Question regarding collimation/secondary offset.

I offset the secondary in my 10" dob. I used a laser collimator to line up the secondary with the primary, then I used the barlowed laser technique to align the primary. However, when I did a star test the secondary reflection was not in the exact center of the defocused star. Is it supposed to be? Since the secondary is offset I thought that might be the reason why.

Chris,
The star test should show the secondary in the center regardless of the offset. The offset does not effect this as this is the optical train alignment which includes the offset. The primary and the secondary are still not collimated completely.
Floyd

Hi Chris,

What's the focal ratio of the 10" newt?

Thanks, John

I'm not sure this is your problem, however one critical element that cannot be done with a laser is to center the secondary in your focus tube first. This is what the sight tube is for (or a cheshire). You can also do this with a plastic 35mm film cannister. Trim the rolled lip off the cannister and drill a small hole in the bottom, then inset it into your focuser with the little hole for your eye.

Here is a link with some illustrations that may help with centering the secondary.
http://makeashorterlink.com/?B6F911E87
One of the things I have found that helps with this task is to lay a sheet of paper in the tube oposite the focuser. This provides a contrasty background and makes it easier to focus your attention on the position of the secondary while you're trying to center it in your view.

I think once this task is done, your laser should bring the rest of the system into alignment. I used to use a laser for collimating my dob and then I bought the Tectron tools for collimation and ended up selling the laser.

Good luck.

Gary Weber