I have found when taking astrophotographs, I occasionally run into a problem that is called vignetting. In my case, it occurs when I take photographs of constellations with short focal length camera lenses, less than 60mm, and if I use a focal reducer on my 10 inch LX-50 to widen the field of view and reduce the focal length to f6.3. In some cases this can become quite severe as can be seen by this raw photograph of M17, the Swan Nebula. This was a 45 minute exposure taken with my LX-50 in my back yard. It is about the longest exposure that I can do from my yard due to sky fog. The uneven field illumination is obvious in this photo. What can also be seen is that the photo returned from the developer with a strong green cast to it. I have done some experimenting with Photoshop image processing software to correct this image. I have been able to develop a technique that can correct both the vignetting and the color. The second picture shows the corrected image. This approach works well with constellation images taken with conventional camera lenses. 

Start by taking the scanned image, as in figure 1. For purposes of this explanation, I have not done any preprocessing on the image to correct the colors. This is a step that I normally do. I have left it out here to concentrate on the vignetting. Use the REPLACE COLOR function in Photoshop to select the bright white from one of the stars. Adjust the fuzziness control so that only the stars and possibly the object are selected. Then set the lightness to -100 (black) and click OK. You will now have an image which is similar to the example in figure 2.. Repeat the replace color operation with any bright objects in the photo. For the example, I next selected the remains of the nebula. Care must be taken to only select the very bright parts of the object. The image will now appear as in figure 3.

Fig. 1. Scanned Original

Fig 2. Closeup of Image Following Initial Replace Color Operation

Fig 3. Image Following Replace Color Operations

This next step is optional for most photographs. However, for images such as this one, where the object being photographed is quite large and within the vignetted region, I will typically fill in the dark region that remains with the same color as the background from the non-vignetted area. The approach that I use to fill in a region is to first create a new layer on top of the image. Next select the color to be used. Then select the region to be filled in. In the new layer, use the PAINT BUCKET tool to insert the color into the selected region. Next flatten the image. This is shown in figure 4. It is best to experiment to see if this provides any benefit to the final results.

Fig. 4. Large Dark Region Filled in

Next, use the Gaussian Blur function in Photoshop to blur the image. I will generally use a setting that is 25% of the dots per inch (DPI) at which the image was scanned at. (If the image was scanned at 400 DPI, I will set the blur value to approximately 100.) If the preceding step was not used, the resulting image will appears like figure 5. If it was used, the image will appear like figure 5a.

Fig. 5. Result of Gaussian Blur on Image

Fig. 5a. Result of Gaussian Blur on Modified Image

Next take the blurred image and invert it using Photoshop's Invert Function. The image, using the figure 5a, now appears as shown in figure 6.

Fig. 6. Inverted Image

Now this inverted image is used to correct the original. COPY the inverted image and PASTE it as a new layer over the original image. Then change the Opacity of this new layer to 50% and flatten the image. The result is shown in figure 7. The final step is to stretch the contrast of figure 7 using Photoshop's Levels command to obtain the corrected image shown in figure 8. At this point additional processing can be done on the image, such as cropping the photo, setting the black point or color saturation to values desired.

Fig. 7. Image with Overlaid Mask

Fig. 8. Final Image of M17

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