difficult double stars are the proper tools. When observing visually, these are what distinguishes between the optical quality two otherwise identical to telescopes.

 

I'm skeptical about daytime testing. The conditions are just too different. I doubt very much you could see a difference between two 10 inch Newtonians. Thermal issues and seeing issues limit what can be seen.  

 

Jon


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#21 Asbytec

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Posted 27 January 2022 - 01:48 PM

It's true seeing during day or night can blur a focus test, and turbulence is likely more prevalent during the day. That's true for any test of the optics, including seeing the Airy disc and diffraction rings. Yes, the object we focus on should be at optical infinity at higher magnification. But it's still a good test for spherical aberration, or more accurately of the caustic focus (blur), without learning to start test. The image of a good scope should snap into sharp focus with good clarity more readily than a lesser scope. The Airy disc and rings can show other specific defects like astigmatism, collimation, etc. If one wants a quick simple test of the scope, generally, better scopes have a more precise focus. You can't quantify it, but you can qualify it. Side by side testing is best.

Edited by Asbytec, 27 January 2022 - 01:53 PM.

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#22 PKDfan

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Posted 27 January 2022 - 05:41 PM

At what level of correction does a snap begin to be formed and is their clear differences to it when ascending very high on the wavefront correction plane?

 

Does it get super 'snappy' able to be snapped in the worst seeing conditions imaginable?

 

 

And confirmed by other means mean DPAC? Or...?

 

Thanks! 

 

 

Cs&Gs

 



#23 luxo II

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Posted 27 January 2022 - 06:21 PM

I would describe it a little differently. Having had two maks with exceptional optics, there are three practical aspects...

 

1. Point objects (stars) and say the moons of Saturn are tight little points, in much the same way as in a very good triplet APO. 

 

2. They handle average seeing better. Using a target such as Saturn and its moons, the fainter moons - mag 9...13 - should be within scopes of comparable aperture, and on a night of excellent seeing, most comparable scopes will indeed show them. But when the seeing deteriorates to average to poor, several will be reduced to an invisible smear in many scopes. With really superb optics however, the moons are still visible as tight little points "doing a little dance".

 

3. In any conditions the maximum useful magnification is higher than scopes with average optics. On pretty much everything. There have been many club nights in average conditions when other observers are struggling at 150-200X, yet I'm running 300X or more and thinking the seeing is quite OK.

 

4. There are things they resolve when other scopes don't. For example, in the southern sky NGC2808 is a large globular cluster similar to Omega Centauri in terms of physical size, but at twice the distance from Earth so it appears half the apparent size, and fainter. I've seen 10" scopes that show it as a smudge and cannot resolve it as stars, yet my 10" mak easily resolves this as stars and at 300X you would be forgiven for thinking you're looking at Omega Cen.

 

And sure, there can be really awful nights when I don't go past 100X.


Edited by luxo II, 27 January 2022 - 06:25 PM.

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#24 Asbytec

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Posted 27 January 2022 - 06:43 PM

 

I am one who believes in focus, high magnification comparisons of the planets and difficult double stars are the proper tools. When observing visually, these are what distinguishes between the optical quality two otherwise identical to telescopes.

 

I'm skeptical about daytime testing. The conditions are just too different. I doubt very much you could see a difference between two 10 inch Newtonians. Thermal issues and seeing issues limit what can be seen.  

 

 

Jon, I believe Roland is talking about difficult unequal double stars. The effect of contrast transfer is the same with difficult planetary detail. He's assessing the level of aberration and the intensity of light in the diffraction rings. Seeing and thermal absolutely affect what we can see, those are more prevalent during the day. Bottom line is, day or night, we need a good image that is limited by the optic and not the atmosphere or the thermal properties of the scope. I think we both agree with him. 

 

 

At what level of correction does a snap begin to be formed and is their clear differences to it when ascending very high on the wavefront correction plane?

 

I don't know. It's something we cannot really quantify, it's more of a qualified answer. Unless we set some standard like 1/4 wave PV of primary spherical. At night observing a star image, we can find the point of best diffraction focus fairly easily. We tend to it naturally in good and bad scopes, so I am not sure that tells us much. But, on extended objects we do not see the diffraction artifact of a point source. We can only estimate how sharp the image becomes as it comes to focus. A perfect parabola with a single focal point there should be no doubt when we are in focus as the eyepiece hits the focal plane. There is very little wiggle room. However, with some level of longitudinal spherical aberration, there is more wiggle room as we pass along the caustic through paraxial focus, best focus, smallest blur, the circle of least confusion, marginal focus, and anywhere in between. To my way of thinking, it's this wiggle room we are evaluating. You can get a good feel for it. 


Edited by Asbytec, 27 January 2022 - 06:52 PM.

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#25 markb

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Posted 27 January 2022 - 06:44 PM

Jon, thanks for the very interesting link. Most unexpected information (to other post readers there is even more at http://www.csun.edu/~rprovin/roland/) and I cannot wait to read further.

 

Jon, I absolutely agree that daytime conditions are very different indeed.

 

But they do carry a surprising amount of value. By all means try some differing quantity scopes of your own in daytime.

 

I had decided not to post further in this thread, but thought I'd suggest that you do try comparing a couple of large reflectors with known figures in daytime, for focus snap, and with critical resolution tests. Especially if you have never done so.

 

One really has to experiment with daytime tests to see what they can, and cannot offer (only first class eyepieces of course).

 

Transmission-no, contrast-partially, resolution and indication of overall figure-yes. But contrast can play a significant role in perceived sharpness. IIRC the photos sites refer to the combination as accutance.

 

Essentially, resolution is resolution, whether in daytime or at night.

 

To the posters that have said that terrestrial and nighttime viewing are very different, that is of course correct, it is just an incomplete view. Daytime terrestrial testing has definite value beyond convenience.

 

Ironically, and unexpectedly, atmospheric turbulence effects have been controllable by target choice, and minimal compared to locations with routinely mediocre, poor or, at my current location, abysmal seeing at night. I would think the miles shorter path to the target is why it seldom is bad enough to interfere. Even here in the Valley of Hellish Heat, oops, I mean The Sun.

 

Daytime optical testing of no value?

 

On the contrary, the most valid binocular tests that I have read, and trust, have purely been on birding sites, critically reviewing mid and high end binoculars.

 

Birders are pickier than amateur Astro users, by a long shot, especially on resolution, contrast and CA.

 

Every binocular that I've tried that birding posters reviewed at the top during daytime, was an excellent performer at night, barring any transmission limitations. That usually shows up anyway, in transmission graphs. The only other reviews that I've relied upon consistently have been some of those on CN by known users.

 

During daytime critical resolution tests on small print I can distinguish differences much harder to separate at night, mounted. Low power of course is a limitation at night, in particular.

 

I have daytime compared refractors, f5-12, maks (my five differed a lot, and I gave up on getting a superb etx125, pending finding a Terabeam) and SCTs, as well as far too many binoculars. I still cannot believe the snap to focus is as effective an indicator of overall sharpness as it is. 

 

+1 on Asbytec whose last posts state much of this better than I have.

 

 

Besides testing fresh shipment when time is short or clouds rule, night testing requires decent atmospherics that some of us rarely see.

 

My testing problem started because my backyard atmospheric seeing eight miles from the NYC line was consistently abysmal. My new home two miles outside the Phoenix city line is, somehow worse.

 

The testing suggested for night time would absolutely be the best way to critically judge optics, but are completely useless under those conditions of poor seeing.

 

I had expected terrible atmospheric turbulence, but the relatively short distances involved minimized turbulence effects, and I also was able to choose different lines of sight with very different amounts of intervening atmospheric turbulence. Incidentally, when testing really critically in enclosed areas, the effects of even breezes can be seen. Everything truly is relative.


Edited by markb, 27 January 2022 - 07:00 PM.

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