Shift Processing Algorithm

This procedure is for images with bloated stars or stars that are elongated due to tracking errors. It is based on a method suggested by Bruce Johnston in a discussion on the SBIG Yahoo group. It is clearly a convolution filter, resulting in information loss. This can be seen in the loss of detail in the galaxy arms in the examples below. Nevertheless, where bloating or streaking are otherwise seriously damaging an image, and deconvolution is not an option, the technique appears to offer a chance to resurrect an otherwise useless image.

Procedure:

  • The image is imported into Photoshop.
  • It is resampled by 2X or 4X.
  • The background layer is duplicated four times (for a total of five layers).
  • One of the new layers is marquee selected (control-A) and shifted one or more pixels.
  • This is repeated with each of the new layers, for up, down, left, and right.
  • The layers are displayed using the Darken option.
  • The layers are flattened.
  • The image is resampled back to its original size.

Here is a Photoshop 6 Action that will automate 1.0 pixel and 0.5 pixel four-way shifts (for bloated stars).

Raw image
1. Raw Image
Uniform 1 pixel shift
2. Shifted 1.0 pixel L/R
Non-uniform shift
3. Shifted 0.5 pixels U/D, 1.0 pixels L/R
Non-uniform shift
4. Shifted 0.5 pixels U/D, 1.5 pixel L/R
Raw Image
1. Raw Image
Shifted 1 pixel
2. Shifted 1.0 pixel U/D/L/R

© Copyright 2002, Chris L Peterson. All rights reserved.