Repeatibility Tests

Summary: The photometric repeatability of galaxies seen toward the Abell 3558 galaxy cluster (z=0.048; Glat = 31 deg) demonstrate that 2MASS satisfactorily achieves the Level-1 photometric science requirements. The most accurate measurements are made with the 7" radius circular apertures, which are the optimum mags for color information for the fainter (i.e., smaller) galaxies. The most versatile measurements are the isophotal mags, whose repeatiblity RMS also satisfies the science requirements. The isophotal mags, however, underrepresent the "total" flux of a source by ~10-20%. The Kron mags, by design, are a better measure of the total flux (but still systematically underrepresent the "total" flux by ~8%), although the RMS scatter is considerably larger than the isophotal mags. We recommend that the elliptical isophotal mags be used as the "default" mag for most purposes, while the "total" mags be used as the measure of the total flux coming from the source.o

a. Sample

Derived from a large repeated-scan data set, the photometric repeatability of the primary 2MASS apertures is presented. The apertures include the circular 7" radius, 20 mag arcsec-2 isophotal, Kron and total (extrapolated) versions. See Large Aperture Photometry: Profile Fitting, Isophotal & Total Mags.

A number of repeat scans of Abell 3558 were acquired during the month of April 2000 to complement the set taken in March of 1998. Scans compiled:

The total area is depicted with this Areal Coverage plot. The scan-scan overlaps of the Abell 3558 core were repeated over 30 times, while the outskirts and most of the cluster area were covered over 15 times; hence, there is more than enough repeats to provide excellent statistics.

b. Detection Repeatability

c. Circular, R=7" Aperture

d. Elliptical-Shape Fit

The galaxy is assumed to be symmetric and elliptically-shaped (again, reasonable assumptions for most resolved objects). We also assume that the shape of the galaxy is preserved from low to high radii -- a crude approximation that is needed for robustness and processing speed. The projected shape of a galaxy is determined at the 3-sigma isophote. The basic algorithm is detailed in Jarrett et al. 2000. An improved method for crowded fields has been deployed for the final 2MASS data processing; described in Improved Ellipse Fitting and Isophotal Photometry in Crowded Fields. The resultant axis ratio and position angle represent the galaxy shape.

e. Isophotal Photometry

f. Kron Apertures

g. Total Mags

h. Notes

[Last Updated: 2002 Jul 15; by Tom Jarrett]