Slow drift while autoguiding

[Gem]

Hi all,

I have noticed that when I leave the scope taking a series of images while being guided, that it slowly drifts one direction over time. The result is that longer exposures are not possible (more than 60-90 seconds). The QHY5 keeps on guiding and the graphs shows all the usual small oscillations, but the overall image still drifts over time.

Is there any way to fix this, or is it normal? I know I am imaging at a long focal length (2350mm), so that longer exposures are more difficult. I would love to be able to image 5 to 10 minutes rather than one...

[RobF]

The likely culprits are polar alignment being off slightly and/or flexure between guiding and main scope Grant. How do you guide?

[Gem]

I do have flex problems I think.

I have an 80mm refractor on a photo bracket with a QHY5. It needs a 50mm focal extender to achieve focus. So it ain't the most solid guiding option. I want to get a better way to mount the 80mm and maybe even change the guide scope to one which doesn't need a focal extender.

The polar alignment doesn't seem to make a huge difference to the problem, so flex does seem to be the issue.

Thanks!

[jjjnettie]

ahuh.... fix the flex and expect exciting exposures. lol

[cventer]

One other possibility is guide star too far off axis from imaging scope. I recently had exactly this issue and after aligning my guider to my main imager the problem went away. More of an issue when using very fast wide fl guidescopes like 50mm guiders.

This issue only shows up on longer than 8 or 10 min exposures on my rig though.

[Starcrazzy]

Try an experiment. Guide with long focal length scope and image with the 80 mm. That May point you in the right direction.

[tlgerdes]

Field rotation.

[Gem]

Thanks all! :)

Plenty of things to try... just need the cloud to go now!

I will give the guiding with the main scope option a try first. I definitely will get a sturdier way of mounting the 80mm guide scope though. Just need to work out which bracket/ring combo is best.

[Peter.M]

If you arent worried about polar alignment goodluck guiding at 2350mm, I would think that your graph will fluctuate beyond the scale of the graph.

With the qhy5 at 2350mm your image scale is 0.46 asec/pix, even with perfect polar alignment and great seeing your graph is going to fluctuate wildly and not really tell you anything.

[Gem]

I do worry about polar aligning, it just hasn't made a huge difference. I always try to polar align, but sometimes I try harder than others. :)

When I am imaging up to now, the QHY5 guider is through an 80mm guidescope, the ATIK 320E is through the main scope. I will try it the other way the sake of experimenting for now.