[Shiraz (Ray)]

Have you actually tried binning the 8300 yet? Got to be worth a try, although I have read in a few places that it has a bright star overload problem when binned, due possibly to limited capacity in the transfer registers - but of course that may be just another of the self perpetuating myths that populate the web. If on-chip binning presents problems, do it in software afterwards - not as good, since you will get extra read noise, but would fix any overload issues and still give a significant gain in sensitivity.

otherwise your options are limited to one of the 7-9 micron pixel Kodak chips. The 4000 or 11002 for example may be relatively affordable from the better second tier makers (eg Atik, Starlight Express). Read up on RBI before buying one though - it can affect some Kodak chips. (edit: but apparently not the interline transfer devices) You need to find an expert on Kodak chips.

Or to throw in something totally from left field, the only other chip that I can think of with big enough pixels is the QHY8/SX M25C, which is a sensitive OSC with usable Ha capability. It would give you a decent field of view and 0.85 pixel scale. would be ideal if was mono, but it might still be just worth looking at if the Kodak based cameras are totally out of reach.