Flexure and field rotation. Please help

[nobbygon]

Hi,

I’ve recently got my imaging rig back up and running. I’ve never really taken long enough subs to encounter this problem but here it is. By the way I’m pretty sure I know what is going on, all I need is some confirmation.

The image attached is a stack of 19 300s subs of m42, 1.5hrs all up. (not aligned.)

1). The stars rotate around the FOV slightly. Would I be correct in saying this is field rotation? This would be caused by a polar misalignment, correct? If so, that’s an easy fix.

2). The fact that the stars have an obvious tail when stacked may be caused by differential flexure. Could anything else be causing this? If it is flex I have a few ideas to reduce it. If it’s something else, I’m open to suggestions.

As I said above, I’m pretty sure I know what’s going on here. I just need someone with more experience than me to confirm and I’ll go away and try to fix it all.

Thanks for any help.
Cheers,
Angus.

[John Hothersall]

Hello Angus, if you stack on a central star field rotation from poor polar alignment will show as rotation in the corners. If you stack the exposures during period of continued guiding they should produce round stars, if they show elongation then this is flexure usually found in the guidescope rings or guidescope focuser slop. Most imagers eventually go to OAG's for trouble free guiding because any problems are a lot simpler to solve.

John.

[naskies]

Hi Angus, what's your actual concern? Most people will dither their subs for a better signal-to-noise ratio, so an unaligned stack is fairly meaningless.

More importantly, how does an individual sub and the aligned/stacked result look?

Were these subs guided? If you weren't dithering, then the fact that the central stars are moving so much suggests a problem with your guiding.

[Geoff45]

Instead of a stack, it would be more helpful to post some individual subs.
Geoff

[nobbygon]

Hi everyone,

My concern is star trails. They can been seen in the images attached. I was guiding with a Orion ST80 and DSI II Pro.

Here’s a single sub of 300s stretched and a crop from that sub. The crop is relatively close to the centre so I’m certain that this is not curvature. Also note that this is uniform throughout the image.

Not all subs are like this however. Approx 50% show similar star trails to this sub.

[Peter Ward]

Angus,

Simple...get some software that will allow you to align, rotate and scale your sub-frames.