XSC Sources with Null Default Mags
The XSC contains sources whose "default" mags have null values.
The XSC default mags are derived from the circular isophotal mags.
Nulls arise from corruptions in the circular isophotal apertures,
often associated with nearby stars, but predominentely due to
signal-to-noise limitations. Since most XSC sources are faint,
the default-mag "null" problem is mostly confined to faint
(SNR < 10) XSC sources.
a. Summary of XSC Nulls
Total number of XSC sources with nulled default mags: 7878 (0.5% of total)
To date, 20% of these sources have been
visually inspected. A large fraction are "false positives" or sources
which do not belong in the XSC (e.g., double stars). The results
are given here:
gals = 43.56%
stars = 7.27%
unk = 22.51%
artifacts = 8.61%
doubles = 15.50%
gfuzz = 0.38%
triples = 2.17%
b. S/N Distribution
The XSC S/N distribution (mag versus estimated uncertainty) for
verified galaxies
is shown below,
where the r=7" circular aperture photometry is used. Note that the
RMS mag uncertainties fall well above the level-1 science requirements
(i.e., roughly the S/N = 10 threshold).
c. Example
Example: XSC source at (ra,dec) = 199.386673 -17.96435022
default mags = null
j_m_7, h_m_7, k_m_7 = 15.255, 15.154 , 14.025
j_m_k20fe, h_m_k20fe,k_m_k20fe = 15.619, null, 14.359
(S/N ratios range from 4 to 7)
[Last Updated: 2002 Sep 19 ; by Tom Jarrett]