Processing: Cropping

A good point at which to crop the image is before any serious processing has been done. Smaller images are easier on your computing resources. Why crop at all? There are a number of reasons:

  • You're interested in only a portion of the image around your target
  • Your polar alignment or tracking wasn't perfect and during the course of data acquisition the field shifted significantly. Or you acquired your light frames over several nights and didn't compose the field exactly the same way each time. When this happens stacking may produced dark bands at the image edges. If these bands are allowed to persist, later processing may be affected. This is particularly true if the bands have different thicknesses for different channels, as is often the case.
  • Your image field wasn't adequately flat, producing star shapes that become visibly distorted near the field corners.
  • There are issues in parts of the image that you can't or don't want to correct and your only recourse is surgical removal.

If you're using OSC cropping is easy using the cropping tool found to the left of the work area. Can't see the tool bar? Window > Tools will make it visible, and it can be docked or free floating, too.

If you are multi-channel imaging each channel that must be cropped identically. The easiest way I've found to do this is to use a couple of the scripts that come with Photoshop.

  1. Load the files to be cropped into layers. This is done using File > Scripts > Load Files into Stack... Use the dialog box that pops up to specify the files you want to crop and click the OK button. Adobe will create an untitled image with layers containing your images. (Adobe Bridge can also put files into layers using Tools > Photoshop > Load Files into Photoshop Layers...)
  2. At this point the images are unstretched and may be too dark to determine how they should be cropped. If you don't need a stretch, skip to Step 5.
  3. Create a Curves adjustment layer using Layer > New Adjustment Layer... > Curves... with mode = normal, color = none and opacity = 100%. The reason we use an adjustment layer is that it is applied to all the layers below it.
  4. Apply an aggressive stretch like this:

  5. Use the visibility icon (the eyes in front of the layers) to turn them on and off and get a sense of how to crop.
  6. With at least one channel layer visible for guidance use the crop tool to make the crop. All the layers will automatically be cropped the same way.
  7. Make all the layers visible except for the adjustment layer (if you used one). If the adjustment layer is kept visible the cropped layers will be saved with the wildly aggressive stretch in effect.
  8. Save the cropped layers as individual images using File > Scripts > Export Layers to Files... Specify a folder into which save the files, a prefix for the file names, and the file format to use. Be sure that Visible Layers Only box is checked, then click the OK button and you're done.

An alternative way to automate crops that is often suggested is to record an action as you crop one channel and then replay the action for the remaining channels.

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