do these calibration frames look ok?

I’m very new to most of this, and am learning the ins and outs. I’ve mostly used Siril’s osc script to process images from darks, lights, biases, and flats in folders. I’ve tended to use background extraction and manual colour calibration to get around parts that felt “weird”, but thought I’d take a deeper look into what’s actually going on.

I’m attaching screen captures of my stacked calibration frames (in Siril’s histogram mode). Do these look ok? I see a semi-interesting pattern of the flat, and a very strange polygon for the dark and bias. Looking at it, I’m almost certain that it’s the lens cap that’s letting in light, so I’ve been feeding it junk darks and biases… or is this sort of pattern expected? To takes darks, I simply put the lens cap on, and take pictures with the same settings as the lights. For biases, I put the lens cap on and set the shutter speed to 1/8000s (the max on my camera - in this case, the Sony A7R iv).

Any input is appreciated.




Well… That’s new. @lock042 have you ever seen that? I haven’t.

The flat clearly lacks dynamic.
You need to analyze statistics, but IMO you need to reshoot your flat frames.

have you seen a dark or offset like that, with the polygon??

reminds me of an aperture…

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Yes, biases and darks are not light frames like flats :).

Thanks, guys.

So, the dark and biases are fine and I shouldn’t be worried?

Also @lock042, if you could kindly elaborate on what you mean by dynamic… What would a “good” one look like / not have that’s in the one posted here?

The statistics for the flat are here:


Is this good / bad / what should I be looking for?

Oh yes. It is easy to see here that your values are strange.
Looks like you are in 8bits!!! If not then, your flat exposure is not good enough. If it is 8bit it is a mistake because 8bit has not enough dynamic.

For the flats, I took a white sheet in front of the lens and put a laptop white screen with max brightness. The exif info for each of the flat frames is: f/1.8, 1/25s, ISO 100 (same as the lights). The histogram of a flat suggests nothing is being clipped. How do I check if it’s 8 bit in Siril? I just used the OSC script to process everything.


@lock042 apologies… I think I gave the stats for my screenshot doh.

Here’s the stats from the file.

if your dslr is a 14bit one. The range of a pixel is [0, 16384]. But here your mean is 879.

Under exposed.

Ah… makes sense…

So in that case, what would be the correct way to take flats? Mt aperture was wide open at f1.8, and my lights and darks were at ISO 100 (any higher ISO was blowing the highlights on the subject with 30 second exposures). Should I raise the ISO just for the flats, or look for a brighter screen? Or should I keep the ISO and aperture the same and manually increase the ISO so the histogram is more to the right (but not clipped)?

Also, do the darks / biases look reasonable?