Guided SA - strange DEC tracking pattern

[thomasv]

Finally just got my SA setup with guiding completed and first tests show promising results. However, I've been trying to interpret the PHD log viewer results and see a regular pattern in DEC (which is of course unguided). Anybody has some insights as to what could be causing this? I would understand a slowly drifting result due to incorrect PA or some sudden random drifts due to the cables pulling, but can't think of a reason for a sinusoidal pattern.

Thanks.

 

[Cornelius Varley]

At a guess I would say periodic error in the ra axis showing as a wave form in the fixed dec axis.

[michael8554]

Try running the Guiding Assistant for 20 minutes, which is about the period of  the excursions.

As RA won't be guided you'll get a true idea of the dec behaviour.

How is your balance biassed? Although Dec isn't guided there will still be play in the bearings to be taken up by judicious ba!ance.

Incidently your imaging lens looks very poorly attached to the mount, do you get differential movement?

Michael 

[thomasv]

Thanks, running the guiding assistant is a good idea, never done this before, so I'll try next time I've got it setup. During this test the RA balance was pretty much neutral, haven't checked the DEC balance, but that should be pretty solidly fixed by the clutch on the DEC bar. Pushing by hand the guide scope and imaging lens, it feels quite solid, but of course it's hard to tell, you only need a tiny differential movement. I don't have any ideas on how to attach the lens better, maybe I should look into fixing both the lens and the camera to a dovetail bar.

 

[michael8554]

11 hours ago, thomasv said:

haven't checked the DEC balance, but that should be pretty solidly fixed by the clutch on the DEC bar

Confused by this. Balance is checked with clutches off, just locking the clutch doesn't make the balance right.

Michael 

[thomasv]

Thanks for the reply Michael. Sorry, I was writing from work, so what I meant was that so far I hadn't thought about checking the DEC balance. I've tried to check it now at home, but on the DEC bar of he star adventurer, even with the clutch fully released (shown on the picture below) there is some friction, so it can only be checked roughly, and at that level it seemed ok. I've then detached the lens/camera assembly from the mount and held by a finger at the fixing point, that showed that it is actually well balanced. 

[thomasv]

OK, it was clear tonight, so managed to run the guide assistant. Looks like Cornelius was right and the main source of error is the RA periodic error showing up in DEC when guiding. (I need to think about this a bit more to try to understand what this exactly means, is it just that PHD thinks the RA and DEC axis are not orthogonal?). I am somewhat surprised at the relatively large 100" peak to peak periodic error, has anybody else measured teir SA, is this normal? Has anyone tried to tune it to improve the PE? But I'm also happy that it can be guided out so well.