Star Testing

I star tested my dob last night. Never done it before. The one website I've been reading says I should see concentric rings of light. Maybe I wasn't at high enough power, but all I got was a large disk with the shadow of the secondary in the middle. Is this what you're supposed to see or should it be these conentric rings the author spoke of? I was pointing at Venus, it doesn't get much brighter than that. Any pointers or free advice, or links to websites would be appreciated.

Hi Leif
I'm certainly no expert, but a planet is not a good choice to do a startest. Pick a bright star near zenith and get upwards of 200x then try again. Here is a site to get some idea of what you are looking for. http://perso.club-internet.fr/legault/collim.html

Hope this helps
Buck

Buy this book and study it.Star Testing Astronomical Telescopes by Harold Richard Suiter.It is the definitive work on this subject.Here is a link. http://www.willbell.com/tm/tm5.htm
This book explains everything about the subject and newtonian telescopes.
Also be aware that when your newtonian telescope is slightly out of collimation and is racked a 1/4 turn on either side on focus you will see what looks like astigmatism. That is the star image will elongate first one direction and then 90degrees the other direction on the opposite side of focus. This tells you that collimation is just a little off. Clear Skies,Scotty.

Try using about 20x per inch and see if thta doesn't help.
Floyd

In my little 4.5" f/9 reflector, I get the concentric rings, as I do in my C8. But in my 14.5" f/4.5 dob, I don't get the concentric rings.

Try the higher power first. But if that doesn't work, then make up an aperture mask, and put it at the front of your scope, such that it's hole misses the spider vanes, and you'll get the concentric ring.
Renato