1. 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

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)
11/27/2018 Simon Hodgkin CASU

3. 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

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 )
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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
11/27/2018 Simon Hodgkin CASU

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)
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

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
11/27/2018 Simon Hodgkin CASU

8. 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

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
11/27/2018 Simon Hodgkin CASU

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
11/27/2018 Simon Hodgkin CASU

11. 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

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
Globular clusters (NGC5053, M3)
Horizontal branch for blue stars
Small, dense cores
Open clusters (Pleiades, Praesepe)
Numerous, large areal coverage
Not many stars
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13. NGC5053: 2MASS J
8 arcminutes
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14. NGC5053 source counts
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15. The Globular M3
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16. M3 Source Counts
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17. Gal Plane: 1000 (red) stars 5h45m+18d (l=190, b=-6)
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18. Galactic Plane Field
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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?
11/27/2018 Simon Hodgkin CASU