WFCAM Photometric Calibration

1. WFCAM Photometric Calibration Simon Hodgkin ++

2. 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)

3. 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)

4. 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 )

5. 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

6. 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

7. 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

8. Secondary Standards • By defining standard fields we: • 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?

9. 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

10. 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. Nightly Calibration • 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 • 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

12. 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 • Mostly red • Globular clusters (NGC5053, M3) • Horizontal branch for blue stars • Small, dense cores • Open clusters (Pleiades, Praesepe) • Numerous, large areal coverage • Not many stars

13. NGC5053: 2MASS J 8arcminutes

14. NGC5053 source counts

15. The Globular M3

16. M3 Source Counts

17. Gal Plane: 1000 (red) stars 5h45m+18d (l=190, b=-6)

18. Galactic Plane Field

19. 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?