Posted By Maurice Clark
I have been trying to rework some of my old astrophotos that I scanned recently and am having some problems with uneven field illumination, mainly vignetting. I have tried using Berry's method of subtracting a blurred version of the image, but the result is too dark. I have also tried using gradients as suggested by Jerry Lodriguss, however that does not work because in many of my photos, the background does not drop smoothly from the centre outwards. The centre is slightly darker, then there is a brighter ring covering about 2/3 of the image before the illumination falls off rapidly towards the edge. Also I find that using this method really gives problems for the colour balance.
Below is one of the images I am trying to improve. It is of comet 2001A2 LINEAR which was a wonderful sight during May and June of 2001. In the image, the tail of the comet extends the full width of the field, however the vignetting makes it very difficult to enhance the tail and make it more visible.
If anyone would like to have a go at this image and get rid of the vignetting, I would be very interested in hearing what you did. I am sure many others would also be interested.
I have Photoshop, Paint Shop Pro, CCDSoft, and AIP4Windows to work with and all suggestions are welcome. I am also posting my results with this image so far.