I have taken a very quick look at the first Mark III image that Tom Droege has released. My notes are sketchy, but may help others with the data.
You might also want to read my notes on a flatfield vector from an early Mark III image.
First, I have modified an earlier program I wrote to convert the Mark III data to FITS format. You might want to look at this ANSI C translation program, which I call markiii2fits.c.
Once I converted the image to FITS format, I could display it. Here's a small image --- you can click on it to get a larger one. The orientation is North up, East to the left (so the scan went from right to left).
You can see very clearly a variation in brightness from top to bottom of the image. There appears to be a bit of vignetting across the field, with the center areas more sensitive to light than the top and bottom (north and south) edges. In addition, the overall levels drop slightly from the beginning of the scan (right side = west) to its end (left side = east).
I don't know enough about the dark current to subtract it properly, so I just tried to do some cosmetic flatfielding by first making a 1-D vector formed by the median along columns. Dividing the raw image by this vector alleviated the vignetting. I then made a second 1-D vector formed by taking the median along rows to remove the variation in brightness with time. Having done so, I had a slightly more pleasing image to work with (again, click on the image to retrieve a larger version).
I find the image covers a field between (1950) RA=7h 53.5m to 8h 05.5m, Dec=-3.5 to -0.5. The bright star near top center is 28 Mon, a star that varies between V=4.68-4.70 with a period of about 0.2 days. The bright star near the lower-left (south-east) corner is SAO 135530, which has a magnitude V=6.8. SAO stars are, in general, prominent in this image.
To get some idea of the limiting magnitude, I made a closeup around 28 Mon, about 50x50 arcmin in size.
I compared this image to the HST Guide Star Catalog and matched up a few stars. Here's the info on 3 particular stars:
ID mag RA (J2000) Dec
------------------------------------------------------------------
box on left 484601744 12.0 8:02:32.9 -01:31:41.6
box on right 484600106 11.1 8:00:08.4 -01:11:22.2
circle double 12.4+13.7
Based on this image, I estimate that the faintest objects that can be detected are V <= 12.5, and that a more reasonable limit for object with decent signal-to-noise is about V = 11.8. But these are rough numbers.
Note that 28 Mon (V=4.7) is saturated, as is SAO 135530 (V=6.8). However, a fainter SAO star (V=8.6) is not. Also note that 28 Mon has a bleed trail about 14 pixels long. SAO 135530 shows no bleeding at all.