Robin Laing writes:
> One thing that I have read is making multiple exposures with digital 
> cameras and then adding the photos together.

One common operation is "stacking": as you add layer N to the image,
make the layer mask's transparency be 1/N.  So the first layer is
the background at 100%, the next layer goes in at 50%, the next at
33%, etc.  This enhances the contrast of a bunch of short exposures
without enhancing the noise much; it apparently also sharpens
lunar/planetary images, by reducing the effect of temporary bad
seeing in one part of the image.

I don't know of a gimp plugin to do stacking, but it would be fairly
trivial to write.  (I'm not really an astrophotographer myself and
have never stacked more than four images, so I didn't look very hard
for a plugin, nor bothered to write one.)

Of course, you have to make sure all the images are accurately
aligned (easy if you have pinpoint stars, not so easy if you're
shooting something with soft edges like Jupiter).  2.2's transform
tool previews should make this important part a LOT easier.  

It would be a bit easier still if there were a way to alternate
between rotation (transform tool) and translation (the move tool)
while previewing without having to actually do the rotation (there's
presumably a quality loss every time you free-rotate an image) but
the only way I've found is to remember the rotation amount in the
transform tool, cancel, select the move tool, move the layer, then
transform again and type in the rotation where you left off.  That
comes up a lot with panoramas, too.  Anyone know a better way to
combine rotation and translation?

Though with a real astrophotography CCD and a rock solid mount
you may not need any rotation/translation.

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