WFCAM photometric calibration

Please send any comments on this document to Simon Hodgkin.

Introduction

The WFCAM photometric calibration concerns the conversion from detector counts to Vega magnitudes in the MKO-NIR system and transformations into other photometric systems. The goal is to measure photometry to 2% accuracy (UKIDSS science requirement).

Looking to produce two papers:

  1. modeling of the end-end system (before WFCAM goes on the telescope) - synthetic colours of representative astronomical objects.
  2. empirical measurement of the WFCAM photometric system (after commissioning/on-sky characterisation).

WFCAM characterisation plan

Once commissioning is completed, we'll complete the WFCAM on-sky characterisation. An agreed set of tests has been drawn up. The tests include the measurement of the following:

  1. detector noise properties
  2. microstepping test
  3. sky emission
  4. fringing
  5. sensitivity
  6. background limit
  7. cosmic rays
  8. persistence
  9. flatfield
  10. scattered light: meso-step field, effect of bright stars
  11. area calibration: chip-chip/channel-channel
  12. astrometry
  13. guiding
  14. photometry: set up secondary standard fields

Primary Standards

JAC are observing standards (the FS and Persson stars) with the Mauna Kea consortium filter set in UFTI (Simons and Tokunaga 2002, Tokunaga et al. 2002). WFCAM will use the same JHK filter system. There are more than 100 UKIRT standards with (JHK)MKO-NIR which will not saturate in a 1s WFCAM exposure (about 50 for a 5s exp). Preliminary results show persistence effects are small (2e-4 after 20s). The UKIRT standards therefore make excellent primary standards for WFCAM. Y,Z and narrow-band filters require extra work.

Secondary Standards

I have chosen fields that fulfil the following requirements:

leading to choices:
  1. Around UKIRT FS - 100s of stars - primary standard measured simultaneously
  2. Near Galactic Plane (5h45+18, 7h15+00, 17h50+00, 20h30+18) 1000s of stars, avoid worst crowding
  3. Globular clusters (NGC5053, M3) - Horizontal branch for blue stars - but need to avoid small dense cores.

A total of around 100 fields selected - mostly equatorial with around 7 in north and 7 in south - need further refinement to make sure there is nothing big and bright on the WFCAM/VISTA footprint. Nearly all these fields are centred on a UKIRT FS.

Further Questions

How photometrically stable is MKO, e.g. with humidity? How does extinction vary with time on a wet vs dry night? Using UFTI data spanning two years I've found that both photometric zeropoint and extinction are independent of humidity to first order.

Nightly Calibration

We must measure enough standards every night to ensure that the photometricity of a (fraction of a) night can be derived from the data, i.e. of the order of hourly. It will not be under the observers control - this will be under the control of JAC.

Overheads
e.g. ( (3x5s) +20s ) x 5 filters + slew + acq
5s S/N=100 J=15 (5 min total)
So it will take about 5 mins to observe a standard field in 5 filters with WFCAM. We need to think about how the standards MSBs should follow the target MSBs. Should we do standards in all filters even if we spend a night observing in only K?