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Cal-R2

"Color terms of griz DECam vs SDSS Stripe-82 photometry for stars with g-r<0.8 are within 0.02 (TBC) of values predicted from synthetic photometry using expected component wavelength responses."

This is primarily a test of how well we understand our filter passband system response curves.

Cal-R3 is essentially a duplicate of this test, but for the Y band.

Prerequisites

  • Nighttime test
  • Photometric conditions
  • Synthetic color terms based on the Pickles (1998) stellar atlas (equivalent) and the measured filter passband system response curves (preferably from a DECal spectrophotometric response scan, but, if necessary, built up from measurements of the various components of the system response -- e.g., the CCD QEs, Asahi-measured filter transmissions, etc.)

Procedure

  1. Observe at least one field within SDSS Stripe 82 in all DES filters.
  2. Match stars from the observed field with those in the standard star catalog (e.g, the STANDARD_STARS table in the DESDM database). (The observed stars should come from images that have already been detrended and cataloged by Sextractor; the observed stars fluxes/magnitudes may be either PSF mags or aperture mags).
  3. Plot the delta mag vs. color for each filter and perform a fit.
  4. Compare values of fitted color ("b") term coefficients with the synthetic values.

Note that, if we follow the default DES operations nightly standard star observing strategy, which calls for observations of 3 standard star fields in evening twilight, 3 in the middle of the night, and 3 in morning twilight, this procedure can be done "for free" by running the standard stars through the DESDM pipeline up through and including the Photometric Standards Module (PSM). For a given filter, the PSM matches stars from the observations to the standard stars in the STANDARD_STARS table and fits a first-order extinction coefficient for the night as well as photometric zeropoints and color term coefficients for all 62 science CCDs (see Appendix B of the document, "Plan for Calibration of the DES in the Early Years," which is in DES-doc#6584).

Verdict

The values of the fitted color ("b") terms must be within 0.02 mag/mag of the synthetic values.

Consequences

If the measured (fitted) values color terms are out of tolerance with their synthetic values, we will need
to identify the source of error. This will likely require involvement by the DECal team.

Other Notes

The form of the photometric equations for the DES follows closely that for the SDSS (see also Appendix B of DES-doc#6584:

g = -2.5log10(counts/sec) + a_g + b_g*( (g-r) - (g-r)_0 ) + k_g*X
r = -2.5log10(counts/sec) + a_r + b_r*( (g-r) - (g-r)_0 ) + k_r*X
i = -2.5log10(counts/sec) + a_i + b_i*( (i-z) - (i-z)_0 ) + k_i*X
z = -2.5log10(counts/sec) + a_z + b_z*( (i-z) - (i-z)_0 ) + k_z*X
Y = -2.5log10(counts/sec) + a_Y + b_Y*( (z-Y) - (z-Y)_0 ) + k_Y*X

where 

g,r,i,z,Y = the calibrated mags
(g-r),(i-z),(z-Y) = the calibrated colors
(g-r)_0,(i-z)_0,(z-Y)_0 = fiducial (reference) colors
a = photometric zeropoint
b = instrumental color term coefficient
k = first-order extinction
X = airmass

For this test, we will be using standard stars in the SDSS ugriz system, not standard stars in the DES grizY system.