Star Test 12" Dob

[Scooter]

Hi Folks,

OK so got the new GSO 12" DOB and been messing around as per my first light thread but I have an issue with star tests.

Now from what I have read doing a star test should give me an indicative idea of the quality of the optics and how good the collimation is (everything I read says the is the definitive test). So do my research on star tests and do some last night and what I get is nothing like what I should get.

Basically I picked a few bright stars (I now read should be mag 1 or 2 stars so will try again tonight on recommended targets) and did them at around 200x (now read should be on 600x so again will try again) and all I get is the back hole with a wide light band around. The large light band around gets more solid or fuzzy with the ammount of off focus and I can vaguely see the spider which gives it a crosshair like appearance.

Now everything I see on the net shows concentric rings so I am worried as mine looks nothing like that. Mine is as per attached pic and its just the single band that increases or decreases in size and fuzzyness with off focus.

Whats the go?

[davidpretorius]

concentric rings appear for me in my 5mm vixen eyepiece when i am just out of focus......... if you go tooo far, then you get the "cd rom" look as you have described.

now depending on seeing conditions, then these sharply defined rings can be blurred quite significantly.

Also i have found the quality of the eyepiece a big factor....the better the eyepiece ie vixens and even better the TV radians, then the better these rings.

http://legault.club.fr/collim.html

this is a good read.

[Scooter]

Thnaks for that, was a good read

What's with the comment "Anyway, if no Airy pattern can be discerned, no high resolution result can be expected (except in big telescopes for whom the Airy pattern is rarely or even never visible)." What are they classing as a big telescope?

Anyway...will check it out better tonight through the scope

[davidpretorius]

refractors tend to show the airy disc better apparently

i have only once seen the airy disc and that was when my mirror was perfectly cooled and the seeing was 8 or 9/10.

what that meant was that i could get my camera to image at around 1600x or more.

so what they are saying is correct, having a perfectly cooled mirror, spot on collimation and fantastic seeing mean you can really bump up the magnification and image........

[Rigel003 (Graeme)]

Legault talks about a 2 step process. Rough collimation can be done at a lower power and patterns similar to that one will help you see that the mirrors are collimated. You do this just inside or just outside focus, but closer to the focus point than you've shown.

The next step is to examine a star's airy disk and diffraction rings at the exact focus point. This requires lots of magnification and excellent seeing. The larger the telescope, the smaller the airy disk, as larger diameter mirrors have increased resolution (they can discern finer detail). It's easy to see the airy disk and diffraction ring in a 4" refractor at 250x. A 12" reflector would require 600x or more and this an unrealistic magnification, almost never supported by the seeing conditions. Uneven cooling and tube currents etc. also complicate the matter with large scopes.

[Scooter]

Thanks Rigel and David

I've had a few goes at star tests and to be honest with the 12" dob I am stuffed if I can track at high mag so it was painfull (and the seeing was probably bad the nights I did it anyway).

I can now get the rings at high mag but as I said just can't keep the star in view for more than a few seconds. Can also see the rings at lower mags so that is good (although it takes my eyes some adjustments to make it go from blurry to clearly defined rings - is that normal?).

They (the rings) seem pretty good but the outside ring has been blazing like a sun so I am assuming that indicates bad seeing?

Will not worry about the airy ring at all me thinks

Appreciate all the help

[goober (Doug)]

I remember reading an article somewhere on an alternate collimation technique for less than perfect seeing. Of course, I can't find it now. The gist of it was you strongly defocus the star, and collimate based on the position of the secondary mirror shadow within the out of focus star.