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Hi Peter,
I think you can safely discard focus or seeing as a cause, simply due to the FWHM results you listed, you can confirm this yourself by looking at the FWHMs for the three stacked channels, I'll bet they are very comparable.
The listing you posted did not show the altitude of the individual frames, but again you can easily assure yourself that it isn't a factor (it would show up in the FWHM anyways).
The bgd count doesn't reveal much if anything about the focus or seeing, just the convolution of camera QE, filter bandpass and light pollution/natural sky brightness where the scope was pointed.
What is more important is the stellar profile. In Maxim there is a graph window which displays the line profile of star for all three colors simultaneously (for color images), and also a graph of intensity versus radius for each channel (you can do these graphs in excel if you can export the data in tabular txt format, but that's a pain). From this you can quantify the amount of halo in each channel independent of stretch or black point level. Don't have Maxim? send me the raw stacked FITS x3 in floating point or 16bit, cropped around the area of interest.
Now the FWHM calculation is not necessarily sensitive to the low intensity halo, but the aggressive stretching we use in processing to bring out faint nebulosity takes no prisoners. So here are some hypothesis to try:
1) greater reflection/scatter from CCD microlenses in the blue
2) reflection/scatter from telescope surfaces
3) reflection/scatter from filter surfaces
4) limited correction of objective lens in the blue
To confirm/deny these:
1) Use a different camera which has a different chip (e.g. KAF, KAI or DSLR, OSC) the latter two nicely avoid changes in seeing or altitude. You may already have images from this scope with another camera?
2) Not being a refractor guy I am unsure here, but I do know the design and AR coatings need to be right in order to avoid ghosts which can show up as haloes. Do others have the same problem with different cameras?
3) Borrow a blue filter from a mate and see if it goes away?
4) I doubt it is this one since you would think the FWHM would be affected, only pick as right answer if everything else is excluded!
HTH,
EB
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