Airy discs

[thing]

Have just collimated my scope after a mirror wash which went OK, but when I defocus the star image I get a nice circular disc but it looks like a small chunk has been bitten out at one edge, also defocusing the other way shows a little 'flare on the disc like a solar prominence. Anybody any idea what this is and how to fix it?

[barkis]

Sounds to me like you have a fairly bad misalignment of your optics. Seems likely your secondary mirror is askew for some reason, or you have somethging intruding into the lightpath. Did you remove the mirror to wash it? Also, sometimes the focuser whenracked right in, can impinge into the cone of light, and show up as a shadow on the mirror. I would do a careful check of both mirrors before you press any panic button.

Ron.

[thing]

I have just done a wash (see DIY section) but collimated secondary and primary before taking a quick look through the clouds. I use a Baader Laser collimator so I'm assuming it's fairly accurate. I also marked the mirror (well....not literally ) before I took it to bits and reassembled it the same orientation that it came out. I'll check it again tomorrow but I'm pretty sure it's spot on.

[Andrew*]

sounds like the focuser tube getting in the way. When viewing the airy disk, rack the focuser out and see if the "chunk" disappears.

[thing]

I see what you're saying but I've never seen it before when checking collimation, thanks for the ideas though.

[Astroman]

Your use of the term "airy disk" is incorrect. The airy disk of a star has to do with the limitations of telescopes resolving a point source. A telescope of given size cannot resolve a point source due to the interaction of light with the aperture, or edges of the optical element. (Mirror or lens) Instead, the aperture creates interference fringes. A "perfect" focused image shows, under perfect viewing conditions, a central bright region, the airy disk, with a dark ring around that, surrounded by another bright diffraction ring. The quality of this disk is determined by the quality of your optics measured as Strehl Ratio, P-V distortion etc., and the steadiness of the seeing.

The image you call "airy disk" is simply the defocused image of the scondary mirror. Defocusing sets up diffraction rings on your image, alternating dark circle, bright annulus, light annulus and so on, depending on the depth of defocus. Alignments and star tests are analyzed using the considerable information present in this defocused image. It sounds to me like your alignment is off somehow. You want to make sure your entire optical train is aligned, as well as just your primary and secondary. If your secondary is aligned with the primary but not the optical axis of the focuser, it could cause what you see. (This was mentioned by AstroPhethean).

HTH.

[thing]

Thanks for that, most of it went over my head but thanks anyway! How do i align the optical train?

Having just thought about it, what if I reassemble the primary with all of the adjustment screws 'zero' set, ie wound out and then start from there? If the mirror is flat against the adjusters and all of the adjusters are at the same level would that be a good place to start?

[Astroman]

Yeah, it's tough describing what the airy disk is. At least your head didn't implode, as most do when I explain it. :shock: :?

Collimation should have been close, if you marked your initial position. But anytime you move the main mirror, it's important to do a complete alignment routine. Which scope do you have, or did I miss it? (Sorry).

[thing]

Celestron C8N-GT, I have a Baader Laser collimator.

[barkis]

I don't know if this video will help you regarding the type of scope you have.

Ron.

http://www.andysshotglass.com/Collimating.html

[thing]

Thanks Barkis, great vid. I'll check the centralisation of the secondary in the focuser, it's the only thing I can think of that might be off. The rest of the collimation is as per the video.