To pretrait or not to pretrait, that is the question...

When doing astronomical images, it is sometimes better NOT to do pre-processing. When the number of dark frames is small for example, as it adds more noise to images than it can remove.

Here are some discussions about this.

Here are 2 images, obtained from the same raw pictures: 30 30-seconds images of m31, taken with Canon 85mm f/1.8.





This one has no pre-processing: hot pixels are visible as small red, green and blue strings (click on it to see it larger).














This one had full pre-processing: dark, flats and offset. When viewed full-size, it doesn't have more signal (it probably actually has less), and overall is doesn't look as good. I only had 1 dark frame to use, as well as only 1 offset and 1 flat field.















For comparision, this one has no pre-processing, but only 10 images stacked together (3 times less). The signal/noise ration is clearly less than the first one with 30 images.