In article ,
Martin Brown wrote:

I have to say that for picture of the moon and sun I would be inclined
to use the sharp focussed green channel as luminance and then tint it
using the chrominance information from the red and blue channels.

Can this be done in GIMP? If not, what program would you recommend?

It sounds like you are saying that even though the blue and red images
may extend beyond the edge of the moon as defined by the green image,
that information will be tossed--the image will be "trimmed" to fit the
green image.

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