Confused about diffraction rings.

[Doc]

I was reading last night about collimation, star testing and diffraction rings.

And am a little confused.

When I defocus my eyepiece I get an image of my secondry mirror. I have never seen diffraction rings, maybe my seeing is not good enough. Have I adjusted it out of focus to much?

I might be stupid here but if I place a crosshair into lets say a 8mm eyepiece and then defocus until I see the image of my secondry and then move the collimation screws on the primary mirror to move the centre of the secondry image onto the centre of the crosshairs wouldn't this have the same effect as a star test and reach perfect collimation.

[themos]

I think that would be a Cheshire, effectively.

I can say that in the year I've owned the 12" Dob, I've only ever seen a good diffraction pattern once. The seeing must be very good (for UK standards) and the thermals in your tube must have subsided. You then need to defocus slightly so that a brightish star goes to 2x-3x the focused width. I was using 450x power.

[Astronut]

Don't defocus too much. The diffraction rings are quite small which is the reason it's recommended to use as high a power as possible. I've only ever seen half-decent diffraction rings once and that was only at mid-power and the outer ring was boiling due to thermals and average seeing conditions - too small to check collimation.

re. defocusing too much so you can see the illuminated primary surface and seondary obstruction and using that to collimate with a crosshair eyepiece - I don't think this would work. In a fast 'scope like your 16" Lightbridge the secondary is noticably offset from centre when it's collimated.

[themos]

I captured the Airy disk of Polaris with a barlowed webcam some time ago. Maybe this will give you an idea of what to look for.

http://ukastronomers.com/ext/video_player.swf?config={videoFile: 'http://ukastronomers.com/themos/video/31.flv', splashImageFile: 'http://ukastronomers.com/images/video_player.jpg', autoBuffering: false, initialScale: 'scale', autoPlay: false, }

[Doc]

themos link doesn't seem to work.

[themos]

it should be better now.

[Psychobilly]

yep works now

[lunator]

Diffraction rings can be very ellusive due to UK skies.

I can usually see them in my 5" refractor but they were a much rarer sight in the 8" newt

Cheers

Ian

[Doc]

Thanks themos at least I'll know one, if I ever see one

[ngc2403]

during my summer project last year i did the collimation of all the university telescopes and only the small C5's showed good rings, they were collimated in a optics lab (a room under the ground with windows and no thermals) were the following picture was taken. it is the stacked image of a large avi

[Milamber]

"... telescopes cannot form a point source image of a point object due to a process called diffraction. Perfect optics actually create a tiny circular disk-like image surrounded by fainter concentric circles, known as the Airey Disk."

"What you get in a [real-world] telescope is a small but measureable primary image called the "Airey disk" surrounded by symmetrical diffraction rings. Central obstructions (even very small ones) have the effect of smearing light from the Airey disk into the diffraction rings, causing a loss of contrast. The effect is worsened when there are vanes holding the secondary mirror, since these cause diffraction spikes, further distorting the star's image."

I think there is beginning to be some confusion here between rings and spikes?

Arthur

[ngc2403]

(click to enlarge)

[Reggie]

I leave my 6" SCT focused for my guidecam, so when I'm aligning my 13mm EP is always defocused, I always get diffraction rings.

If seeing is your issue then take a look at metaguide, it smooths out seeing using a webcam to give you a better look at collimation in less than favourable conditions.

[Doc]

I'm beginning to unserstand this now.

Thanks for alll your replies it really helps.

Metaguide I'll google this to see what it is....