Absolute Photometric Calibration Resource Needs

Number/name of Requirement or Goal: Cal-R2, R3, R6, R7, G7, G8-12

These requirements are all to insure that the absolute system spectral response is not anomalous due to some component flaw, and is stable to the extent expected from atmospheric variations. Cal-R7 checks that we are able to observe the fundamental calibrator The goal activities are to begin the careful derivation of absolute calibration and characterize the kinds of zeropoint variation that will occur so that the first-year calibration tactics can be refined.

Almost all of these are achieved by repeated observations of one or more fields of stars with known grizY magnitudes. This will have to be on Stripe 82.

DECam observations

  • Field(s) to observe: Somewhere on Stripe 82
  • Number, duration of exposures per filter: Each visit will be 2x100s observations in each filter
  • Dithering / tiling pattern: Dither by ~CCD scale, not critical
  • Sky conditions needed (seeing, photometric, moon phase, etc:): see below
  • Other timing constraints: Visit this field at every clear twilight (potentially 1 exposure per filter, shorter) and at least once during every clear night. On one night, visit once per hour to test zeropoint stability. Visit at least once during a non-photometric night to exercise software for placing zeropoints on non-photometric images.

Do we need more data to determine extinction coefficients each night?

Additionally, for Cal-R7, we will require observations of BD+17 field, possibly defocused, but preferably with shutter parameters adjusted to allow 0.1s exposures to be unsaturated without defocusing. Will need to verify that this shutter mode yields constant exposure time across vicinity of BD+17. This will need to be 4x0.1s (TBC ???) exposures in each filter. Probably will get done during Commissioning but needs to be done on a clear night when the Stripe82 field is also observed.

Also, a complete set of DECal flats across each filter, to be taken during a cloudy night.

Data processing needs

Which of the following products would you need to conduct the test:

  • Raw images available at some off-mountain site? No
  • Images with cross-talk, debias, trim, dome-flat correction? Yes
  • Images with an "illumination" or "star" flat applied for better relative photometry? Yes
  • Single-exposure object catalogs? Yes
    • with astrometric solution accurate to: 1 arcsec
    • and photometric zeropoint accurate to: This is what is being determined here
    • Only need bright stars, or also want galaxy magnitudes? Bright stars

Telescope or other engineering data

Will need RASICam, GPS, any other sky-monitoring information to be acquired during all these observations.

External data sets

*Stripe 82 stellar magnitudes and colors in grizY
*BD+17 spectrum
*Stellar spectral libraries

Codes and scripts for analysing the test data

  • Potential alteration to shutter parameters for BD+17
  • Code for derivation of zeropoints, color terms, and extinction coefficient given DECam object catalog and overlapping standard catalog.
  • Code to determine variation of zeropoints, color terms over course of SV.
  • Code to regress zeropoints against stellar FWHM
  • Code to derive photometric calibration for non-photometric images via overlap with photometric images
  • Synthetic photometry code for predicting color terms and BD+17 magnitudes.
  • Code for turning DECal flats into synthetic photometry corrections at any point on the array
  • Code for processing sky-monitoring data and correlating with observed zeropoints and color terms.


Are there real-time decisions to be made during the observations? Yes - whether it is clear enough to be considered a photometric night

How quickly can / must the data be available and analysed? 24-48 hrs for return of stellar photometry from images

Who will do it, and where? Can be off-site

Other important resources or things to note