A large number of H only false sources with high chi^2 have been found . These sources are near the snr 10 threshold, and often have no frames with aperture photometry measurements over 3 sigma, and are mostly concentrated at a few positions in cross-scan, predominately in the southern hemisphere.
A typical H only source was followed up:
990208s_016 ra/dec = 89.249834 -32.495682 h_m = 14.934 h_psfchi = 7.83
rd_flg = 010 ndet = 000500 j_pix_flg = 0105
This false source was found to be due to a hot pixel (206/59) adjacent to a patch of 6 masked pixels. The following image show the offending frame (hf0160140) on top of the coadd with the false source position marked on the coadd:
This hot pixel was not detected in frame 140 due to being adjacent to masked pixels, and thus was not identified or blanked as a solo. The hot pixel was detected in the coadd, and extracted by PROPHOT. The only frame with a detection > 3 sigma was not photometered due to the masked pixels in the aperture and thus n/m was 0/5. These false sources are easily identified by the combination of cross-scan location, high chi^2, and n of 0.
The coadded image bump is off-center because no values were interpolated from that frame when the kernel was centered on the hot pixel due to the adjacent dead pixels. The only effects of the hot pixel were when it was on the edge of the kernel. PROPHOT uses all unmasked pixels, and got the full effect of the hot pixel.
This particular hot pixel, responsible for most of these H only sources, is bistable during scan 990208s_016 as shown in this plot. The level shift is about 600dn.
This pixel displays similar behavior in R1, but with a reduced amplitude of about 50dn.
There are likely to be instances of this type of problem in J and Ks, which may be responsible for other single band sources.
This page last updated on Nov 5, 2002.
Gene Kopan - gene at ipac.caltech.edu
