Darks vs no darks

One of the features of my new ASI2600mm camera is the low noise levels compared to other cameras. Recall that the 2600's dark frames typically have median ADU values of about 500 (Test of the 2600). And that is consistent over the typical range I use for exposures in my imaging (60-600sec).

Earlier this week I imaged some RGB and Lum subs of M98. Clouds rolled in and ruined the complete set of 60 Luminance subs so all I got was the 20 each of RGB. Not a whole lot of subs to work with, and with short 60sec exposures the amount of data was really low - in fact, only about 20-80 ADU over the noise level of 500.

To make matters worse, my attempts at taking the flat calibration frames produced odd horizontal banding on the images so I couldn't use any flats in my post-processing. Later I found out in my research that the 2600 doesn't like short exposures using a flat panel for illumination. If I had reset the camera gain to 0 (from the 100 I used for imaging) I would have had better results.

So I decided to run a little experiment. I processed each set of 20x60sec RGB subs, one with darks (but no flats) and one without darks (again, no flats). Darks are typically used in processing to calibrate the subs by removing the camera's inherent noise from the images. Since the 2600 has very little noise I wanted to see what I would get if I didn't use dark calibration in the processing.

The results are shown below. The image on the left is the 20x60 sec RGB stack processed normally, but without darks. The one on the right is with dark frames. Neither, of course, had flats, but I cropped the images to remove the vignetting and the dust bunnies weren't obvious in these short exposure images. They look pretty much the same don't they? And indeed, I was just a little surprised at the fact that they did look very similar. The low noise is really a great feature of this camera.

M98 - No darks vs darks
20x60 sec subs EdgeHD11/ASI2600

Now, if we zoom in on the images you can see that the image processed without the darks starts to show some significant noise. The image with the darks is much cleaner. (these images have been lightened to show the background noise better.)

So, I will continue to process with darks and flats. But it was an interesting experiment nonetheless. As for the final image of M98 - although I really want to retake this one with proper exposure times and processing to bring out all the lacking detail and brightness, this image will be used in the M110 contest (award on TSS for imaging all 110 Messier objects). I only have eight more to go!