This is a picture of NGC5907, a 20 minutes exposure on July 28th 2001 with my 13cm Opticon SCT. It shows dark spots caused by dustspecks on the CCD and optics. Also there are signs of uneven ilumination of the field and noise due to uneven sensibility of the CCD's pixels. But it is possible to correct these blemishes.
This is a so called "Flatfield" image, an image made at dusk when the sky is evenly dark blue.
Its shows all dust doughnuts, vignetting of the optics and, not obvious, the differences in sensibility of the pixels as grain.
This is the first image, divided by the flatfield and the result of that operation multiplied by the mean of the flatfield to get back to the original brightness.
It's a miracle! Even the background is less grainy than in te upper image.
This image was made at half resolution, 10 images of 2 minutes were combined using the DRIZZLE command in Iris.
In that way resolution is restored to almost full resolution of the CCD!