Flat-fielding technique exists which produces galaxy magnitudes that vary by less than 0.02 mag RMS when measured on distinct exposures with large dithers.


  1. Single-epoch (or deeper) exposures of truth field, e.g., on Stripe 82 or in CFHTLS W1. Undithered between grizY filters, and under photometric conditions.
  2. Multiple single-epoch (or deeper) exposures of any field, taken using standard DES tiling offset pattern (e.g., 10 tilings per filter), under photometric conditions.
  3. Corresponding standard DESDM single-epoch processing of the above data, resulting in SExtractor catalogs with model fitting photometry and detmodel colors.


Use IDL code (to be written) to carry out the following:
  1. From truth field exposures, derive star flat and color term corrections to transform observed galaxy magnitudes on each DECam CCD to a TBD fiducial DES system (e.g, average over all CCDs), using the truth field system (e.g., SDSS) as an intermediary.
    • Use of truth field exposures intended as a quick way of constructing star flat and color term corrections, as opposed to more cumbersome way using extensive grid of offset DECam exposures.
    • Use DESDM model and/or auto mags, plus detmodel colors.
  2. For multiple tiling-offset exposures, apply above transformations to derive galaxy magnitudes in the fiducial DES system, then compute residual magnitude offsets relative to the mean for each galaxy, and finally average these residual offsets, as a function of CCD number.
  3. Plot average residual magnitude offsets vs. CCD number, and compute the rms of the average offsets.


Motivated by science requirement R-10, verify that the resulting rms over the CCDs of the residual magnitude offsets is < 0.02 mag. May need to first subtract off in quadrature the photometric calibration errors inherent to the truth field data itself (e.g. ~2% for SDSS).


If the above is not met, we can try the following:

  1. Check sky background values and photometric zeropoints in the data to look for signs of non-photometric data, and remove any from the analysis.
  2. Compare star flat and color terms derived from galaxy photometry here against those used in tests Cal-R1,R2,R3,G7 to look for signs of any inconsistencies/problems.