SCT Collimation with an Artificial Star

Somewhat new to the hobby. Definately new to collimation in general. I have two dobs (8" and 10") both odysseys and I use a catseye collimator on them. It works really well. I also have a Celestron SPC-8. I have been dabbling in astrophotography and think that it needs to be collimated. To my knowledge it has never been collimated, and I "liberated" it from my dad who wasn't using it and who was the original owner.

Living in the seattle area (under the flightpath of the airport no less) my seeing for lack of a better word sucks. So I built an artificial star using a 62.5micron Fiber Optic network cable.

Now I kind of understand how the Dawes limit works but I am having trouble applying it to using an artificial star.

I get the feeling that about 50 meters would be adequate but I don't think this is quite right either. Can someone tell me how far away I should place the artificial star? How much does it really matter?

Thanks in advance.


Addendum - heres a pic after I made some very very minor adjustments.

Attached Image:

The final adjustments for collimation are made on and IN FOCUS image.

The artificial star must not be larger than the Rayleigh Limit, or the Airy disk size for your scope. You have an 8" scope. The radius of the Airy disk for your scope is 5.45/8 = 0.68 arcseconds. For a very bright light, the central visible disk within the Airy disk will be larger than that.

You would want the test point source light to form an angular image smaller than the Airy disk. I think your light source is too large, or you are too close.

However, there are some parameters to take into consiideration based on the f# of the scope. there is an appendix section in the back of Suiter's book "Star Testing Astronomical Telescopes" that gives all the parameters.

edz

edz

Zach, for an 8" SCT it is "normally" recommended to set the artificial star around 100 feet (+/-) of distance... 8)
I did this once for my 10" SCT collimation, and worked fine...

Hope this helps,

Zach, For my C11 I used a 50um pinhole 100 feet away using a bright LED flashlight in a wooden box set on a camera tripod. After spending at least 10 hours understanding how the knobs affect my collimation, I achieved what I thought was the ultimate alignment. Sadly, moving to a real star over head resulted in a collimation shift. For my 20 year old scope, my best collimation is done on a nearby bright star EACH night. Sounds crazy, but a few tweaks can be done within 10 minutes now that I know how to do it. Hopefully your scopes will hold the collimation better than mine. Good luck!

Regards,
Steve