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

• Completeness (internal repeatability) and reliability are satisfactory
• Circular, R=7" aperture photometry: SNR=10 Thresholds: J=15.1, H=14.1, Ks=13.5
• Elliptical isophotal K-band fiducial photometry: SNR=10 Thresholds: J=14.8, H=14.0, Ks =13.4
• Total (extrapolated) photometry: SNR=10 Thresholds: J=14.5, H=13.0, Ks=12.5
• Isophotal photometry is systematically ~10-20% fainter than the "total" measurement
• Kron photometry is systematically ~8% fainter than the "total" measurement

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:

980319s, 87-94, 101-105
980328s, 88-94
000406s, 54 - 58
000407s, 55-58
000408s, 48, 55-58
000411s, 55-59
000412s, 55-57
000419s, 44-48
000421s, 41-45
000422s, 42-46
000423s, 41-45
000424s, 42-46
000425s, 42-46
000428s, 49-50, 57-58

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

• detection repeatability
  white points are uncontaminated; green points are contaminated; red points are false extended sources (usually double stars)

  Key Points: Internal completeness and reliability satisfy the Level-1 Science requirements

c. Circular, R=7" Aperture

• R=7" Photometric Repeatability

  Key Points: SNR=10 Thresholds: J=15.1, H=14.1, Ks=13.5; note the slight overestimation of the estimated uncertainty for K-band (one culpret: stellar confusion noise)

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.

• Axis Ratio Repeatability for 3-sigma isophotal aperture,
  with a minimum radius of 7"
  meanJ(b/a) == mean J-band axis ratio for (0 < /ba/ < 0.75)
  sig = RMS in meanJ or meanK or meanS
  rmsJ(b/a) == statistical RMS in the b/a repeatability
  meanK(b/a) == mean K-band axis ratio for (0 < /ba/ < 0.75)
  rmsK(b/a) == statistical RMS in the b/a repeatability
  meanS(b/a) == mean axis ratio for the "super" (J+H+Ks) coadd
  rmsS(b/a) == statistical RMS in the b/a repeatability of the "super" (J+H+Ks) coadd
  

e. Isophotal Photometry

• Isophotal Radius Repeatability (20 mag/arcsec^2)
  ditto with zoomed scale

• Axis Ratio for 3-sigma isophotal aperture, with a minimum radius of 7"
  meanJ(b/a) == mean J-band axis ratio for (0 < /ba/ < 0.75)
  sig = RMS in meanJ or meanK or meanS
  rmsJ(b/a) == statistical RMS in the b/a repeatability
  meanK(b/a) == mean K-band axis ratio for (0 < /ba/ < 0.75)
  rmsK(b/a) == statistical RMS in the b/a repeatability
  meanS(b/a) == mean axis ratio for the "super" (J+H+Ks) coadd
  rmsS(b/a) == statistical RMS in the b/a repeatability of the "super" (J+H+Ks) coadd
  

• Elliptical K-band Fiducial Isophotal Photometry (20 mag/arcsec^2)

• Circular K-band Fiducial Isophotal Photometry (20 mag/arcsec^2)

  
  Key Points: SNR=10 Photometric Thresholds: J=14.8, H=14.0, Ks=13.4;
  The uncertainty in the isophotal radii is typically 5-10%;
  The axis ratio RMS is ~5%

f. Kron Apertures

• Kron Radius Repeatability, GALWORKS V3

• Kron Elliptical Fiducial Photometry, GALWORKS V2

• Kron Elliptical Fiducial Photometry, GALWORKS V3

  Key Points: SNR=10 Thresholds: J=14.9, H=13.8, Ks=13.0;
  The RMS scatter is larger for the V3 version -- the reason being that the Kron apertures are now allowed to grow larger (closer to the total radius, hence increasing the scatter).

• Kron vs. Isophotal Elliptical Fiducial Photometry, GALWORKS V2

• Kron vs. Isophotal Elliptical Fiducial Photometry, GALWORKS V3

  Key Points: The Kron V2 (older) version is systematically 0.05 mags brighter than the isophotal mags, with a (incorrect) trend in brightness. The Kron V3 (improved) version is systematically 0.05 - 0.15 mags brighter than the isophotal mags, with no systematic trends in brightness. I.e, the systematic trends are greatly reduced in the newer (V3) version compared to V2.

g. Total Mags

• Radius of Total Aperture, GALWORKS V3

• Total Integrated Flux, GALWORKS V3

  Key Points: SNR=10 Thresholds: J=14.5, H=13.0, Ks=12.5;
  The formal (estimated) uncertainties in the photometry (denoted with the purple dashed line) overestimate the true uncertainty at the faint end (J > 14th mag, K > 12th mag). Overall the repeatability in the "total" mags looks excellent.

• Isophotal vs. Total Integrated Flux, GALWORKS V3

  Key Points: The isophotal mags are systematically 0.1 to 0.3 mags fainter than the total mags, with a more typical value ~0.2 mag. Hence, we conclude that the isophotal mags systematically underepresent the total flux by about 20%. The large scatter is probably due to intrinsic properties of the galaxies (e.g., morphology). Note the mag trend seen in the J-band.

• Kron vs. Total Integrated Flux, GALWORKS V3

  Key Points: The Kron mags are systematically 0.05 to 0.2 mags fainter than the total mags, with a more typical value ~0.08 mag. Hence, we conclude that the Kron mags systematically underepresent the total flux by about 10%. Note the mag trend seen in J.

h. Notes

• We have eliminated inscan duplicates (which are meaningless for these tests); but we have included scan-to-scan dupes (which are meaningful).

• We treat sources that are "contaminated" separately from sources that are "clean". Contamination means that the photometry flags are tripped (non-zero), indicating that another source is nearby and was masked accordingly (with isophotal substitution to recover the lost flux).

• We apply the Gscore threshold = 1.4 as the primary star-galaxy discriminant.

• For small radii, we adjust the axis ratio such that the minimum semi-minor axis is 3 arcsec. This modification is needed to counteract the circularizing effects of the PSF. An example of a dynamically adjusted axis ratio is given here.

• Some examples are given here, including images (before/after star subtraction) and radial profiles.

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