Dark Calibration Frame Scaling

Rho Ophiuchi Complex

I wasn't sure what dark calibration frame scaling was exactly and why you would need to do this.  So, I did what I normally do in these situations; I researched it.  I found information on the Diffraction Limited website which I will share here...

The reasons we need to take and apply dark calibration frames to our image data are (1) to reduce the amount thermal noise in our images and (2) when we take long exposures, we increase the amount of dark current noise. 

These are the primary reasons that the dark calibration frames and the light frames need to match in exposure time and temperature for dark frame calibration to be effective.  Temperature matching is particularly important because the rate at which you get dark current noise varies with temperature.  For those of us who use temperaturee cooled dedicated astro-imaging cameras, the process to take dark calibration frames is pretty easy.

However, if you're using a DSLR or other camera that does do not have cooling, taking effective dark calibration frames will be a problem.  Temperatures drop and change during the night and depending on the weather or season, these changes can be considerable.  You can take dark calibration frames throughout the night to minimize the problem and the temperature differences, but you will still have problems getting good consistent dark frames to calibrate your data.

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So, this is where dark frame scaling is used.  Basically, you use a dark frame whose temperature and exposure time are as close to correct as possible and scale it to the temperatuee and exposure times you need.  Processing software such as PixInsight has algorithms to do this.  It's also recommended to take and use bias frames since these calibration frames are constant and does not scale with exposure time.

For those of you in the community who know a bit more on this topic than I do, I'd appreciate any further information you have to share and PLEASE correct me if I've said anything misleading or flat out wrong here.

Clear skies...

Astro Image Credits:

IC 4605 (https://app.telescope.live/click-grab/all?target=ic 4605); Telescope Live 1-Click Observation Data, Processed using PixInsight by Reggie Jones

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