Help with comet stacking

[rogerg]

Hi all,

Along with being useless at LRGB/CRGB imaging I'm also useless at stacking RGB comet images. I can never get rid of all the stars, to do a stack which shows static stars + stacked comet :mad2:

As I understand the theory, one stack is on the comet with median and sigma clipping to remove the stars, the other stack is on stars with similar to remove the comet, then the two stacks are layered. Layering isn't a problem but getting rid of the stars/comet is grrrrr :mad2:

See attached for my best attempt at a 20x180s stack, done in PI using median, windsorised sigma clipping. I get the same result in CCDStack using sigma clipping and mean stack. I've fiddled with the sigma clipping values quite a lot and haven't managed to remove the stars.

What am I doing wrong ?! Please! :)

Regards,
Roger.

[alocky]

Hi again Roger - haven't done this since comet Lemmon, but will be revisiting this after tomorrow's dark sky trip for Terry's latest. I seem to recall the trick was to pick the minimum rather than average value in the stack.
I assume you've figured out the comet tracking script in PI to register the subs for both stars and comet?
Cheers,
Andrew

[astronobob]

Looks good Roger, I havent done much in comet stacks but hope you sought it out, Looks real promising data there !
All the best ....

[rogerg]

Yeap I have the comet aligned no problem so can do the stack of comet or stars, it's just a case of somehow getting rid of the stars :) Minimum 'eh ... interesting. That could make sense if the noise is not the dominant feature.

[gregbradley]

Also you need to have sufficient time gap between the subs to allow movement for the stacking algorithim to reject the stars as outliers.

As I recall median will do that but the time gap has to be long enough. Too close together and it won't reject the stars.

Greg.

[rogerg]

Yes, I had read that the separation is important. I've taken a continuous stream of images but in stacking tried excluding 2 out of every 3 images such that stars are not overlapping. This hasn't made a significant improvement over stacking all with median and sigma clipping.

This gets me wondering if the root of my problem is the NABG camera and/or my optics. If the light were to disperse more than otherwise might for each star due to either factor, then greater separation would be required. Having sufficient separation to keep the stars apart but not necessarily the full extent of their halo would logically leave halo's like I have. Hmm.

[rogerg]

Scrap that thought about NABG – I’m dealing with my Canon 6D here not my ST8.

[cometcatcher]

There are ways of getting rid of stars. I believe Fred is the expert at that. ;) I'm not sure how he does it, but I find a minimum filter to be somewhat effective.

But I'm having trouble combining the two also, especially in a rich starfield.