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A lesson learned from DarkLibrary

I thought my the darks I’d been taking were good.  Not only had I amassed a pretty good library of darks, but I made sure that they were taken on nights when the temperature was consistent.  I checked the hourly temperatures the next morning and threw out the frames that might be out of range.  I assumed that if I took the lights on a 50 degree night, darks taken on another 50 degree night would match.

I was wrong.

I was wrong because of the location I was using to take my dark frames:  the ground.  I generally take them on the floor of my open garage or on my porch.  I figured that as long as the concrete wasn’t heated up all day by the sun, it would equilibrate with the air relatively quickly at night.  It’s a pain to manually get the EXIF temperature data from the raw files, but the DarkLibrary software makes it very simple, and it also reads your DeepSkyStacker file list to help match appropriate dark frames.

With this new tool, I had a look at a recent set of lights I took of M51 on a night that it ranged from about 45-48 degrees F.  The EXIF data shows that the sensor temperature recorded was a range from 8-11 degrees C, so pretty much the same as the air.  The next night, it was hazy, so I figured I would take some darks.  The temperature was only a few degrees warmer, but stable, maybe 50-52 degrees F.  I put the camera on the porch and plugged it in via the AC adapter, and let it take shots all night.  The next day, I looked at the EXIF data, and I was shocked:  20-25 degrees C!  Sitting there on the concrete, the camera was about 20 degrees F hotter than it was when up in the air attached to the scope.  That is a huge difference, and it explains some problems I’d been having with noise in my shots at warmer temperatures despite using over 100 darks.

Thank you to the creator(s) of DarkLibrary for making it simple to really match DSLR lights and darks by temperature!  It uncovered a stupid mistake I didn’t even realize I was making.

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