[GeoffW1 (Geoff)]

Hi,

I recently bought a 300mm Bintel Dob from Zuts, and am very pleased with it (many thanks to a friendly colleague). I thought that with such a monster OTA the inverse law must apply, and so I should put a bit extra effort into collimation.

Accordingly I got out my laser collimator (Bintel model) and went ahead. I like Barlowed collimation, and used that method. Everything was smooth, a star test was good, and I was able to enjoy the increased aperture.

Tonight I was keen for a repeat, and put the scope outside early. In between BBQing, I thought I would collimate again to get a feel for the behemoth. What ?? Miles out!! The laser spot was on the edge of the primary mirror!! It can't vary that much in one day surely.

So I took the laser collimator into the garage to find out what was going on. I have a little wooden jig (previously shown here) to check the laser alignment and on first rotation it was enormously out, all over the garage door. Had something gone loose? No, it had not. What then?

I then found something we need to know about these devices. You can align these laser collimators all you like, but if you then give the laser on/off screw an extra turn or 2, it can ruin everything! I don't claim they are all like this, but mine is.

Now colleagues, this might not be news to you, but it was to me. Turn it on too much, and you are, um, politely put, not where you thought. Argh.

Thus enlightened (was that a pun?) I think the only repeatable, reliable thing to do if you have a laser collimator, is to turn it on just a smidgen, no more than just enough. How many times was I kidding myself I wonder?

I also found another interesting wrinkle on barlowed collimation which I will submit as an article, because it needs a few photos.

Cheers

GeoffW1