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Henry's simple guide to testing spotting scopes? (1 Viewer)

henry link

Well-known member
Hi Kumar,

Thanks a lot for putting me on the spot. ;) OK, the way I evaluate a spotting scope can take many hours if I test for everything, but I start with one "simple" test I think I've mentioned a time or two here - the star-test.

If the star-test of a particular scope I'm thinking about buying isn't up to whatever standard I've decided it needs to meet then that's the end for it. No other tests are needed.

After doing many, maybe thousands, of star-tests over the last 30 years the thing comes pretty naturally to me now, but I've been surprised by how much can go wrong or be misinterpreted when somebody first tries it, so here are a few pointers to get started.

It's easier to use an artificial star than a real one. On a sunny day there are many possibilities for artificial stars that consist of tiny glitter points of the sun reflecting from small shiny convex surfaces, like a ball bearing, a Christmas tree ornament, curved trim on a car, insulators on power poles, etc. Place the scope at least 20 or better 40 meters from the shiny object. Try to avoid hot ground surfaces between the two, so the air will be reasonably steady.

Place the glitter point in the center of the FOV at the highest possible magnification. In a perfect scope the glitter point will look like a tiny disk with a single thin dim ring surrounding it. Slowly defocus in one direction, then the other. In a perfect scope you will see a bull's eye of equally bright rings, increasing in number with more defocus and looking identical on both sides of focus. The degree of defocus most revealing of optical defects is between 2 and 5 rings. Defocus too far and all scopes look similar.

The typical problems revealed are things like too much spherical aberration, astigmatism, pinching, coma and defects in the roof edge of the erecting prism. There are many sources on the internet which show the visible forms these defects take in a star-test. You really don't want your very first star test to be the scope you're looking at in the store the one you just bought online, so I would suggest reading up on the subject and practicing with whatever scopes you can find to look through before you go to the store or place the order. It's often possible to do a quick star test of somebody else's scope in the field if you can find a tiny glitter point of the sun (even dew drops will work in a pinch).

The next most useful test is a resolution test, but that requires an accurate distance measurement between the scope and a resolution chart and enough magnification to reveal the resolution limit of the scope. Many spotting scopes don't provide enough magnification for a person with 20/20 acuity to see the resolution limit of a good scope.

In the end, even with all its potential pitfalls the star-test is still the single best thing we have to quickly evaluate the quality of a scope.

Henry
 
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mskb

Well-known member
A Perfect Ten response, thanks Henry! :)

The often linked telescope-optics.net website was too hard for me to parse through. So I have been grabbing the star-test images from your various spotting scope reviews, along with your descriptions and trying to study them side by side. I was able to get an understanding of the overall experimental setup for the star test and gather some intuition with interpretation.

I am doing all of this to characterize the performance of a 66mm scope I have in hand. I have tried star tests, but like you allude to, it has been quite hard to setup the experiment to generate sufficiently clear results for interpretation. I have tried resolution measurements as well at 60x and it was clear (with great help from David) that I need to be a lot more careful with distance measurements.

After such repeated experiments setting up star tests on ball bearing-like structures on electric poles, and observing the vertical and horizontal bars on the resolution chart, I feel like my current scope does come with astigmatism, and under-corrected spherical aberration at the very least.

But I can't shrug off this constant feeling of a lack of confidence in my results, mainly because of the challenges in setting things up appropriately. Hence, my original question. My apologies if you had felt uncomfortable by explicitly asking a question to you via a forum thread!

One final question: based on the results, how does one decide if a scope is a lemon? Basically, I would like to get a sense of (a) when I could consider a star test result to be really bad, and (b) If my ideal resolution reference is 119/D for a D diameter spotting scope, what level of jitter around that point is acceptable?

Cheers, and once again thank you Henry!
Kumar
 

mskb

Well-known member
@Pileatus, Thanks!

This is my first scope, and it is precisely the problem of gauging its performance in the field that I resorted to doing these measurements.

Clearly, the image quality (e.g., contrast, the sharpness feeling) at higher X is lower than what I can achieve at the basic zoom of 20X. The focus is snappier at 20X than it is at 60X. BUT these are things I have noticed with all scopes in store/outdoors, with the caveat that I haven't used them in the field as long as I have with mine. Not to mention those frequent hazy days, which makes concluding anything very hard.
 

Pileatus

"Experientia Docet”
United States
@Pileatus, Thanks!

This is my first scope, and it is precisely the problem of gauging its performance in the field that I resorted to doing these measurements.

Clearly, the image quality (e.g., contrast, the sharpness feeling) at higher X is lower than what I can achieve at the basic zoom of 20X. The focus is snappier at 20X than it is at 60X. BUT these are things I have noticed with all scopes in store/outdoors, with the caveat that I haven't used them in the field as long as I have with mine. Not to mention those frequent hazy days, which makes concluding anything very hard.
What make/model are you working with?
 

mskb

Well-known member
Hello @Henry & others,

I was finally able to do a much clearer star test following @Joachim's idea of setting up an artificial star with a flash light (wrapped with an alum foil, pin-hole). The day was gorgeously non-hazy! I am still trying to photograph the result with a more powerful flash light and a higher magnification (e.g, binocular focused at infinity) to share the exact result with you here, but the following is what I am seeing so far:

The artificial star was placed about 100' away. De-focussed about 4-5 rings on either side. At 60X, the rings appeared concentric. Relative to the diffraction rings inside of focus, those outside of focus appear about 30-40% less bright, center appears "filled up" with white. The rings become more discernable as we move outward from the center. They appear purplish with outermost ring tending to more white.

Any thoughts on the result above?

Thanks a bunch, everyone!
Kumar
 

jring

Well-known member
Hi,

concentric circles shows we don't have astigmatism (ellipses changing direction from inside to outside) or coma (non-concentric).

Rings more visible inside of focus than outside means undercorrected, which is common. The question is, how much.

Joachim
 

mskb

Well-known member
Thanks Joachim.

1. How do we figure out by “how much”? Are there reference figures at various degradation settings that I could use? I think the telescope-optics website had these black and white references for two “lambda” settings but not sure how to go about comparing them to mine with color deviations. Henry’s Razor HD / Zen ED2 review makes me think this undercorrected spherical aberration could be of the spherochromaticism type, although it doesn’t appear fully purplish like his image (in my case, outer rings tend to be more white).

2. Also, I missed mentioning one thing. As I defocused on either side, I consistently felt that the diffraction rings started out elongated and rotated (in orientation relative to each side of focus) but settled to concentric circles by 4 rings. Could this imply potential astigmatism / is this normal? I want to use a more powerful torch light to image all this, and be absolutely sure. The one I used with this 66mm scope is a 500 lumens. I want to move to a 1500 lumens type so that the rings are hopefully visible via a binocular to further magnify and image.

Thank you for your insights,
Kumar
 

jring

Well-known member
Hi,

telescope-optics.net shows examples of primary SA for an unobstructed aperture at lambda/4 (which would make the objective just about diffraction limited, provided no other aberrations are present) and the much smaller lambda/8 (which I would be perfectly comfortable with). Very experienced star testers tend to use an artificial central obstruction for unobstructed telescopes too in order to quantify spherical aberration and then measure the amount of defocus at which the shadow of the central obstruction becomes visible...

Spherochromatism is a type of chromatic aberration and is certainly not the cause of the different patterns inside and outside of focus you see (except for the purple fringe). You could use a green led flashlight for your tests to get a best case scenario at the wavelength your scope is probably optimized for...

As for the slightly elongated patterns when just out of focus - it might be a hint of astigmatism, but if you don't see it with 4 circles, it's probably less of an issue than the SA...
In general, aberrations get less visible farther out of focus - if you defocus far enough, most instruments will show a perfect pattern ;-)

Joachim
 
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henry link

Well-known member
Two to three rings is best for detecting astigmatism. If you see oval shaped diffraction discs that close to focus there is astigmatism. The more elongated the ovals appear and the farther from focus they linger the worse it is but it will seem to disappear with more defocus.

Your description of the spherical correction might apply to something as good as 1/4 wave or something as bad as 1/2 wave. Manufacturers are likely to consider 1/4 wave all they need to do, but a while back someone posted here that a tech at Leica had told him that 1/2 wave was their minimum standard and that is very poor. If I'm going to spent big money I want at least 1/8 wave. I've seen that in spotting scopes, most recently in a Nikon Monarch ED. I personally wouldn't accept a 1/2 wave scope at any price, but 1/4 wave would be OK if there are no other defects and the price is low enough. There ought to be some examples somewhere on the internet of the appearance of different aberrations in star tests, including various levels of spherical correction (1/8,1/4,1/3,1/2, etc.) like the ones in Suiter's book.

I wouldn't get too hung up on spherochromatism, especially if you are using an LED light source, which unlike sunlight, may have a very skewed color spectrum. Just treat what you see as if it were all white light. It is possible to test for monochromatic spherical correction over a narrow band. I have a very narrow band green filter (539 nm) I sometimes use for that and have even approximated narrow color bands with ordinary colored Christmas tree ornaments as artificial stars.

Unfortunately, illustrations of pure single aberrations don't necessarily prepare you for the real world where there may be several different aberrations piled on top of each other, each adding to the total image degradation. That's when it's good to have a reference scope of known high quality set up next to the scope being tested for comparing image quality and resolving power.

The more I write the harder it seems to define a hard minimum standard for what you should require from your scope. It's easy if the star test reveals sensibly perfect optics or an outright lemon, but the gray area in between will always be a judgement call.

Henry

PS: Looks like Joachim was posting a similar response as I was writing.
 
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mskb

Well-known member
Hello Joachim and Henry,

Thank you for correcting my error on spherochromatism, describing what "obstructed" means and strongly indicating the comfort range on the lambda scale. In some sense, this is precisely what I was after. So I greatly appreciate your help!

Just looking at the "primary" SO images over at the T-O website here, I thought my scope is perhaps closer to a quarter of a lambda. However, after reading @Henry's post on the possibility of my descriptions applying to 1/2 a wave as well, I wanted to be sure. Henry's mentions took me to Suiter's book's images (Figs. 10-9 -- 10-10, unobstructed, ). After looking at those, yes it could well be 1/2 a wave as well!

Clearly, I should repeat the experiment with a more powerful flash light, image and share with you all. The result will conclusively decide if this scope goes back.

Thanks again, Joachim, Henry and others! I greatly appreciate your help!
Kumar
 

jring

Well-known member
Hi,

diffraction limited or a Strehl value of 0.8 (which is basically a measure for the diffraction generated by the sum of aberrations of an existing instrument as compared to an imaginary perfect example - which only has the diffraction of the objective aperture and obstruction, if present) is an accepted minimum standard for budget astro scopes.
So if your $500 chinese ED80 doublet can do that (and is quite probably significantly better), premium sports optics suppliers should be able too.
Lambda/2 peak-valley or P-V in any single aberration as a quality criterion is a bad joke - it basically makes the objective useless except for burning bugs...

That being said, with a diffraction limited instrument a normal sighted user should be fine at the relatively low maximum magnification of 60 or 75x for most spotters. If you intend to use extenders to get to 100x or beyond or have exceptional eyesight, you might want a better example...

Joachim
 

mskb

Well-known member
Hello @Henry, @Joachim, and others, Thanks for your exceptional guidance, I was able to make good progress in making a decision. I present the details below. I end with a couple of questions. I would greatly appreciate all of your insights as always!

Instead of being restricted to just the scope that I have, I decided to spend several hours at our local birding store comparing
(a) focus snaps at 60X and
(b) star-testing to the best of my ability.

The scopes that I had access to were the ATS65, ATS80, Kowa 883, store's Kowa 663M and my 663M.
(1) My focus snap ratings 60X: ATS65 > Kowa 883 > store's 663M > ATS80 >= my 663M.
(2) With respect to star tests on electric resistors’ highlights: astigmatism, spherical aberrations and coma seemed relatively easier to notice, so that's what I recorded. I repeated the test on various such highlights across a range of distances >= 100'. The results follow:
(a) The store's ATS80 copy left me with a similar distaste like my 663M copy. The diffraction rings on one side of focus was extremely mushy, the worst in this regard among all the scopes compared.
(b) The store's current Kowa 663M copy was excellent in contrast/clarity at 60X than mine. It did carry astigmatism like mine. Given that the diffraction rings were more similar on either side of focus, I felt it was better corrected for spherical aberration as well. It also had smoother focus, perhaps adding to its better performance over mine.
(c) The Kowa 883 with its dual focusser certainly showed some astigmatism and coma. However, it was a great deal better in its ring patterns than the ATS80.
(d) Finally, the ATS65 copy was a whole league better than the poor performing scopes here with respect to the diffraction rings. The focus snap was very nice.

The questions:
(1) All scopes however did have relatively dimmer rings on one side of focus, compared to the other. Why is that so?!! Is that all spherical aberration?
(2) Given that the store's current Kowa worked better than mine, I am inclined to send back my copy. But I still need to figure out if I want to take the store's Kowa as an exchange over my copy / do a return and move on to something ele. This is going to be a hard decision for the following reasons (besides the >=$600 cost difference):

(a) Moving from the store's 663M to ATS65, perhaps due to a combination of FoV, brightness and eye-relief reasons, I had a jump in the feeling of a relaxed view at lower and moderate zoom ranges (20-30X, 30-50X). However, at higher zoom ranges (50X-60X), the Kowa was more easier with eye positioning.
(b) Swaro being Swaro basically produces an image with the same color balance & characteristics as my current 8.5x42 binoculars … soo… I am worried if it would get boring after spending all that money.
(c) Moving on to the Kowa 883, this time from the ATS65, I felt the relaxed feeling doubled/tripled! At this point, it became very clear that if comfortable viewing / easy eye positioning / FoV / brightness are of primary concern, it is hard to settle with 60/65mm scopes - whoever the manufacturer is! However, this drama in the view from the 883 comes with ~1.1lb weight increase, and >=$600 price increase over the ATS. It is thus very clear the 883 is too much for us to be traveling/hiking around with.

Given the very noticeable inferiority of 60/65mm scopes’ relative to the 80ish mm scopes with regards to image quality (especially on those cloudy days) and comfortable viewing anyway, it does beg the question: should we settle for the store’s Kowa 663 for the price, and spend on an 883 or something similar for local birding? I am very confused!

Thank you for any insights you might add!
Kumar
 
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Hermann

Well-known member
Moving on to the Kowa 883, this time from the ATS65, I felt the relaxed feeling doubled/tripled! At this point, it became very clear that if comfortable viewing / easy eye positioning / FoV / brightness are of primary concern, it is hard to settle with 60/65mm scopes - whoever the manufacturer is! However, this drama in the view from the 883 comes with ~1.1lb weight increase, and >=$600 price increase over the ATS. It is thus very clear the 883 is too much for us to be traveling/hiking around with.

That's the point - looking at the ease of view and the brightness is only one side of the equation. Sure, theoretically bigger is always better, at least when it comes to scopes. But you pay a penalty - bigger scopes are heavier. And it doesn't end there: For a bigger/heavier scope you need a bigger/heavier tripod with a bigger/heavier head. And the differences in weight can be quite significant. A lightweight head weighs in at something like 600 gr., a bigger head that can support one of the big boys is something like 900+ gr. That's a 300 gr difference without taking the tripod into account.

Only you can decide which way you go - do you need (or want!) the greater ease of view of a big scope? Or do you prefer a smaller, lighter setup? In the end it all depends on your own preferences and your style of birdwatching.

That's the reason why some people use two different scopes nowadays, a big scope if there's no or only little hiking involved, and a small scope for all other occasions. I actually use three different scopes: An 80mm scope for when I don't expect to move too far from the car, a 60mm scope as my go-to scope for everything, and a small, lightweight 50mm scope for when I expect to do a lot of hiking. I've got different tripods for all three scopes as well. The weight of the three setups is something like 5.5 kg for the big scope, 3.7 kg for the 60mm, and about 2.1 kg for my small 50mm scope. So if we manage to go to Norway again this summer I'll definitely take the 50mm scope as there'll be a lot of hiking in mountainous terrain - and even then I'll likely leave the scope in the cabin much of the time.

So, to sum up: If your birding involves a lot of hiking I'd take a medium-sized scope. It's the most universal size IMO. And if I were to get a new scope today, I'd look at the Nikon Monarch 60mm scope and the Swarovski 65mm.

As always, YMMV.

Hermann
 
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jring

Well-known member
Hi,

good to hear that you had a nice day testing optics... now the performance of your 663 example has been put into perspective and it seems to be underwhelming, so you should try to return it - if still possible.

Regarding your questions:

ad 1) remember, the star test is a very sensitive test and very small defects can be seen by an experienced observer. A little bit of SA is common and very experienced hobby astronomers who have star tested hundreds or even thousands of scopes tend to remember the handful of perfect examples.

ad 2) with the results you mentioned, I would only consider the ATS65 - if the 883 in second place showed visible coma and stig (both are more damaging than SA), the rest seems to have been in the OK to not so great range...
I would try to test the ATS with the 25-50 wide angle zoom and see if that works for you (some users without glasses experienced kidneybeaning with that one). If it works it would trade the 50-60x range with its tiny exit pupil of 1.3mm and less for some more field, especially at lower magnifications... If not, you could always try to find a used 30 wide EP for the ATS along with the 20-60x zoom.

Or, now that you know what to look for, keep searching for that used cherry and get that...

Joachim
 

henry link

Well-known member
Or, now that you know what to look for, keep searching for that used cherry and get that...

Joachim

I've been in that position for years. I can't bring myself to pay a large amount for a scope that's much below sensibly perfect because I know of a few designs that have demonstrated the potential to produce superb scopes, if only the individual units are made well enough. I've seen a few of those myself and also read credible descriptions of near perfect star-tests from others. As Joachim said, such scopes are probably rare, but then my friend strolled into a store and and left with a near perfect Nikon Monarch 82ED on his very first try!

Kumar, in my experience specimens of the 65mm Swarovski are likely to be pretty good to excellent. I haven't seen any lemons, but I'm sure there must be a few. I've found Kowa's 883s to be more variable, with a larger possibility of getting a lemon. I definitely wouldn't buy one (or any other scope) with any significant amount of astigmatism or coma.

Henry
 

Binastro

Well-known member
In testing hundreds of scopes, mostly astro and thousands of lenses I have only found a few essentially perfect ones.

However, when it comes to professional broadcast lenses and things like Technicolor lenses, they are likely to be nearly all cherries.
New these types of lenses may cost $10,000 to $200,000.

There were some telescope makers who routinely made near perfect optics. Jim Hysom, Horace Dall, and two other British professional opticians. Also some Dutch, French, German, Japanese and U.S. makers.

However, in actual use, even for planetary viewing, some defects are O.K. as the atmospheric conditions are usually the limiting factor.

Long focus telescopes are more forgiving.

B.
 

mskb

Well-known member
Thanks eveyone!

@Hermann, If we do get access to a Nikon 60, we will make sure to try it. Based on Henry's review, we will give the ~.66lb heavier (relative to the 65s here) Nikon 82 ED a close check as well. If we aren't swayed by it, it is likely going to be a travel scope, and the decision comes back down to the 663M for the price or the ATS65.

@Binastro, thank you for the cautionary note on going on, what might become, an endless search. :)

@Joachim, thank you as always for the very specific, kind response.

@Henry, @Joachim : given your thoughts, the ATS65 does appear mighty interesting. If you'd allow me to be more detailed here on question 1, with respect to the ATS65: The rings were highly concentric, practically nill astigmatism and coma even at 2-3 rings, and no purple rings outside of focus as was the case with the Kowa. I could still very clearly and crisply count the rings on either side of focus all the way up to the center! The ONE thing that so far hasn't let me call it a cherry is that I did notice the rings outside of focus rings were ~30% less bright. What might be causing this? Is this all spherical aberration alone? Could atmospherics be playing a more major role?

Cheers and thank you so much!
Kumar
 
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