Does my master flat look weird to you?

[storyface]

Hi everyone. I've had some comments lately that my images don't appear to be properly calibrated, esp. as regards flat field calibration images. I use the BatchPreprocessor script to do all calibration, alignment and stacking. I decided to look at my most recent master flat produced by the script. Zooming way in (more than 1:1), there's a very regular checkerboard pattern that was a surprise to me to find there. See attachment.

I usually take 25 or so flats with the same Canon 6D and ISO setting that I image with, using an artist light panel through a t-shirt and aiming to get the histogram right about midway, left to right. I'm kind of surprised to see such a regular geometric pattern in the master flat. Individual flat frames just look like smooth raw files. Is this an indication that something is wrong in my calibration process, or am I just looking to closely at the sausage being made?

Appreciate any feedback, and thanks.

[pfile]

it is the sausage - for DSLRs and other One Shot Color cameras, the calibration files are kept in so-called CFA format (color filter array - undebayered and essentially the raw data coming off the sensor). in fact, calibration must always be done on non-debayered files since the debayering process is a form of interpolation.

so anyway, its supposed to look like that. if you want to see what it looks like as a 3-plane RGB file, apply the debayer process to it. i think canon cameras are all RGGB but dont hold me to that, it has been a long time since i used my DSLR for astro.

rob

[Raymond]

Hi; I'm using a Canon 500d and this pattern looks quite normal to me. As Rob says, it is important to use the non-debayerd files to calibrate your light images.

If you want to know how the debayered and stretched flat can look like, you can have a look last image at the bottom of my page:
https://www.ltam.lu/physique/astronomy/projects/flatbox/flatbox.html
It looks quite horrible - but it corrects for most of the vignetting and dust on the sensor.

Raymond