What could cause a scope to go out of focus mid-shoot?

[BrendanC]

Hi,

Last night I took over 500 subs of M66, the Leo Triplet.

After a lot of wrestling with DSS, I realised why it wasn't playing ball: about 80% of the subs were out of focus.

The first load of subs are pin-sharp until suddenly, the stars go blurry. The rest of the subs are useless. It's very vexing!

Nothing had changed - I'm not guiding or anything, my AZ mount (I know, not ideal, but I'm having fun with it) was just tracking, my camera (a modded EOS1000D) taking subs of 30 seconds. Go through them and you have sharp, sharp, sharp etc, then suddenly, blurry, blurry, blurry from then on.

I use the Skywatcher autofocus (not with the actual auto focuser module, just the hands-free DC focusing device), so there's no active focusing going on either.

Any ideas what could cause this?

[carastro]

When I first started imaging I had a similar problem, and eventually discovered that my drawtube was slipping (even when locked).  I have been told that autofocussers don't allow this to happen, but as you say your autofocusser is 

Quote

not with the actual auto focuser module

then this might be worth considering.  change of temperature can cause focus to change as well but it sounds like your problem is more serious than that.

Carole 

Edited April 16, 2020 by carastro

[BrendanC]

Ah, very interesting, thank you. I hadn't considered that. My drawtube does actually have a locking mechanism which I don't believe I've ever used, so that's definitely worth checking. Thanks!