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Home > Articles > How To > Advanced > How to Subtract Dark Frames with Photoshop, Photoshop LE, or Photoshop Elements

How to Subtract Dark Frames with Photoshop, Photoshop LE, or Photoshop Elements
By Michael Covington - 11/25/2004

Figure 1
Whenever you take a long exposure with a digital or CCD camera, there will be a few "hot pixels" which register lighter than they ought to, because of electrical leakage. They look like stars, but they are sharper and often more vividly colored.

To get rid of hot pixels, take a picture just like the first one, but with the lenscap on. This is called a "dark frame" and should be taken during the same session, while the camera is at the same temperature, and all settings should be the same.

Then subtract the dark frame from the original picture. You can do this with almost any astronomical image processing program, but it's also easy to do with Photoshop.

The following instructions apply to any version of Photoshop 5 or later, including Photoshop 5 LE and Photoshop Elements.

(1) If possible, work with camera raw files (.RAW or .CRW) and convert them to TIFF using your camera's software or a camera-specific Photoshop plugin. Photoshop can open
a lot of raw files by itself, but it doesn't always know what adjustments to apply, and results can look bad. If your camera only produces JPG files, that's not a fatal
problem. Keep going...

(2) In Photoshop, open both the picture and the dark frame.

(3) Go to the dark frame image and click on its title bar.

(4) In the toolbar, choose the Marquee tool (dotted box).

(5) Right-click on the dark frame image and choose Select All.

(6) In the menu at the top of the screen, choose Edit, Copy.

(7) Go to the picture image and click on its title bar.

(8) At the top of the screen, choose Edit, Paste. This pastes the dark frame over your original picture as a new layer. Don't panic - your picture is hidden, but not gone!

(9) At the top of the screen, choose Window, Show Layers. (This may already be set.) Then look at the layer blending controls shown in Figure 1.

(10) In the layer blending controls, change Normal to Difference. Voila - your picture is back!

(11) Set the opacity to 50%, or a different value as needed if the dark and light exposures are not a perfect match.

(12) At the top of the screen, choose Layer, Merge Down (or Flatten Image).

(13) Save the resulting picture.

Figure 2
Figure 2 shows what dark frame subtration accomplishes.
At the left is a star field with some hot pixels in it; at right, the corrected image.

A useful variation might be to blur the dark frame
very slightly before using it, to cover up some nonlinearity in the camera.

Michael A. Covington
Author, Astrophotography for the Amateur



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