Off-Axis Guiding and Camera Rotation

[Euan]

I'm putting together a new imaging train at the moment and at the heart of it is a TS OAG. Like so:

Atik 383L+ >> Filter Wheel >> OAG / QHY5 >> Reducer / Camera Rotator

Now, sitting looking at this I just realised that whenever I use the camera rotator it's going to change the angle of the whole train including the QHY5, so I will need to re-calibrate the guiding in Maxim everytime

How do SBIG's with built in autoguiders work then if they are used with motorised camera rotators? Off to find out more....

[Euan]

Found a great paper on it from DC3: http://acp.dc3.com/RotatedGuiding.pdf

...for a GEM, flipping alters the relationship between the error and correction vectors. And it should be clear that rotating the guider after calibration also changes this relationship. Both of these require either guider recalibration or corrective inputs to MaxIm.

The "corrective inputs to Maxim" is the changing of the guide angle with respect to the original angle it was calibrated at. Now this is sort of do-able with an automated camera rotator, but not with my setup.

So with a manual rotator, it's too rough to guess the changes I will just need to recalibrate each time.

...or buy an automated one

[FraserClarke]

So with a manual rotator, it's too rough to guess the changes I will just need to recalibrate each time.

...or buy an automated one

I don't think you need to be too accurate with the rotation angle

I reckon if you get it to 10-15 degrees, it should be OK. I presume most of the guide corrections will be small (<1 pixel), so the fractional error you'll get from having the wrong angle is probably not too much of a concern. I've just run a system where we had a 18 degree offset between the rotator and the calibration at one point, and it ran just fine. It was a very good drive system with only minimal slow corrections to make to be fair!

Could you make a reference mark / rotation scale on your setup? With that, you might be able to judge it to a couple of degrees just by eye?

[Euan]

Could you make a reference mark / rotation scale on your setup? With that, you might be able to judge it to a couple of degrees just by eye?

I actually only ever do either 0 or 90 degrees depending on which object I'm imaging, so just making a mark then tweaking the guide angle might be OK then.

Except for the one night I forget to change it...