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No Airy Rings?


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When I do a star test, I always get the image of my secondary mirror's spider vanes collapsing to a star. As long as it collapses in symmetry I am happy. No matter how much I back off of the focuser, I just can't seem to get the concentric airy ring effect. I have an Astromaster 114EQ and have used my 10mm and 6mm eyepiece to try to get the effect, but still only get the spider shadow. Am I doing something wrong? Thanks.

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Your scope has a barlow built in and this may affect results. I cant say for sure never having tried to star test.

When you try for airey rings you have to back the focuser off a fraction to defocs. Its a very small amount. Just a whisker really.

It may be that the design of your scope, which is. Alled a Bird Jones may have an effect bu I dont know.

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No, it's not the design of the scope, you're just defocusing too much and you're expecting the wrong thing and the seeing is getting in the way (as Steve points out). Point the scope at Polaris (so the star doesn't move) and use your highest power eyepiece. The 6mm will yield 167x which is almost 40x per inch. Plenty for what you need. Focus on the star. Look at it and note that you don't see a point but an extended disk. Exactly what you see will depend on your aperture and on the seeing quality, but it will look something like what Steve's link illustrates. The more stable it looks the better. Unless it looks at least like about Pickering 5 or better you won't be able to star test.

Note that as well as looking like an extended disk, there's a ring around the star. That's the first ring of the Airy disk: http://www.iue.tuwie...ofer/img597.png The second one is faint and you're unlikely to see it. When you use a lower power the Airy disk is smaller and you don't see the above effect. This is because the Air disk has a fixed angular size and the more you magnify, the larger it looks. The Airy disk defines your resolution limit: http://www.telescope.../resolution.PNG and is smaller in a larger telescope. That is why a larger telescope provides better resolution. So the Airy disk is what you see when you're at focus.

Ok, so that's the Airy disk. When you defocus slightly (as astrobaby points out) you are viewing the Airy disk at different focal depths: you're taking cross-sections through it at different "depths": http://en.wikipedia....ation-slice.jpg (http://en.wikipedia.org/wiki/Airy_disk). So what you you saw at focus is a cross section taken at the centre of that cone. What you see away from focus are cross sections with increasing numbers of rings. Each time you turn the focus knob a little you add a ring (you can see why that's the case by looking at the image in the previous link). The cross sections you see will resemble these: http://www.telescope.../star_test2.PNG So these patterns are what the Airy disk looks like when it's out of focus. They will look small and turbulent. If you can see the shadow of the secondary mirror you've gone too far.

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No, it's not the design of the scope, you're just defocusing too much and you're expecting the wrong thing and the seeing is getting in the way (as Steve points out). Point the scope at Polaris (so the star doesn't move) and use your highest power eyepiece. The 6mm will yield 167x which is almost 40x per inch. Plenty for what you need. Focus on the star. Look at it and note that you don't see a point but an extended disk. Exactly what you see will depend on your aperture and on the seeing quality, but it will look something like what Steve's link illustrates. The more stable it looks the better. Unless it looks at least like about Pickering 5 or better you won't be able to star test.

Note that as well as looking like an extended disk, there's a ring around the star. That's the first ring of the Airy disk: http://www.iue.tuwie...ofer/img597.png The second one is faint and you're unlikely to see it. When you use a lower power the Airy disk is smaller and you don't see the above effect. This is because the Air disk has a fixed angular size and the more you magnify, the larger it looks. The Airy disk defines your resolution limit: http://www.telescope.../resolution.PNG and is smaller in a larger telescope. That is why a larger telescope provides better resolution. So the Airy disk is what you see when you're at focus.

Ok, so that's the Airy disk. When you defocus slightly (as astrobaby points out) you are viewing the Airy disk at different focal depths: you're taking cross-sections through it at different "depths": http://en.wikipedia....ation-slice.jpg (http://en.wikipedia.org/wiki/Airy_disk). So what you you saw at focus is a cross section taken at the centre of that cone. What you see away from focus are cross sections with increasing numbers of rings. Each time you turn the focus knob a little you add a ring (you can see why that's the case by looking at the image in the previous link). The cross sections you see will resemble these: http://www.telescope.../star_test2.PNG So these patterns are what the Airy disk looks like when it's out of focus. They will look small and turbulent. If you can see the shadow of the secondary mirror you've gone too far.

Great indepth explanation as usual. I'll try again tonight. Thanks again.

Ron

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No, it's not the design of the scope, you're just defocusing too much and you're expecting the wrong thing and the seeing is getting in the way (as Steve points out). Point the scope at Polaris (so the star doesn't move) and use your highest power eyepiece. The 6mm will yield 167x which is almost 40x per inch. Plenty for what you need. Focus on the star. Look at it and note that you don't see a point but an extended disk. Exactly what you see will depend on your aperture and on the seeing quality, but it will look something like what Steve's link illustrates. The more stable it looks the better. Unless it looks at least like about Pickering 5 or better you won't be able to star test.

Note that as well as looking like an extended disk, there's a ring around the star. That's the first ring of the Airy disk: http://www.iue.tuwie...ofer/img597.png The second one is faint and you're unlikely to see it. When you use a lower power the Airy disk is smaller and you don't see the above effect. This is because the Air disk has a fixed angular size and the more you magnify, the larger it looks. The Airy disk defines your resolution limit: http://www.telescope.../resolution.PNG and is smaller in a larger telescope. That is why a larger telescope provides better resolution. So the Airy disk is what you see when you're at focus.

Ok, so that's the Airy disk. When you defocus slightly (as astrobaby points out) you are viewing the Airy disk at different focal depths: you're taking cross-sections through it at different "depths": http://en.wikipedia....ation-slice.jpg (http://en.wikipedia.org/wiki/Airy_disk). So what you you saw at focus is a cross section taken at the centre of that cone. What you see away from focus are cross sections with increasing numbers of rings. Each time you turn the focus knob a little you add a ring (you can see why that's the case by looking at the image in the previous link). The cross sections you see will resemble these: http://www.telescope.../star_test2.PNG So these patterns are what the Airy disk looks like when it's out of focus. They will look small and turbulent. If you can see the shadow of the secondary mirror you've gone too far.

At the risk of thread hijack (sorry but found the above post very interesting).

Looking at those images labelled 8 in the final link, some display a flattened section. I'm getting something similar in my 8" SkyWatcher DOB when doing the star test. For me, the flattened part of the circle is not as wide and extends approx across the 4-5 o'clock positions.

That flattening in the final link (numbered 8), what is that describing?

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Looking at those images labelled 8 in the final link, some display a flattened section. I'm getting something similar in my 8" SkyWatcher DOB when doing the star test. For me, the flattened part of the circle is not as wide and extends approx across the 4-5 o'clock positions.

That flattening in the final link (numbered 8), what is that describing?

The image comes from this page: http://www.telescope-optics.net/diffraction_pattern_and_aberrations.html Image 8 represents a form of tube current.

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You can tell if it is actually a tube current by moving your telescope gently up and down. If the image moves on the same axis as the flattening then it will indeed be a tube current. They can take a surprisingly long time to disappear, particularly on a clear night after a warm day. Without a fan, on some nights, your mirror may never reach equilibrium with the falling air temperature.

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That's an interesting point because I was about to say that I get the same flat spot, in the same position, summer or winter, out for 10 mins or out for three hours. The scope is kept in the garage recently, too.

So I will give your suggestion a try. Thank you.

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Interesting and educational reading this stuff :), for some reason that last link Umadog gave doesn't work for me, in any case the link to the site now in my bookmarks is http://www.telescope-optics.net/ and also a good related one http://www.handprint.com/ASTRO/ae1.html

I am always interested in the theory of such things. All it is showing at the moment how rusty my knowledge of optics is :) apart from the basic stuff I did Uni many years ago but I never used in practice, so it is all but forgotten. Astronomy will give me a good reason to get into it.

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  • 3 weeks later...

My MAK150 produces fairly textbook rings. Neither my ST102 or TS 8"/F4 do. If "blobby" ring images are anything to go by, the former at least, shows "surface roughness"? Heheh. "You pays yer money", I sense? I am consoled that both scopes are reasonably usable in practice... :)

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Good Stuff - Printed for posterity. Kind of what I had expected. I once heard dark muttering about a well known (respected) SCT only being "1/4 wave". Though, in the past, this limit was considered something of an achievement. As a consolation this may suggest collimation is a bit more lenient? That said, worthwhile doing one's best. I notice my budget astrograph (albeit laser checked) shows a distinctly assymetry in the startest... which I believe I know how to fix! (With a ruler?) As ever, a question of relative contributions to an overall? :)

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Good Stuff - Printed for posterity. Kind of what I had expected. I once heard dark muttering about a well known (respected) SCT only being "1/4 wave". Though, in the past, this limit was considered something of an achievement. As a consolation this may suggest collimation is a bit more lenient

Why? Errors in the optical train add up. Even if a scope doesn't have great optics, it will always perform better if it's collimated well.

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The total "error" on a complete setup is some sort of root of squared errors summed? But if one error dominates, it is harder to see the effect of lesser ones. With a budget scope, one would work on improving the greatest deficiency first? Lack of finish in (adjustment) mechanics, seems a frequent candidate? Not trying to make a rigourous point though... :)

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Since most of the light focused by your mirror ends up in small disc comprising 80 odd percent of it, the remaining light is distributed in very much fainter rings surrounding

that focussed spot. The larger the mirrors diameter is, the smaller that spot is, so your small objective I'm afraid will probably need optical bench conditions to see them

You could buy or obtain an artificial star to do your testing, but you need your mirror a long way off to do the test, and out of moving air, a steady temperature, thermal equilibrium, and all the other perfect conditions that need to be applied.

There is a tendency for a few scope owners to go a bit overboard when it comes to analysing their telescopes performance. The most practical way for you to satisfy yourself, is to use the instrument on objects that will give you an Idea of it's theoretical capability, but again, there are so many variables to affect the result you get, and you may feel disappointment. You should treat the exercise as a long term one, to lessen the chance of you dismissing a perfectly good telescope, because the conditions are not going to favour the perfect result you are looking for.

I don't want to seem as though I am trying to throw doubt your way, but I think you should just keep the scope collimated, and enjoy it for what great sights t can deliver to your eye. Please don't let it become an obsession to find a perfect diffraction pattern. I think you will be frustrated, and unnecessarily so.

Ron.

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The total "error" on a complete setup is some sort of root of squared errors summed? But if one error dominates, it is harder to see the effect of lesser ones. With a budget scope, one would work on improving the greatest deficiency first? Lack of finish in (adjustment) mechanics, seems a frequent candidate? Not trying to make a rigourous point though... :)

I see what you're saying, and I agree totally that the mechanics are really critical. But it's always worth optimising collimation regardless of optical quality. The problem is that optical "quality" is a vague term. I think the complication comes from what effect the error in the optics has. Errors are quantified as a function of spatial frequency using a modulation transfer function--MTF (vhttp://www.telescope-optics.net/mtf.htm). So the MTF tells you how much of each spatial frequency is transmitted. The errors add multiplicatively in MTF space according to this equation:

final MTF = perfect MTF * (MTF_error1/perfect MTF) * (MTF_error2/perfect MTF) ....

This holds if the errors are independent. A misaligned Newtonian produces the largest transfer drop at lower spatial frequencies. Say the error in your optics is mainly at higher frequencies on the MTF graph...

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The larger the mirrors diameter is, the smaller that spot is, so your small objective I'm afraid will probably need optical bench conditions to see them

He won't need bench conditions, just moderately good seeing and a high power objective. Because the OP has a smaller objective he will more easily be able to see the Airy disk because it's larger. In a larger scope the disk tends to be broken down by poor seeing and so stopping down the aperture is the only way to see the disk under most circumstances.

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