Where are my stars?

I've recently been trying out some of the more advanced features in XEphem, which is the big-iron of astronomy software, at least to amateur astronomers.

In this screenshot I've taken an image I made of Vega and used the really cool WCS solving feature of XEphem. You just tell it the approximate center coordinates of your image, and how big of an area your camera and telescope combination images. Next, it searches its star catalogue for stars matching the pattern in your image, and it figures out exactly where your image was taken.

With the image "solved" you can then overlay catalogue data and analyze your image.

So there is Vega in the constellation Lyra, and XEphem has marked the objects in my field. With this I can do things like determine how faint the faintest objects I imaged are, look for things that shouldn't be there, and other geeky things.

Of course, I complained on the XEphem mailing list that it's not identifying some of the objects near Vega. What gives? Well, a fellow from NASA by the name of Allan Meyer responded:

" The HST GSC guide star catalogs were laboriously compiled over many
years by scanning and digitizing rather deeply exposed Palomar
48-inch Schmidt plates. They were after fairly precise positions
down to about 16th magnitude.
Inevitably star images immersed in the diffuse fog of over-exposed
nebulosity could not be used, so there are big holes where extended
nebulae such as M42, and face-on spirals like M83, wiped out the
foreground stars. For the same reason GSC has gaps very near the very
brightest stars. Also note that they did not need to be complete to
a set mag. limit, they just needed a sufficient density of candidate
stars for the HST guide sensors, so the effective mag. limit (at
least for GSC 1) is higher in the galactic plane, more like about 14."

The star catalogue I'm using is the one they developed for the Hubble Space Telescope's guiding system, and he's saying it leaves out things around very bright objects, like Vega.

I suppose if it's good enough for the Hubble, it's good enough for me!