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Sig-R2

grizy zeropoints are within 0.05 mag of expectations. [DECam TD-7] in docdb#806 provides a specification for detector QE ([g,r,i,z]:[60,75,75,65%]), [DECam TO-15] provides a specification for average filter transmission of >85% in griz.

Prerequisites

  • Photometric conditions
  • Spectrophotometric standard(s) with predicted DES grizy magnitudes known with <0.02 mag uncertainty.

Here are candidates for SV supplied by Douglas Tucker: [currently these based on PreCAM filters, to be updated soon to DECam filters]

Name RA DEC g r i z Y
2M003618 00:36:16.27 +18:21:13.00 --.--- 19.087 16.641 15.023 14.432
C26202 03:32:32.88 -27:51:48.00 16.701 16.336 16.244 16.237 16.258
GD50 03:48:50.19 -00:58:32.00 13.816 14.360 --.--- --.--- --.---
2M055914 05:59:19.40 -14:04:49.40 --.--- 22.735 20.295 17.015 16.004
LTT9491 23:19:35.38 -17:05:28.50 14.013 14.199 --.--- --.--- --.---
FEIGE110 23:19:58.40 -05:09:57.00 11.557 12.127 12.495 12.815 12.974

The best candidate for SV appears to be C26202.

  • DECam zeropoints

Here are values for the DECam zeropoints calculated by Huan Lin [email from 11 Sept 2012] based on the latest transmission curves from docdb#6229. The u-band count rate is from the data on docdb#6600.

Filter Zeropoint
u 92
g 590
r 588
i 566
z 446
Y 159

The units for the zeropoints are electrons per second for an AB=20 mag object.

Procedure

  • Observe one or more spectrophotometric standard stars in grizy. They should be observed with 5-10s exposures (to eliminate shutter timing uncertainties) and near zenith (to eliminate need to know airmass coefficients), unless these quantities are already well established. Use at least a 3-position dither pattern per filter to insure that the star measurements are not compromised by bad pixels, cosmic rays, etc.
  • Process these images with bias subtraction, overscan, and perhaps flat fielding.
  • Measure standard(s) with aperture photometry (e.g. with the IRAF phot task) and derive the zeropoints, e.g. g0 = g - gi where g0 is the g-band zeropoint, g is the apparent magnitude in g-band of standard, and gi is the instrumental magnitude.
  • Use relative sky brightness across the focal surface to confirm there are no gross deviations between chips, as the star(s) will likely only be on one of them.

Verdict

Compare the measured g0, r0, i0, z0, and y0 zeropoints to the expected values g0e, r0e, i0e, z0e, y0e.

Failure is if any measured zeropoint deviates by more than 0.05 mag from the expected value in the sense of a smaller zeropoint.

Consequences

If the zeropoints suggest the throughput is lower than expected, quantify implications for survey and discuss with the camera team.

Result: Passed

An extensive series of photometric standard stars were observed during the commissioning and SV period. These indicate that the throughput is within 5% of expectations. The SV Photometry page contains a summary of zeropoint measurements from select nights.

Other Notes

Further spectrophotometric standards are available from docdb#6203

DECam system throughput data are from docdb#6229

There is an ETC for DECam at the NOAO DECam Web Page