This has been brought up before (maybe even by me? ). It turns out it would be quite a bit of effort to extract it into a stand-alone application, or that is what I recall hearing. I do agree though, it is a good one now, and quite snappy, too.
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Er this is embarrassing, my first paragraph only states the existing implementation. I meant to say that first the KStars interface would need to be hidden, eventually, and second that the program instance would need to be reused for each new image opened, instead of creating a new one each time.
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Your proposal of a command line switch for kstars would be perfectly acceptable, if it can launch quickly (on my machine, kstars does not launch quickly), and if it's easy to associate .fits files with a program that needs a command line switch (without having to resort to a shell script). The second requirement can easily be satisfied by making a symlink (or hard link) of fitsviewer to the kstars executable, then when kstars starts up, it can check to see if it was launched as kstars, or as fitsviewer.