Nikon Coloured Rings

[MarkS]

I've just emerged from another rabbit hole - investigating the problem of large scale "donuts" and concentric coloured rings that many people find in astro-images shot with Nikon DSLR cameras.

The answer is that it is caused by Nikon's lossy data compression.  What was really interesting is that once the detailed cause was understood it was possible to undo the damage and produce image files free from rings.  I even wrote some (Windows) software to do this.

This has probably been the most interesting and ultimately the most satisfying of all the astrophotography technical investigations I have ever undertaken - even more interesting than the "Sony star eater".  The satisfaction came from finding a solution and doing a better job of decoding the files than Nikon or Adobe. It was very time consuming but was a perfect winter project for the incessant cloudy nights.

I won't repeat it all here but if you're interested, you can read the whole thread here:
https://www.cloudynights.com/topic/746131-nikon-coloured-concentric-rings/

A flash of inspiration one night led ultimately to the key explanation (histogram gaps vs histogram bucketing) which is here:
https://www.cloudynights.com/topic/746131-nikon-coloured-concentric-rings/?p=10844563

It should be noted that recent Nikon cameras have the option of lossless compression.  Using lossless compression there is no problem.

Mark

[The Thing]

Fantastic sleuthing. You've got far too much time on your hands Mark!

[RobertM]

I agree with Duncan on both fronts.  I must have missed that CN topic - there's so much interesting (and even more uninteresting) stuff on those forums !  That's another one on my reading list, the last one was over 1000 pages and took me ages !

Regarding doughnuts (or donuts), our local doctor has prescribed them for my daughter to help her put on weight.  The doctor said it's the first time she's ever told anyone to eat them !  Personally I would only ever want to have them freshly cooked at the seaside yum yum ! 

Robert

[RobertM]

Just finished that topic.  How strange that these camera manufacturers choose to do these things.  You should be careful that they aren't trying to out do each other in finding algorithms you can't understand - a kind of algorithmic arms race.  Personally I think they should give up trying to find a new 'lossless' compression algorithm and use something that works.  It would also save a fair amount of disk space on CloudyNights but then, of course, it might not be so much fun