Project

General

Profile

Gal-G6

Errors in SExtractor galaxy magnitudes and colors (1.5”, 2”, 3” diameter aperture mags, model mags, auto mags, detmodel colors) have been calculated from repeat DECam observations of a fixed field, and compared to SExtractor-estimated errors.

Prerequisites

  1. Consecutive pairs of undithered single-epoch (or deeper) exposures of the same spot on the sky, for each of grizY under photometric conditions
    • Consecutive and undithered exposure pairs are desired in order to: (1) minimize differences in observing conditions, and (2) avoid any effects due to the dependence of system throughput on position over the focal plane.
    • Typical supernova exposures should meet above conditions of consecutive, undithered exposures in a given filter, but need the data to be taken in photometric conditions to avoid changes in observing conditions
  2. Standard single-epoch DESDM processing of above data, resulting in SExtractor catalogs with model fitting photometry and detmodel colors.

Procedure

  1. Use IDL code (to be written) to compute 68-percentile errors (sigma_68) of pairwise magnitude measurements of galaxies, as a function of magnitude (for simplicity, choose mag_auto). Do so for aperture (1.5", 2", 3" diameter), model, and auto magnitudes, and for detmodel colors (g-r, r-i, i-z, z-Y).
  1. Also compute the average of the corresponding magnitude and color errors from the DESDM catalogs, for the same (mag_auto) magnitude bins.

Verdict

Compare sigma_68 values against the corresponding average DESDM errors and verify that they agree.

Consequences

If sigma_68 values and average DESDM errors do not agree, we can:

  1. Check for potential systematic trends in the pairwise magnitude/color differences vs. RA,Dec, which will likely indicate non-photometric conditions.
  2. Compute sky background, seeing FWHM, and photometric zeropoints in the data, in order to derive independent estimate of the magnitude/color noise to verify against the sigma_68 values and the DESDM errors.