I think I'm starting to understand what's going on here. LinearFit does, indeed, work on two images of the same piece of sky. That will make it tricky - although not necessarily impossible - to use this tool to make one panel of an image match the other panel more closely.
It looks like Fabian Neyer and Juan had a conversation about this already:
http://pixinsight.com/forum/index.php?topic=4138.msg28827#msg28827From what I can tell, Linear Fit is designed to work with two images of the same piece of sky, which have been co-registered to each other (using something like Star Alignment). It looks at a pixel in the reference image, and asks `how bright is the corresponding pixel in the other image'? A large number of these comparisons are made, and we could imagine LinearFit as being like a person who's plotting the comparisons as points on an X-Y graph. Pixel values in the reference image might give the X-coordinates, and the corresponding pixel values in the `to-be-fitted' image would give the Y-coordinates.
Since CCDs are, in theory, linear in their response to light, we might reasonably expect that the graph would be a straight line, with some nominal amount of scatter around that line. I don't know if LinearFit does something like a linear least-squares fit, but it probably does something at least a little bit like that. Armed with a knowledge of the slope and Y-intercept of the line, it can transform the brightness values of the `to-be-fitted' image into the brightness values that they *would have had* if that image had been captured in the same way as the reference image. If either image has been stretched into the non-linear realm, then I assume all bets are off.
Given all this, I'm going to need at least some overlap between my frames - which of course I do have, since I'm mosaicking. I think a little jiggery-pokery with Star Alignment, and maybe cropping, can give me frames on which to perform LinearFit. Next, I would need to get LinearFit to tell me the operations it recommends performing on the `to-be-fitted' image. (Maybe those show up in the Process Console?) Then perhaps I could use PixelMath to alter the `to-be-fitted' image.
My remaining concern: Doesn't `Frame Adaptation' do all this? Is Frame Adaptation simply an under-the-hood version of LinearFit, applied to the overlap regions of the mosaic panels? If I'm using FrameAdaptation, and I'm still getting a bad-looking mosaic, maybe the gradients across and between my images make mosaicking hopeless.
Possible lesson that might come from all this: If it seems like a good idea to learn how to shoot and process a mosaic during bright-Moon time... errrr, maybe not so much.
- Marek
