Help with star-testing scope (1 Viewer)

[tenex]

I'm no expert but have tried several times now to star-test my new Meopta S2, using a 5mm NAV eyepiece for 88x. I just don't get the sort of clear pattern I see illustrated for example here. It looks nicely round, but no distinct rings, even when only slightly defocused, just a fuzzy blob, a bit sparkly or grainy, with possibly a brighter outer rim on one side of focus, not so much the other. What could be going on:

(a) bad seeing? (on different evenings, various stars at moderate to high altitude, and this is only 88x)

(b) shipping damage somehow? (it does seem to have been dropped in transit, though the outer box wasn't damaged)

(c) if that doesn't seem likely, just a poor sample of the scope? (it looks great in daytime use, even at 88x)

 

[henry link]

I would suggest setting up an artificial star that will allow more control over air turbulence. My backyard set-up consists of a small shiny black glass Christmas tree ornament, but any small shiny sphere that will return a tiny glitter point of the sun will do. I have it placed about 2 meters above a grassy lawn and about 40 meters from the the telescope, which is on my deck about 3 meters above the ground. The higher off the ground the better for the sphere and the scope and grass is much better than pavement for the surface between the two. Morning is better for testing when the air is about the same temperature as the ground.

If you still don't see any diffraction rings try making a small stopdown mask to place in front of the objective lens. Most aberrations should be well corrected once the lens is stopped down to 30mm or less. Use a magnification that gives about the same size exit pupil you're using now, about 30-35x for a 30mm aperture. That should allow you to see what a very good star test looks like in your set-up.

Henry

 

[tenex]

I don't have a suitable sphere, and we have increasingly cloudy weather with snow ahead. I did just try a foil pinhole over a flashlight, at the longest distance available across the yard (~80 ft), and the result was the same. Given such varied circumstances and identical results, I'm starting to wonder about my vision, and in any case giving up. Stopping down the scope wouldn't provide the desired evaluation of it, and if it's conditions here that are so consistently bad, optical perfection becomes irrelevant. My curiosity wanes when I'm not getting anywhere, and the scope does appear to work well for our purposes. (This eyepiece at 88x is great for ID'ing birds so far out on a reservoir complex that we can barely see them.) Thanks for your advice though.

 

[jring]

Hi,

your vision should not really matter with a star test. If the scope works well for you at 88x the SA you see cannot be too bad...

Joachim

 

[tenex]

Thanks Joachim, that's what I think too. I simply didn't expect to see so much granularity obscuring the "pattern" no matter what I do. But ultimately it's not important, this just sounded like an interesting test to try. What would one notice in optics with unacceptably high SA, inability to get sharp focus?

 

[jring]

Hi,

too much SA (or basically any mix of aberrations) will make it difficult or impossible to reach good focus at 60x or even lower. Difficult means that you don't really have a clearly defined point of best focus but rather a fairly wide range of least blurriness.

Having a crisp image at 88x means that your spotting scope is probably above average.

Joachim

 

[henry link]

Hi Tenex,

I'm perplexed by your difficulties with star-testing. I've never heard of a telescope that doesn't display diffraction rings on one or the other side of focus. Even with high spherical aberration there should be strongly defined rings on one side of focus contrasting with no rings at all on the other side.

I can only think of two possibilities for your experiences. One is that your pinhole is too large to form a point source at 80', but that seems unlikely if it looks like a small pinhole when viewed up close with the naked eye. The other possibility is that when you defocus you are too quickly skipping over the the area with 5 or fewer rings. The diffraction disc formed by 5 rings will be quite small at the exit pupil you're using. Some people trying this for the first time tend to rack the focus so far out that the rings disappear.

One reason I suggested the stopdown was as a way of making the the rings obvious, so you could see what they're supposed to look like. Provided your light source is bright enough they should be very obvious with your scope stopped down to 40mm or so and viewed at 88x. It's a shame to get this close to star-testing, which is so informative, and then abandon the effort.

Henry

 

[Volvonium]

star testing can be a challenging read and it requires good, steady skies to make a definitive assessment. Even then, one side of focus should show some rings and some of the more obvious aberrations. Bright outer ring on one side sounds like turned edge, but shouldn't affect things much at typical terrestrial observing magnifications.

I've read that prisms can induce spherical aberration, but i would imagine well considered field scopes have their objectives figured to address this, resulting in a well corrected overall system.

I agree that if you can reach sharp focus at 88x, the optics are fine.

 

[kabsetz]

One vision-related possibility for difficulties in seeing diffraction rings in a high-magnification star test is cataracts. I had to swap from using my dominant left eye for scope viewing to my right a few years ago. What prompted it was noticing that narrow exit pupil star tests on my reference scope were becoming worse, more granular and with distinctly fuzzier rings. I first thought that something had happened to the scope, but with my right eye everything was as it should again.

I have it relatively localised, so it does not show up on diffraction rings at large exit pupils or with naked eyes (I am farsighted, so I see diffraction ring patterns whenever I look at point sources without glasses). Since training my right eye to become dominant succeeded, I have not had surgery for the left eye yet, but may well do it eventually.

Kimmo

 

[tenex]

I'm not aware of cataracts (only some floaters now), and have done this with both eyes and seen no difference. I just gave the test one last try, in the morning, with as tiny a pinhole as I could make, and do think that helped a bit. I can at least see a difference now: there are distinct rings on the far side of focus, not so clearly on the near. The "bright outer ring" is only somewhat brighter than others, and the only discernable(?) feature left in the near side pattern. I think this is a quite acceptable result, together with the nice daytime view the S2 has already given at high magnification.

The diffraction pattern is indeed quite small, and still suffers from the same strange sparkly or grainy effect as before, which was also seen by Juhani in his Televid 82 in this previous post:

...As I wrote earlier, the inside focus is totally different compared to the outside focus; there cant be found no clear circles and the pattern "sparkles" in an odd way. It's hard to even see the shape of the pattern... The outside focus shows bright outer ring and dimmer inner circles. Judging by that and as Joachim and Henry concluded, that is evidence of undercorrection of spherical aberration....

He didn't mention "sparkling" in the Zeiss also tested in that thread, or two Kowas in another, nor has anyone commented on it here. Can this effect (which complicates the test) somehow be a product of prism design in certain scopes (Leica, Meopta) but not others?

 

[GeorgeL]

I have mild cataract in both eyes and due for surgery next month and I know they’ve been affecting my seeing through my scopes, especially telescopes. I’ve often asked my pre-teen grandkids to take peeks through my astro scopes and they clearly see more detail, when looking at planets, for instance.

 

[henry link]

I'm not aware of cataracts (only some floaters now), and have done this with both eyes and seen no difference. I just gave the test one last try, in the morning, with as tiny a pinhole as I could make, and do think that helped a bit. I can at least see a difference now: there are distinct rings on the far side of focus, not so clearly on the near. The "bright outer ring" is only somewhat brighter than others, and the only discernable(?) feature left in the near side pattern. I think this is a quite acceptable result, together with the nice daytime view the S2 has already given at high magnification.

The diffraction pattern is indeed quite small, and still suffers from the same strange sparkly or grainy effect as before, which was also seen by Juhani in his Televid 82 in this previous post:

He didn't mention "sparkling" in the Zeiss also tested in that thread, or two Kowas in another, nor has anyone commented on it here. Can this effect (which complicates the test) somehow be a product of prism design in certain scopes (Leica, Meopta) but not others?

If the sparkling is in motion it is almost certainly air turbulence, either inside the scope tube or in the air between the scope and the pinhole, most likely the later. I can have a problem with that in my outdoor set-up when the air is either much warmer or much cooler than the ground. I have a 10 meter indoor set-up that eliminates that problem, but most scopes have more spherical aberration at 10 meters than they do at long distances, some much more, so that close set-up is more accurate for other aberrations.

 

[jring]

Hi,

nice that a result has been achieved - so we have a bit of overcorrection and maybe also a little edge problem. But nothing major and the views are great.

PS: I agree with henry that the sparkle (or basically anything change of the diffraction patterns) has to be some kind of air movement.

Enjoy your scope!

Joachim

 

[BoldenEagle]

The diffraction pattern is indeed quite small, and still suffers from the same strange sparkly or grainy effect as before, which was also seen by Juhani in his Televid 82 in this previous post:

He didn't mention "sparkling" in the Zeiss also tested in that thread, or two Kowas in another, nor has anyone commented on it here. Can this effect (which complicates the test) somehow be a product of prism design in certain scopes (Leica, Meopta) but not others?

I see this "sparkling" more on the outside pattern than on inside at least with Kowa and Zeiss and as far as I can remember, also Leica. So, if it is seen only on the other side (outside) of the sharp point with all of these scopes, it would be strange if it would be produced by the prism, added with the fact that at least Zeiss didn't show any obvious prism defects (Kowa certainly did, can't recall Leica's performance on that). I'm not expert on these things so those who are, can comment on that.

It may be easiest to just see the star test pictures I took on the Kowa and Zeiss threads so one could see what I mean by that "sparkling", which is more evident on the outside pattern, as the inside pattern has more distinct and less spiky rings, especially the outermost ring as neither of these scopes showed very clear inner rings on the outside pattern due to some SA...

As for the SA in these 3 scopes, Leica clearly was the worst one as it's outside pattern was just one "sparkling" round shape without any obvious rings, there was not even visible outer ring to be detected. It's view was ok at 50x max. mag. I think but when using higher mags. with the 1.8x extender, the view became too blurry say beyond 70x to see anymore details.

My Kowa 883 sample does a lot better, as with the extender view stays reasonably sharp up to 96x max. mag. allthough optimal resolution may appear little before that. Zeiss is even better but I have no eyepiece to reach more than 75x with it. I still haven't done any specific resolution tests with these scopes, just compared them carefully side by side.

But as Joachim already wrote, if you can have sharp view @88x mag., I also wouldn't be too worried about SA...

Regards,

Juhani