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How to identify bad darks?

Calibration Dark Frames Artifacts Help Needed
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Arny avatar
Arny avatar

I just took the annual update of my set of darks for my 2 cooled astro cameras - and everything ran fine during the day under a dark hood all caps closed.

Stacking them to masterDarks with WBPP worked fine and the resulting masterDarks as well as the single frames looked fine without gradients when looked at in STF.

BUT when I did a testrun calibrating good imaging data where the old darks had worked perfectly well, all images came out of WBPP with massive gradients.

So I assume I must have picke up some stray light when taking the darks, as it was during the day.

=> Has anyone got an idea or best practice to identify poor darks earlier in the process?

I tried 2dplot script and found a tiny gradient across some of the darks - but nothing that would add up to th eeffect I noticed in the wbpp processed images.

Arny

Mikołaj Wadowski avatar
Mikołaj Wadowski avatar

If your darks are too bright, your flats will not correct properly, which is probably why you’re seeing gradients.

If you have a dark frame you’re sure had no light leaks, compare it with other dark frames. The median value of a dark frame should always stay the same for a given temperature and exposure length, so any significant deviation would indicate stray light is hitting the sensor.

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Craig Towell avatar
Craig Towell avatar

Yeah look at the average pixel value of the darks, they shouldn’t vary more than a coupe of ADU compared to a good dark frame.

E.g. with my 533mm the good darks are ave 2875 so when I’m taking new darks I know any more than that and light is getting in somewhere.