There is no difference... in case of square pixels i.e. Pixel size = Pixel size X = Pixel size Y (all in μm).
This is not the case for pixels, which are rectangular i.e. Pixel size X != Pixel size Y
However I don't know how Pixel size is handled in the latter case

OK, hat's what I thought it was without reading up on it.
I was just a bit worried because someone here suggested to use the sensor size in mm for "Pixel Size X/Y".

Yes it was me and it works for DSLR (been doing DSLR on Indi for over 2 yrs) and still no one has answered why it does work - I do not use UM in X and Y only in Pixel Size.

Still awaiting an answer from developers (e.g. @KNRO) - Yes Radek it does allow any floating point number - but it what you do with it and decide what the value is in coding that counts as you know

I will start a new thread!

OK, I got the solution for the FITS issue. Posting it here in case someone reads this thread later.

I had also posted the question in the DSS group on groups.io and got a pretty good reply there.

The issue is that DSS does not know the real bit depth of FITS files. It *does* know it for RAW / CR2 files. So DSS scales the CR2 files automatically to 16bit (in my case from 12bit). But since it does not know the bit depth of FITS files it does not scale them which makes them look 16 times darker.

The solution is to set "Brightnes" in the DSS FITS settings from 1 to 16 (for a camera with 12bit sensor). This will multiply each pixel value by 16 which is the same as scaling 12bit to 16bit (i.e. left bit shift by 4 bits). For a 14 bit camera you would need to set it to 4.


Full topic here if anyone is interested. Not sure if this can be viewed without an account.
groups.io/g/DeepSkyStacker/topic/73054745#23793

Sorry I missed this one...
I have taken another look at your flats. I was wrong. It's not that bad While looking at pixel values on your flats I was thinking of 16bit camera
Camera pixel depth is 12bit (ADU 0-4096), minimum on your flats is 318 and maximum is 1978, average 869 (21% of maximum). So you're placing histogram peak at 1/5 of depth range. It is quite ok, I would normally push it to 1/4 - 1/3. If they work ok you can leave them at 1/5 though.

Thanks for taking the time to look at the flats again.
Do you think it is a big problem that they are very blue?

I use mono camera so have little experience with OSC/DSLR but I can imagine that white balance for flats could be an issue. It's probably coming from color temperature of your flattener led.