Your flat and its weave pattern actually look typical to me, assuming your image of it is under a big stretch. That weave pattern should not be present in your calibrated lights if your flat-fielding is working as it should.
There are some oddities in the flat, however, in addition to the strange shading. There's also a diagonal line in the lower right quadrant. What do your other flats look like from the other filters?
Please give details about your flat-fielding process.
it is stretched to point out the problem, the lines are present in every flat, light, i assume that it comes from the filterwheel
- pointing the 17" CDK to 90°
- put on top the artesky flat field box (open truss ring with secondary)
- dimm the light
- taking flats ( first ones where 2 seconds>shutter in picture)
(second ones where 7 seconds, full well 30%)
(third where 20 seconds full well 40% with more dimming)
- tried Masterflats, without Calibration, with DarkCalibration, with Dark&Bias, with Bias only (even tried normal Masterbias and Superbias on all those steps)
- tried also with multiple ways to calibrate the lights (Darksubstraction, Biassubstraction, Bothsubstraction)
- normal Calibration is (flat - optimized Dark)-Bias
((Light-Dark)-Bias) * mean(flat) / flat
if you want to try out the several ways to calibrate an image, i already uploaded all the "raw" files
there you can see that when you correct it in a "good" way it will end with my result (except the left structure and one donut beneath m51, result of flats taken 2 weeks later)
That is a very "funky" looking flat. Is this the linear master flat data or did you enhance it somehow? You've got a lot of spatial variations and some sort of rectilinear "weave" pattern going on--as well as a sharp edge in the lower left corner. I'm not familiar with the "flat field generator from artesky". Is that a "normal flat box? Are you using a t-shirt with it? I've never seen a flat that looks like this. Have you ever been able to get flats to work correctly? I'm wondering if this is a new problem or if this is a problem that has always existed. You've either got an equipment problem or you are doing something wrong but from the information you've provided, I can't figure out what's going on. So, let's start with the flats. Exactly how are you taking and processing the flat data?
I've attached a re-sampled flat from my ML16200 camera on my C14 so you can see what a good flat often looks like.
PS If I mess around with stretching my ML16200 master flat data to enhance contrast, I can see a similar faint "weave" pattern. That same pattern does not exist in my ML16803 master flat data. So, did you stretch the flat data that you posted?
yes it is stretched for better visualization, its a normal flat box with dimmer, 560mm in diameter, no t-shirt, we moved the box in circles between some flats
to see wether the structure is coming from it, we cant saw that its was changing, as a bonus : we took some sky flats, before the flat box arrived, and the structures were there (but shutter problematic)
this is nearly our first light, so we don't have experience with this setup
Which camera are you using? How long are the flat exposures?
fli ml16200 20seconds