Flats not fully helping in stacking using Hyperstar 6"

[Ehud]

Thanks Robin.
" In order to compare this sort of issue sensibly from one session to the next, you have to take care to use the same level of stretching in the image - even really good flat correction will show flaws if the image is stretched heavily enough, whereas mediocre correction can look quite reasonable on an unstretched or lightly stretched image" -

How can I ensure same level of stretch, or disable stretching?

[turfpit]

Ehud

Whatever offset (black level) I use for my bias frames, I use the same value in my lights, darks & flats. See the last page of the doc. A real world example is here viewtopic.php?t=7696 , see post #9 (once you understand this you are on your way). It took a few years for me to get to that level of understanding. Following my own advice gets me images like this viewtopic.php?t=8322

This is not ideal but I took your master flat PNG and converted it to FITS format using PIPP. This is the resulting file displayed in FITS Liberator v3.
Note the Max value of 47,000. A figure around 30,000 (28000-32000) is what I aim for.

master_flat.JPG (128.44 KiB) Viewed 317 times

The answers to your problems lie in the Image Headers of your bias, flat, dark and light FITS frames.

Dave

[Ehud]

Thanks Dave.
this means that if now I am trying to have the flats histogram in the middle, I actually need it to be a bit below the 50%?

[turfpit]

Around 50% is good.

Next time you image, save everything as FITS as this will help debugging any problems. Draw up a table with bias, flat, dark, light down the left and offset, gain, exposure across the top and things will make sense (or not).

Dave