How to adjust feather touch focuser for collimation?

Question

I have a Starlight Instruments feather touch focuser for my Dobsonian telescope.

While trying to collimate the secondary mirror with the focus all the way in, I noticed that my Howie Glatter laser collimator drifts away from the center of the primary mirror as I adjust the focus to all the way out. The collimator fits snugly into the focuser.

Clearly this is not good as collimation is now dependent on the focus position. While I could work from the mid-point of focus, I'd rather resolve this issue.

How can I adjust my focuser such that it is aligned with the telescope and collimation does not drift as I adjust focus?

I noticed that adjusting the secondary has reduced this issue, now the drift as focus moves is more subdued, but still noticeable.

Answer 1

Okay, so you basically have the best laser collimator on the market (it could get even better if you add the tuBLUG attachment). So let's try something else.

Maybe your secondary is still out of whack? You don't have to mess with the focuser as long as the secondary is:

1. Directly under the center of the focuser
2. Axially aligned with the focuser and primary.

So, one is translation on the tube's axis (up-tube / down-tube) and radially, the other is pure tilt.

Here's an article, it's biased towards the CATSEYE (which is not a bad bias to have), but it will show you the idea. Howie Glatter also has a pretty long tutorial on his site, regarding collimation, and it does mention the secondary.

http://www.cloudynights.com/item.php?item_id=2019

At the beginning there's a link to a newer version of the tutorial, but I feel the older version is a bit more clear. YMMV

Answer 2

(The Stackexchange format is not very helpful here, as I think this should be a series of a few steps to troubleshoot what's really going on. Finding the right answer in one shot would be a bit of a random process.)

Make sure your laser collimator is itself collimated. Plenty of cheap lasers are miscollimated and cause all sorts of trouble.

Here's the principle, but the implementation is a bit too low-tech:

http://www.stark-labs.com/craig/llcc/llcc.html

I'd say, instead of those silly nails, cut two big V shapes out of thick hard plywood, glue/nail them to a thick solid board, and rest the collimator on the two wooden V shapes.

Turn on the collimator and mark the position of the laser spot on a wall a few meters away. Now gently rotate the collimator 90 deg then go back to the wall and examine the spot. Did the spot move at all? If it did, the collimator is out of whack. If you're lucky, you could adjust your collimator until it's lined up correctly. If you're unlucky, get a better collimator - I've a Howie Glatter laser collimator, it's great.

Once you confirm that the laser is perfect, troubleshooting could continue.

In fact, all laser collimator owners should do this test. Otherwise, your "collimation" is meaningless.

EDIT: Is there any play/wobble in the focuser itself? Does the laser go in snugly? (the focuser could hold the collimator askew if the diameters don't match)