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

[jring]

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!

Hi Henry,

two perfect scopes for a single person would be like winning the lottery twice... after all you have your Stowaway, which is not only a rare beast in general but also a perfect example...

Your friend probably had beginners luck - a bit more than me when I bought my first TSN-3, but I won't complain. It is certainly not perfect but probably in the same league as the ATS-65 example mentioned above - nice snap of focus at 60, can go higher with an extender and has just a little bit of SA.

Now that I think of it, my perfect example is probably the old 8" f8.5 mirror I have bought for the price of a Pyrex blank a few years ago... it looked quite good when I did a Ronchi and when a friend and me drove to an optics tester to have his large mirror tested, we also had the tester do a few quick i-grams of my 8" and it came out a bit north of Strehl 0.95...

I really should get some sonotube and give this beast some starlight... should be nice on planets...

@Kumar: as you describe the pattern for the ATS 65 it's just a little bit of SA. Maybe have another look at the lambda/8 image for unobstructed primary SA on https://www.telescope-optics.net/star_testing_telescope.htm and decide if it looked like that or even less pronounced. lambda/8 would already be a great scope.
The CA you saw in the Kowa is probably due to the Kowa being quite a bit faster at f5.7 than the Swaro at f7. The color correction of the 883 is still quite good for such a fast instrument.

Joachim

 

[mskb]

Hi @Joachim, thank you!

The rings weren’t as diffused as the 1/8th pattern shown on the website. They were just mostly “dimmer” with a very slight haze to the outer ring. So I am unable to figure out if it’s 1/8 lambda or more or less from the pictures.

Thank you,
Kumar

 

[mskb]

I just looked up Suiter's pictures on spherical aberration we talked about earlier in the thread. It appears counting the rings all the way up to the center is possible even at 1/4th wave and to an extent, even 1/3rd wave. I am unable to quite ascertain the "dim"ness of the rings from Suiter's pictures. Is there any other way to be sure?

I will try to get a picture next time when I am up at the store. For now, thanks a lot everyone for the incredible help!

Cheers,
Kumar

 

[jring]

Hi,

in general, while Suiter is a very good book on the procedure of star testing, the examples in there are just drawings. The examples on Vlad's page were calculated with aberrator, a free software for simulating diffraction patterns with different aberrations. I would trust that more - you could get it and make some more images for your aperture and focal ratio.

There is also libraries of real star test images with discussion to get a hang of what things look like...

http://aberrator.astronomy.net/scopetest/html/refractor125_1.html
http://interferometrie.blogspot.com/

But in the end of the day, if you want to quantify small amounts of SA with a star test beyond saying it's pretty good, you'll need a 1/3 D central obstruction and a way of measuring the amount of defocus (or at least have a way of getting an equal amount of defocus both sides of best focus - does the ATS have any marking on the focus wheel?).
Or of course use other methods like Ronchi or Foucault testing - but these are less practical with spotting scopes as they bring obstructions to the focal plane.

Finally, measuring such tiny amounts of SA is more of an academic exercise for a spotting scope as with the low magnifications of 60x it is going to be used, the difference of 1/8 or 1/10 wave is not going to be visible. Even with the usual 1.6x extender at 96x or so that is doubtful. If the 65mm ATS was an astro refractor one would use it routinely up to 130x - some specialist observers would go higher for planetary or much higher for double stars.

Joachim

 

[mskb]

Hello @Joachim,

Those examples on the aberrator website were incredible1 I have been dying to get a hold of something like these for a while. Thank you very very much!

I have a small question. In ( http://aberrator.astronomy.net/scopetest/html/refractor100_1.html ), I find the rings outside of focus to be not as well defined as those inside of focus. The rings outside focus also have the "dim"ness to them compared to those inside focus. Yet it receives high ratings from the author! Similarly from ( http://aberrator.astronomy.net/scopetest/html/refractor125_1.html ), ignoring the cut-out effect, the "outer" outside focus ring appears more brighter than that inside focus especially at fewer rings of defocus. The rings outside of focus are also less well defined than those inside. Again, the author gives it high ratings.

I am sure I am missing something - could you elaborate?

If I understand any insights you might offer from above examples, I might be bold and say the Kowa 66 examples were better than 1/5 wave (based on: http://aberrator.astronomy.net/scopetest/html/refractor90_1.html ). Certainly better than 1/2 wave from here (http://aberrator.astronomy.net/scopetest/html/refractor100_2.html). But without understanding those images from high quality instruments it is difficult for me to extrapolate.

Also Joachim, are there other websites like this to grab more example images? I will also be downloading the aberrator to try things out. So useful! Thank you!

Sincerely,
Kumar

 

[jring]

I have a small question. In ( http://aberrator.astronomy.net/scopetest/html/refractor100_1.html ), I find the rings outside of focus to be not as well defined as those inside of focus. The rings outside focus also have the "dim"ness to them compared to those inside focus. Yet it receives high ratings from the author! Similarly from ( http://aberrator.astronomy.net/scopetest/html/refractor125_1.html ), ignoring the cut-out effect, the "outer" outside focus ring appears more brighter than that inside focus especially at fewer rings of defocus. The rings outside of focus are also less well defined than those inside. Again, the author gives it high ratings.

I am sure I am missing something - could you elaborate?

Hi,

the first example you mentioned also has an interferometric test result with a Strehl value of 0.993 (in green) given. The theoretical maximum for the Strehl value is 1.0 btw - but real objective lens designs are unable to reach this even if they were made perfectly.

I'd be very happy with either of the two examples you questioned. I don't have any optics at that level.

If the Swaro ATS 65 you saw really is equal or better than those, go - no - run to the store and get exactly that example.

It should also be said that for both examples the magnification chosen is quite high. The usual rule of thumb is to use an eyepiece with a focal length in mm roughly equal to the objective focal ratio. So here an 8mm EP would have been optimal, 6mm results into more magnification and thus a slightly more sensitive test.

Joachim

 

[mskb]

Thanks @Joachim for the clarifications.

- Kumar
PS: Some interesting observations are coming. I will update the thread next week with those once I am sure.

 

[mskb]

Everyone, An update after some time. Further checks on the scope copies we discussed above (two K663s, and one ATS65) makes the final decision a whole lot tougher. I would really love your helpful thoughts/insights!

(1) Although the focus snap works as: ATS65 >= store's K663 > my K663, doing resolution checks with a line chart shows that the ranking is reversed, quite a bit: my K663 >> store's K663 >> ATS65: (A) ATS lost a whole pair of blocks (vertical + horizontal) to my K663. Furthermore, the ATS was quite liberal with its CA even on the center of the view. The effect was less noticeable on wild subjects other than a line chart. (B) The store's K663 resolved the vertical half of the final block resolved by my K663. Some star testing also indicates this could be due to the added astigmatism in the store's K663. (C) Furthermore, my K663 beat two other K663 copies I had a chance to check along the way.

(2) Star testing all these scopes indicate that my K663 has quite circular and well defined rings. One side of focus, the rings are purply and fainter yet still countable. There is a slight added intensity in the rings off-center on one side, which makes me think it could be coma potentially (an inexperienced star-tester here). But the store's K663 also shares the same issue with significant notes of astigmatism.

(3) These results might indicate that we might potentially be dealing with a close to cherry K663. However, what keeps me from settling with my current K663 is that I feel like there is something else going on with either the objective / the EP. It just isn't as easy to look through as the ATS / store's Kowa, and perhaps this is why I am ranking the focus snaps the way I did. It is almost like its hard to keep the image stable by the scope being too sensitive to eye positioning. So far I have only observed the following:
(a) I think there is a small reduction in ER with my current K663, but I am not sure if that is the only reason for the discomfort.
(b) In general, when one moves their eye away from the eye piece, there is a central bright circular spot in the view that shows up across all scopes. This white region is "stronger" or "more spikier" in my K663 than others.

That's about all that I have after going back and forth with these scopes. Would you recommend I simply go for the store's K663 despite the drop in resolution? Or are there any further checks I can do to make sure what's going on? I am super super confused.

Any help is greatly appreciated! Thanks all!

 

[jring]

Hi,

I would strongly recommend to keep the scope you have if the star test is that good. What you describe seems to be an EP problem so try to address this. Is there any chance to take your body to the store and test it with their EP?

PS: not quite sure what non-uniform brightness of diffraction rings means... coma is usally non-concentric rings...

Joachim

 

[mskb]

That's extremely helpful Joachim! I have access to the store's scope as well, and can interchange EPs and check. Are there any particular checks you might recommend?

For the coma, I have been very unsuccessful capturing relevant images of these star tests. I will give it a go this week as well.

 

[jring]

That's extremely helpful Joachim! I have access to the store's scope as well, and can interchange EPs and check. Are there any particular checks you might recommend?

For the coma, I have been very unsuccessful capturing relevant images of these star tests. I will give it a go this week as well.

Hi,

basically you want to check if you can get the easier eye positioning of the 663 example from the store with your scope by switching over the EP - ideally without a worse result in the star test (although that is not too probable).

Joachim