WFCAM Photometric Calibration Simon Hodgkin 10252021 Simon Hodgkin

WFCAM Photometric Calibration Simon Hodgkin ++ 10/25/2021 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 – k. J (Χ – 1) strictly k. J = k. J’ + k. J’’(J – K) 10/25/2021 Simon Hodgkin CASU

Flies in the ointment • spatial systematics • • • 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) 10/25/2021 Simon Hodgkin CASU

Scattered Light in WFI V-band observation of a Landolt standard field. Right hand panel shows the results of applying a quadratic correction term: Δmag = a ( ζ 2 + η 2 ) 10/25/2021 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 10/25/2021 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 1 s WFCAM exposure (about 50 for a 5 s 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 (2 e-4 after 20 s) The UKIRT standards therefore make excellent primary standards for WFCAM Y, Z and narrow-band filters require extra work 10/25/2021 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 (H 2, Br-γ, CO) require observations of flux standards first 10/25/2021 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 Spaced every 2 hours in RA Equatorial (good for VISTA) δ=+20 degrees (X=1. 0) Range of colours How many stars? 1000 s? 10/25/2021 Simon Hodgkin CASU

Initial Strategy Observe UKIRT standards with each chip: 1. • • Meso-step star field across array: 2. • 3. Chip-to-chip gain Colour equations Spatial systematics Begin programme to define secondary standards 10/25/2021 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 10/25/2021 Simon Hodgkin CASU

Nightly Calibration • Overheads • E. g. ( (3 x 5 s) +20 s ) x 5 filters + slew + acq • • • Frequency • • 5 s S/N=100 J=15 (5 min total) 30 s S/N=100 J=16 (11 min total) 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 10/25/2021 Simon Hodgkin CASU

Possible standard fields 1. Around UKIRT standards • • 2. 100 s of stars, measured simul. with primary std Mostly red Near Galactic Plane (5 h 45+18, 7 h 15+00, 17 h 50+00, 20 h 30+18) • 1000 s of stars, avoid worst crowding • Mostly red 3. Globular clusters (NGC 5053, M 3) • • 4. Horizontal branch for blue stars Small, dense cores Open clusters (Pleiades, Praesepe) • • Numerous, large areal coverage Not many stars 10/25/2021 Simon Hodgkin CASU

NGC 5053: 2 MASS J 8 arcminutes 10/25/2021 Simon Hodgkin CASU

NGC 5053 source counts 10/25/2021 Simon Hodgkin CASU

The Globular M 3 10/25/2021 Simon Hodgkin CASU

M 3 Source Counts 10/25/2021 Simon Hodgkin CASU

Gal Plane: 1000 (red) stars 5 h 45 m+18 d (l=190, b=-6) 10/25/2021 Simon Hodgkin CASU

Galactic Plane Field 10/25/2021 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? 10/25/2021 Simon Hodgkin CASU