WFCAM Photometric Calibration Main points: Long term strategy Actual plan Nightly observing strategy Dominant effects to monitor/measure: Spatial systematics Colour effects Chip-to-chip variations Extinction Time dependence ? Airmass dependence Choice of fields Primary standards Secondary standards Mechanics Funny filters Simon Hodgkin ++ 11/27/2018 Simon Hodgkin CASU

Overview Conversion from WFCAM counts to Vega magnitudes at airmass unity in the MKO-NIR system The goal is to achieve this to 2% accuracy (UKIDSS) J = ZPJ – Jinst – kJ (Χ – 1) strictly kJ = kJ’ + kJ’’(J – K) 11/27/2018 Simon Hodgkin CASU

Flies in the ointment spatial systematics extinction colour dependence scattered light flatfield errors variable pixel scale geometrical – vignetting/secondary reflectivity extinction colour dependence extinction time dependence chip-to-chip gain dependence chip-to-chip QE colour effects filter colour terms (4 filters) 11/27/2018 Simon Hodgkin CASU

Scattered Light in WFI WFI V-band observation of a Landolt standard field. Right hand panel shows the results of applying a quadratic correction term: Δmag = a ( ζ2 + η2 ) 11/27/2018 Simon Hodgkin CASU

Scattered Light in INT ? Landolt field centred on each chip (chip#4 twice – offset for clarity) Plot of distance from rotator centre with size of bar equal to delta magnitude No evidence for systematic variation in delta-magnitude with spatial position. Note that ESO WFI at CASS with multi-element corrector, while INT WFC at prime with fewer reflections 11/27/2018 Simon Hodgkin CASU

Primary Standards JAC are observing standards with the Mauna Kea consortium filter set in UFTI (Simons and Tokunaga 2002, Tokunaga et al. 2002) >100 UKIRT standards with (JHK)MKO-NIR which will not saturate a 1s WFCAM exposure (about 50 for a 5s exp) http://www.jach.hawaii.edu/JACpublic/UKIRT/astronomy/calib/fs_izjhklm.dat WFCAM uses the same JHK filter system 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 11/27/2018 Simon Hodgkin CASU

Funny Filters YZ define new passbands YZ can be bootstrapped into the Vega system Requires observations of primary standards over a wide range of colours to define the ZP Can then tie secondary fields into the same system Narrowband filters (H2, Br-γ, CO) require observations of flux standards first 11/27/2018 Simon Hodgkin CASU

Secondary Standards By defining standard fields we: Choosing fields: Beat down the noise Allow for variables Can measure spatial systematics Can measure colour-terms (can change) Calibrate 4 detectors simultaneously Choosing fields: Spaced every 2 hours in RA Equatorial (good for VISTA) δ=+20 degrees (X=1.0) Range of colours How many stars? 100s? 1000s? 11/27/2018 Simon Hodgkin CASU

Initial Strategy Observe UKIRT standards with each chip: Chip-to-chip gain Colour equations Meso-step star field across array: Spatial systematics Begin programme to define secondary standards 11/27/2018 Simon Hodgkin CASU

Long Term Strategy Repeat measurements of secondary standard fields (>3 measurements per field) Monitor spatial systematics (esp. as WFCAM comes off/on) Monitor colour equations 11/27/2018 Simon Hodgkin CASU

Nightly Calibration Overheads Frequency Extinction E.g. ( (3x5s) +20s ) x 5 filters + slew + acq 5s S/N=100 J=15 (5 min total) 30s S/N=100 J=16 (11min total) Frequency Depends on stability of a photometric night: hourly ? 2-hourly ? Extinction Do we measure it – nightly/hourly/at all ? Or do we observe standards close to targets 11/27/2018 Simon Hodgkin CASU

Possible standard fields Around UKIRT standards 100s of stars, measured simul. with primary std Mostly red Near Galactic Plane (5h45+18, 7h15+00, 17h50+00, 20h30+18) 1000s of stars, avoid worst crowding Globular clusters (NGC5053, M3) Horizontal branch for blue stars Small, dense cores Open clusters (Pleiades, Praesepe) Numerous, large areal coverage Not many stars 11/27/2018 Simon Hodgkin CASU

NGC5053: 2MASS J 8 arcminutes 11/27/2018 Simon Hodgkin CASU

NGC5053 source counts 11/27/2018 Simon Hodgkin CASU

The Globular M3 11/27/2018 Simon Hodgkin CASU

M3 Source Counts 11/27/2018 Simon Hodgkin CASU

Gal Plane: 1000 (red) stars 5h45m+18d (l=190, b=-6) 11/27/2018 Simon Hodgkin CASU

Galactic Plane Field 11/27/2018 Simon Hodgkin CASU

Pre-WFCAM observations? How photometrically stable is MKO, e.g. with humidity? How does extinction vary with time on a wet vs dry night? Enough data to investigate this? 11/27/2018 Simon Hodgkin CASU