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Cal-R6

"Magnitude zeropoints in all filters are reproduced to <0.02 mag (griz) or 0.04 mag (Y) over the SV period on cloudless nights."

This is primarily a test of the stability of the throughput and gain of the DECam CCDs. It is also a test of the form of the nightly photometric equations (see Other Notes of Cal-R2).

Prerequisites

  • Twilight/nighttime test
  • Photometric conditions

Procedure

  1. Observe 3 standard star fields during evening twilight (between 18 deg and 12 deg twilight), 3 in the middle of the night (around 1AM local time), and 3 during morning twilight (between 12 deg and 18 deg twilight). For each set of 3, one should be at low airmass (X=1.0-1.2), one should be at high airmass (X=1.8-2.0), and one should be at intermediate airmass (X=1.4-1.6). At least one standard star field taken during the night should be in SDSS Stripe 82. Observe the standard star fields in all 5 DES filters (grizY), regardless of moon phase.
  2. Run the data through the Photometric Standards Module (PSM), which is part of the DESDM pipeline. (If the DESDM pipeline is not available, other code will need to be developed).
  3. Compare the magnitude zeropoints fitted by the PSM over the nights of the SV phase.

Verdict

The values of the fitted magnitude zeropoints must be within 0.02 mag (griz) or 0.04 mag (Y) over the SV period on nights determined to be cloudless.

Consequences

If the variation in measured (fitted) values for the magnitude zeropoints is out of tolerance, we will need to identify the source of error. This might require re-formulation of the DES photometric equations (see Other Notes section of Cal-R2).