Removing vignetting
In PhotoShop 5 
by Lorenzo Comolli
One of the first problems that rise when you try to process a deep sky image in a graphical software is the vignetting: in fact increasing the contrast of the photographed subjects, it is increased also the contrast of the background sky, resulting in an unpleasant manner the vignetting.
I was inspired vaguely by the technique of the flat-field used in the elaboration of the CCD images, I have created two techniques for reducing the problem. But it's to be noticed that nothing could replace an image whithout vignetting in origin!

Figure 1
The problem has risen me in the processing of images taken through a Schmidt-Cassegrain telescope and a f/ 6,3 focal reducer. Here to side (figure 1) we see the image of M27 like it result  after the scanning, without any processing. The tracking is not perfect therefore the stars are not good circles. The photo was taken from Tradate (VA, Italy) from 300m a.s.l. exposing for 30 minutes on Kodak PJM 640ASA. The vignetting is evident, also because it was taken from the center of a city and in presence of the full moon. If it is processed as it is, the results would be very ugly for the vignetting.

Technique 1
The first procedure is the one that gives the best results, but unfortunately it's not always usable: his employment is limited to the objects that occupy a limited portion of the frame. The basic idea consists of getting an image of the original strongly blurred (mask), and then of subtract it from the original frame. In this way you remove not only the vignetting, but also the dominant color of the background sky, reducing it to a pleasant black/grey neutral color.

But let's see how to realize this idea:

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Technique 2
This tecnique has to be used with images of objects that cover a big part of the frame (i.e. the Orion nebula or the Lagoon nebula). For showing the differences with the technique 1, I have used the same image of M27. The basic idea is of digitally creating the mask to subtract from the original. This is obtained with the function that creates the tones between two colors.

But here is in detail the procedure:

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Final comments

To first sight the two techniques could seem complicated, but once understand the leading idea, they are very simple and fast to use. In my opinion the first technique it is better because it doesn't allow you errors like the not perfect centering of the mask and the not exact scaling of the tones of colors in the mask, but, as visible from comparing figure 5 and 10, the results of the two techniques are very similar. However, if possible, it's preferible to use the technique 1 because it's also easier to perform and then correct also non simmetrical gradients, like that caused by the prisms of off-axis guider.

Good elaborations!

Coments from Fabio Falchi
Before elaboration
Bigger image: 85 Kb
After elaboration
Bigger image: 125 Kb
Hale-Bopp, Pleiads and California taken and elaborated by Fabio Falchi

Here is how Fabio Falchi elaborated his image:

Send me your comments or critics on the technique here exposed, e-mail at: (Send me also any correction for my not very good English!)

HTML Editing and Publishing by Lorenzo Comolli. Email me at
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