If you use anything but stars scalable to magnitude you are going to
get a helluva mess even in fields as small as a quarter degree.

SDSS data release 4 goes down to mag 24, fainter than the USNO A2.0
and/or USNO B1.0 stars you may also be plotting, that's a lot of stars
at times, even in very small regions, even if they have a tendency to
identify nearly everything fainter than mag 20 as a galaxy.

Cheers

John