Project

General

Profile

Ops-R3

Establish Standard Survey Calibration Procedures
  • The pre-night observing sequence (bias, dome flats, darks) is defined.
  • The post-night observing sequence is defined.
  • The standard star observing sequence (pre-, post-, and potentially during the night) is defined.

Prerequisites

There are two goals for the nightly calibration observations
  1. Observe standard stars to perform an intermediate calibration of the survey for the first 1-2 years
  2. Obtain sufficient instrumental data on a nightly basis for final calibration of the survey, that is for the global relative calibration
The prerequisites for the intermediate calibration are
  • Finalize the calibration plan for the first 1-2 years (docdb#6584)
  • Identify suitable standard star fields
  • Calculate the expected apparent magnitudes of these stars for DECam (Note that standard star fields and their magnitudes on the PreCam system are available as docdb#6203)
  • Determine if dark frames are required, and if so on what cadence
  • Determine if dark frames may be obtained during the day (i.e. negligible light leaks)
  • Determine if bias frames may be obtained during the day (i.e. negligible light leaks)
  • Determine the stability of nightly dome flats
The prerequisites for the final calibration are
  • Intermediate calibration requirements (except for the intermediate plan and standard stars)
  • Finalize the calibration plans for the full survey
  • Software to perform the global relative photometric calibration
  • Identify and track atmospheric conditions for each image (RASICAM, GPS, TmCam)

Procedure

Intermediate calibration
  • Observe standard stars at the start and end of several nights at a range of airmass
  • Process these data and calculate a photometric solution for each night
  • Determine if this solution is stable (requirement is 2.5%, goal is 1.5%) with standard star observations at the start and end of the night
Final calibration
  • Observe a field to full depth with the final survey cadence, preferable with each tile on a different night and a range of airmass
  • Perform the global relative calibration with these data
  • Determine if this solution is stable (meets the 2% rms requirement or 1% goal)

Verdict

Failure of the intermediate calibration is defined to be rms errors of greater than 2.5% (goal 1.5%) for a series of standard stars observed at a range of airmass on a single photometric night.

Failure of the final calibration is defined to be rms errors of greater than 2% (goal 1%) for repeated observations of a single field with each tile obtained on a different night and at a range of airmass.

Consequences

The process for identifying errors in the intermediate calibration includes:
  • Reconsideration of the instrument calibration data (i.e. stabilty of biases, darks, dome flats?)
  • Quality of the records of atmospheric conditions
  • Quality of the standard star photometry (check with repeated observations of the same stars)
  • Data processing pipeline (check with alternative reduction software)
The process for identifying errors in the final calibration includes:
  • Confirm that the intermediate calibration meets the requirement for each individual night
  • Examine the global relative calibration software

Result

Here is a draft of the calibration data needed for the survey:

Daily Calibration Data:
  • Dome flats and bias frames
  • Standard star fields in evening and morning twilight: Three standard star fields at a range of airmass (at X=1, X=1.7-2, and an intermediate airmass)
  • Measurement of atmospheric water vapor
  • Record cloud camera data and RASICAM data
Monthly Calibration Data:
  • DECal (monitor filter performance)
  • Dark frames (monitor light tightness, cosmic ray rate)
  • Photon transfer curve (monitor gain, readnoise, linearity)
  • Observation of BD+17 in i-band
Other Recommended Monthly Activities:
  • Create a new fringe correction frame for z and Y to monitor changes and long timescale variations
  • Check the quality of the star flat with observations of a standard star field
  • Check the cross-talk coefficients
  • Check the bad pixel mask
Comments on calibration activities:
  • The dome flats appear sufficiently stable that only one set per night are necessary. A lower cadence would be fine to produce flat fields alone, although dome flats in all filters each afternoon are valuable because they provide a good shakeout of the entire system before the start of each night. They are also desired to create 'superflats' for the SN field processing.
  • The standard star fields should be changed as infrequently as possible to produce a single, consistent calibration set over the lifetime of the survey.
  • The Monthly Calibration Data and Other Recommended Activities should be obtained and analyzed as soon as possible after any relevant changes to the telescope or instrument (e.g. changes in baffling, readout electronics, any warm up and cool down cycle).

Other Notes

  • The Plan for Calibration of the DES in the Early Years is docdb#6584
  • Overview of the Calibration System is docdb#4378
  • The DES 5-year photometric calibration requirements are from docdb#20