Remove low-frequency background

Question

I have a series of images which are vignetted - an artefact which I want to reduce from the images (see the smoothed and down-sampled image).

My approach is to subtract a heavily-smoothed version from the actual image to remove the inhomongeious background brightness, however the results of that were far from satisfactorily. Changing the black point of the image obviously does not work either.

Is there a way I miss to deal with such problem?

Information on the lens vignetting behaviour are not available beyond what I see on images.

An approach I can imagine to work is to subtract a frequency-filtered version of the image where I only retain the very low radial-symmetric frequencies from the original, thus keeping the high-frequency content of stars and galaxies intact. Yet I fail to find any way to do so safe than writing such plugin myself. Do I miss any approach with gimp (or another programme which I can get working on linux)?

Answer 1

Two techniques:

The old technique (Gimp or Photoshop)

• Duplicate your image
• Set the top layer to Grain extract
• Apply a Gaussian blur to the top layer, enough to make vignetting disappear, but before halos appear around bright objects.

• The result will have a mostly gray background so use Curves or else to restore the blackpoint and add a bit of contrast.

A more modern technique (Gimp does it, I don't know if PS does it...)

• Apply wavelet decomposition to your image. This transforms it into a stack of layers, each layer representing a level of detail. The bottom one (aka "Residual") is the very low frequency stuff.
• Bucket fill the Residual with black
• Optionally, duplicate the fine detail layers to give some "pop" to the stars

Answer 2

Old school method: stop down your lens a bit.

Many relatively wide-angle lenses (say, 35 mm on a 24x36mm frame or sensor) will vignette when wide open, but will cover the frame evenly when closed down one or two stops -- and some lenses that "don't cover the frame" (like a common 105 mm made for 6x9 cm film used in a 4x5 inch camera) will gain significantly in coverage when stopped down to around f/11 or smaller.

since your image looks very much as if you're sky-tracking, the option probably exists to reduce your aperture by one or two stops, and increase exposure time accordingly. Assuming the center brightness isn't due to air transparency problems or light pollution, this will likely improve the overall evenness of exposure.