Sean Esopenko

DeepSkyStacker: Calibration

June 17, 2010

When shooting for DeepSkyStacker you’re supposed to get the best results when you calibrate your camera.  Unfortunately I didn’t calibrate my camera at all when I shot the night for the first time so out of curiosity I shot about 8 dark frames at ISO 1000, F2 and 2 minutes each.  By the time I got to the 8th exposure my camera’s CCD was quite warm and you could see it in the images.

If you look at the right-hand side of the image, you’ll see a large red splotch developing and innumerable bright dots scattered about the image. Shooting these dark frames gives the software a reference point to subtract the hot pixels from the stacked image. I suspect that DeepSkyStacker may consider these hot points “stars” and it could mess up the registration of the stacked exposures, thus giving a good reason to shoot these dark frames.



I also shot about 35 flat frames. Here you can see a “Flat” frame taken, where I stretched two layers of a white t-shirt and shot on ISO 1000, F2 on Exposure mode.  The camera metered for 10 second exposures.  The purpose of shooting flat frames is to provide the software with a reference point to eliminate the vignetting around the edges.

This is a stack of about 35 flat frames. If you look carefully you’ll notice that the center of the vignette is not in the center of the frame. I don’t know if the lens’s vignette is off center or my method of shooting the flat frame was incorrect. Either way it looks like something I’ll have to figure out down the road.

So if you look at this image, you can see the CCD heating up on the right-hand side as well as the strong vignetting on the edges. Next time I shoot the night sky, I’ll be taking about 20 reference frames for flats and 20 reference frames for the darks so that the software can virtually eliminate both problems. The FAQ for DeepSkyStacker recommends that you stack not only your light frames, but your calibration frames as well for improved image quality. If you only include a few calibration frames it won’t be able to average out the noise and you’ll introduce even more sensor noise.

It’s been quite difficult finding documentation on exactly what Light, Flat, Dark and Dark Flat frames are. Light frames are the shots of the stars, the ones with an actual subject. Flat frames are shots of a smooth, evenly lit surface in order to accentuate the lens’s vignetting. This can be done with a soft-box (an aparatus placed over the lens) or with white cloth stretched over the lens hood. I haven’t figured out the exact difference between dark and dark flat frames, however, and will experiment with these the next time I shoot the night sky.

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