Michael Atkinson

Thanks. I'll share darks - but it'll be a while (weeks) due to my current situation. Currently I can't get to ufile.io. I'll keep trying!

Thanks for these investigations. Interesting w.r.t. the median implementation in DSS... I did an hour or two of searching and couldn't find any mechanism to efficiently (from a memory usage perspective) determine true median. I wonder how they do it? I did briefly look at the harmonic mean which, whilst being more expensive then arithmetic mean, could be efficiently calculated. It would help with my cosmic ray outliers, but I think it might suppress hot pixels and amp glow too much . Maybe those...

Apologies for delay responding - I'm away from home. Here's my original master dark, from 50 frames (300s, gain 100, temp -5 - I use gain 100 for NB): https://1drv.ms/u/s!AgyMCIfwIgxggroONm70hlAXAdqCeg?e=VCPt58 Newbie mistake - 300s is too long and I'd be better using 180s. I need to redo my darks accordingly! If you zoom 1:1 and look top left there are a couple of what I believe to be cosmic rays visible. These were a lot brighter in the original sub but the mean calculation even with 50 frames...

For swapping dark with DSS, you have to be very carefull that no flipping occurs. Furthermore DSS uses a very rare TIFF format setting , which ASTAP can't read and also...` The rabbit hole has just widened! Good points. I'll stick with ASTAP although may briefly look into processing darks as lights, keeping a keen eye on the fits headers. Can you share a few dark frames where cosmic rays are visible? If I'd been sensible I wouldn't have deleted the subs until I concluded this query, but I did :-(....

Thanks for the response Han. What I'm seeing makes sense then ;-). I'm pretty certain these are cosmic rays, and in some cases cascades. I need to think about how median use could be noisier, esp in a low noise sensor. I'd have thought that excluding such huge bright outliers could only remove significant pixel to pixel variation and ultimately result in less noise? But now I have the confirmation I can try creating master darks for my library either by using the lights approach you outline or back...

I now have found others detailing how taking the median of pixel values effectively ignores outliers caused by cosmic rays. DSS documents that it defaults to a median calculation for master darks and bias. For Astap I can only find Appendix 1 (stack process) stating that images are calculated using: (image- {1/n∑ [darks]} ) / master flat which implies a mean is used. But this could be simplifying the description and I guess I have to explore the source :-(

With a view to better understanding calibration I recently took a deep dive into the data I've been using. I'm lucky to have an ASI2600 so am not plagued with noise or amp glow but nevertheless have been using a full set of calibration frames. Pixel peeping into my master dark - from 50 300 second exposures (gain 100, temp -5C) has perhaps led me down a rabbit hole... The master showed some bright spots. I still had the constituent exposures and soon found that the bright spot only appeared in one...