WFCAM Photometric Calibration Simon Hodgkin ++ 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 http://www.ast.cam.ac.uk/vdfs/docs/photom.pdf 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) Looking to produce two papers: (i) modelling of the end-end system (ii) empirical measurment of the WFCAM photometric system J = ZPJ – Jinst – kJ (– 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 (time variable?) 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: mag = a ( 2 + 2 ) 11/27/2018 Simon Hodgkin CASU

WFCAM characterisation plan detector noise properties microstepping test sky emission fringing sensitivity background limit cosmic rays persistance flatfield scattered light: meso-step field, effect of bright stars area calibration: chip-chip/channel-channel astrometry guiding photometry: set up secondary standard fields 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) WFCAM uses the same JHK filter system >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 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

Secondary Standards By defining standard fields we: Choose fields: Beat down the noise Allow for variables Can measure spatial systematics Can measure colour-terms (can change) Calibrate 4 detectors simultaneously Choose fields: Spaced every 2 hours in RA Equatorial (good for VISTA) =+20 degrees (X=1.0) 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

standard fields for WFCAM fields centred on primary standards from Persson et al. (1998) and UKIRT FS (Harwarden et al. 2001, Leggett et al. 2003) equatorial strip + overhead (=1) =+20: MKO, =-25: Paranal used 2MASS catalogue to select fields with >100 stars per chip (to J=16) 64 equatorial, 13 northern, 12 southern further refinement required 11/27/2018 Simon Hodgkin CASU

WFCAM/VISTA Standard fields 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

UFTI standards data span Jan 2000 to Oct 2003 only selected nights with >=4 stds in each of J,H,K delta AM >0.5 11/27/2018 Simon Hodgkin CASU

Humidity 11/27/2018 Simon Hodgkin CASU

How many standards? 11/27/2018 Simon Hodgkin CASU

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. Overheads 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 Hourly - this will not be under the observer’s control Extinction Do we measure it – nightly/hourly/at all ? 11/27/2018 Simon Hodgkin CASU