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Photoshop Techniques for Astrophotography

Correcting for Vignetting

Note: This is a major revision of the original article that I had written on this topic. I believe this is a much simpler yet equally effective technique for dealing with vignetting.

Uneven field illumination (i.e., vignetting) is a common problem with long exposures on film. Oftentimes, the center region of the image is notably lighter than the peripheral areas. For example, look at the image of the Veil nebula below. It was a 45 minute exposure on Kodak PPF-120 film taken with a Takahashi FS-102. The vignetting is clearly visible.

There have been several proposed fixes to this problem (e.g., Jerry Lodriguss, James Foster) and my goal here is not to re-present them. However, I would like to suggest an alternative solution. The technique that I have found to be successful in dealing with vignetting is similar to many others in that it involves the use of layers. If the reader is not familiar with using layers in Photoshop, I would suggest checking out some of the help files available on the subject (e.g., Adobe on-line support).

In a nutshell, my approach involves: 1) Duplicating the Layer, 2) Selecting the vignetted area using Color Range, 3) Creating a Layer Mask and adjusting it ,4) Adjusting the image and 5) Flattening the layers and saving the corrected image.

Begin by opening the image that you would like to adjust and click on Select > Color Range. Using the eye dropper, select a representative portion of the image that is in need of correction (the lighter area). Be careful not to select any nebulosity or other subjects that you want to keep intact. Adjust the Fuzziness slider until the area of the selected portion is closest to that of the vignetting itself. Note that the selected areas appear white in color (see below).

Click on OK and the selection will be visible in the image itself. Leave this as is and click on the Add Layer Mask icon in the Layers pallete (it's the rectangle with a white circle inside of it). You could also go to Layer > Add Layer Mask > Reveal Selection. The selected area will appear on the mask. To make the mask active, Alt-Click on it (see below).

You can now make adjustments to the layer mask to make it more usable. I'd suggest blurring it (Filter > Blur > Gaussian Blur) and using Brightness/Contrast (Image > Adjustments) until you are satisfied with the results. You will end up with something similar to the image below:

At this point is when you will need to make the layer's image active by clicking on it. You can then use Curves to darken the masked region of the image until it blends smoothly with the background.

In this particular image, there is still some green hue in the center and bottom. I discarded the layer mask and went back to Select > Color Range to select the tinted area and adjust the green channel using Curves.
As a final touch, you could flatten the image (under Layer > Flatten Image) and apply some finishing touches to the overall image. It's important to save your work under a new name. You may want to keep the original image for comparison (see below).

Before Correcting for VignettingAfter Correcting for Vignetting

P.S. For those of you who are very familiar with Photoshop, you can create an Action to automate much of this process.

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