[Kstars-devel] USNO NOMAD catalog artefacts near bright stars

[Akarsh Simha]

The USNO NOMAD catalog seems to have artefacts near bright stars. I'm
guessing that these owe their existence to diffraction spikes and
glare, as is seen in the POSS plates. AFAIK, NOMAD uses the POSS
plates to find stars.

For comparison, consider the query on NOMAD here:
http://www.nofs.navy.mil/tmp/fchTdJ3EC_fch.html

The same field, with the same central star in the POSS [Blue] plates:
http://archive.stsci.edu/cgi-bin/dss_search?v=poss2ukstu_blue&r=18+12+24&d=-07+17+44&e=J2000&h=15.0&w=15.0&f=gif

Notice how some stars are spurious and some magnitudes are
over-estimated in NOMAD, possibly due to the diffraction spikes.

The same artefacts are found in KStars -- all bright stars usually
have a string of stars flanking them, that are usually aligned
approximately with the north-south direction. I guess these artefacts
are the result of diffraction spikes. It is therefore unlikely that
these disagreements with DSS images are bugs in KStars -- they are
more likely catalog artefacts.

Regards
Akarsh