Hi Hartwig,

(By the way, a warning: most image files tend to be far larger than
this. Sending them to me by private e-mail is probably a good idea.)

There were a couple of problems, but I eventually had success with
the following settings in Charon:

Report file name mpc.log Saturation point 65530
Cutoff point 0.900 Focal len (inches) 51.00
Image offset 1 Pix width (microns) 27.00
Scale tolerance 0.100 Pix height(microns) 27.00
Search distance 0.30 Image is uninverted
Observer code 0 Match to GSC-ACT
Time offset 0.0 File date gives start of exposure
Maximum residual 1.0 Mag limit: 14.00
Number stars used 10 E:\GJG\M48.ST6
White stars on black Object name: GSC 4859 1360
RA/dec in decimal seconds Simple centroiding
Video mode: 320 x 200 Max tilt angle: 0.00
Linear (first-order) fit Photometric band:
Centroid size 7 Save settings

The image has been "processed" a bit much from the standpoint of an
astrometry program, and that caused some of the trouble. Charon assumes
that zero-value pixels are "bad data", and this image is almost nothing
but zero pixels. An image offset of one makes sure that all pixels have
non-zero values. Do that, and Charon suddenly finds stars in the image.

The high 'saturation point' is necessary because it looks as if the
image has been stretched immensely.

The focal length stored in the image header is 51 inches; the actual
length, as measured by Charon, is 46.6 inches. (This sort of mismatch
is very common with Schmidt-Cassegrains. I forget the exact reason, but
the focal length cited by the manufacturer almost never matches the real
focal length.) If you reset the FL in the image header to a correct
value, you can drop the 'scale tolerance'.

The pixel width is not a typo; it really _is_ 27 microns, even though
the ST6 has 23x27 micron pixels. The image was processed to produce square
pixels. (Don't do this if you're planning on doing astrometry!)

After all this, I still had trouble until I did a run with a very
large "search distance". Turns out the image is off to one side of M48.
I chose a new target object of GSC 4859 1360, which is near the center
of the image. (Charon _can_ handle fairly large search distances, and
I know of at least one citizen who uses it as a "finder" system: take
a quick image, feed it to Charon with a large search distance to find
out where you are, then fine-tune the scope pointing. But this can be
a little tricky and very slow. In normal use, I suggest keeping the
'search distance' and 'scale tolerance' parameters as low as possible,
both to keep the processing fast and to discourage Charon from finding
incorrect matches.)

-- Bill