First light with a QHY8L, horseshoe RGB artefacts

[JSeaman]

Last night I had my first go with a QHY8L, it didn't go well!

I lost about 4 hours chasing my tail with USB ports not working and eventually managed to get ~4 hours of data on M101. I came to process this morning and have these horseshoe shaped RGB artefacts as shown on the picture. I have checked each sub and there is no sign on there of them. 

I'm sure you'll agree it's not a very good picture, I focussed with a Bahtinov and guided but the stars look bloated and the artefacts don't help. Any thoughts would be welcomed

Mount: NEQ6 Pro
Imaging: QHY8L on an Evostar ED80 Pro Outfit, f/7.5, 600mm
Guiding: QHY5L-IIC on a Startravel ST80 400mm
Software: Nebulosity for the QHY8L, PHD for QHY5L-IIC, DSS to stack, Pixinsight and CS3 for post process
Subs: 47x5 minute lights, 27x5 minute flats

Maybe it is noise and that the cooling wasn't working? the FITS say it was at -18 degrees but it was set to -20 and I can see dots on the subs (pic attached to try and show) as though it wasn't cooling. Any thoughts welcomed

 

[wimvb]

Imo this is insufficient pixel rejection. Try restacking with more aggressive rejection settings. The shape is then due to changes in fov between exposures. In Pixinsight you can blink the full set of calibrated but unregistered subs, and see the stars move in a similar but opposite direction, or the hot pixels if you blink the registered subs.

[sloz1664]

You have a bad bout of hot pixels  These canbe removed. The process is dependant on your calibration software

Steve

 

[JSeaman]

Thanks for the feedback, could you be a bit more specific over what settings to tweak please? Note I'm stacking in DSS not PI

[wimvb]

Pixel rejection is controlled by a metod call windsorized sigma rejection, or kappa-sigma rejection. This is part of the image integration (stacking) process. In PixInsight you would lower the threshold for bright pixel rejection. In dss there is a similar setting.

Both PixInsight and dss also have a process called cosmetic correction. This process removes bright and dark pixels prior to image integration. In dss you can activate this as part of the stacking process. In pixinsight it's a seperate process. It should be covered in Harry Page's video tutorials, and in kayron Mercieca's (lightvortexastronomy) tutorials.

[JSeaman]

Awesome stuff, just what I needed - I was using averaging not kappa sigma, switched over and it's stacked properly - many thanks!

[wimvb]

You're welcome. But now we'd love to see the final image, of course. 

[JSeaman]

Oh sorry, of course - still bloated stars and needs more data + darks etc. but for my first go I'm happy enough..

[JSeaman]

I had a much better evening tonight, I found that Nebulosity gets really tangled up if you hit 'Abort' and it doesn't actually recover in terms of TEC control for me. That explains a lot of the problems I had the other night. Working around it is easy too

I managed to generate a decent set of darks and bias files and got all my polar alignment tightened up a bit so ready for the next clear night. There was enough time to get 2x10 minute snaps of M42 and one of those had a satellite in it but it's obviously going to be much better than my SLR ever was. I wish I had 3 hours of lights rather than 3 hours of darks but hey ho!