WFCAM Photometric Calibration

Simon Hodgkin ++

4/30/2020 Simon Hodgkin CASU

Overview

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Conversion from WFCAM counts to Vega magnitudes at airmass unity in the MKO-NIR system

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The goal is to achieve this to 2% accuracy (UKIDSS)

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J = ZP J – J inst – k J ( Χ – 1)

strictly

k J = k J ’ + k J ’’ (J – K)

4/30/2020 Simon Hodgkin CASU

Flies in the ointment

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spatial systematics

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scattered light

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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)

4/30/2020 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 ( ζ 2 + η 2 )

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Scattered Light in INT ?

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Landolt field centred on each chip (chip#4 twice – offset for clarity)

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Plot of distance from rotator centre with size of bar equal to delta magnitude

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No evidence for systematic variation in delta-magnitude with spatial position.

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Note that ESO WFI at CASS with multi element corrector, while INT WFC at prime with fewer reflections

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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 4/30/2020 Simon Hodgkin CASU

Funny Filters

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YZ define new

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passbands YZ can be bootstrapped

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into the Vega system Requires observations of primary standards over a wide range of

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colours to define the ZP Can then tie secondary fields into the same

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system Narrowband filters (H2, Br γ, CO) require observations of flux standards first

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Secondary Standards

By defining standard fields we:

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Beat down the noise Allow for variables Can measure spatial systematics Can measure colour-terms (can change) Calibrate 4 detectors simultaneously Choosing fields:

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Spaced every 2 hours in RA Equatorial (good for VISTA) δ=+20 degrees (X=1.0) Range of colours How many stars? 100s? 1000s?

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Initial Strategy

1.

2.

3.

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Observe UKIRT standards with each chip: Chip-to-chip gain Colour equations

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Meso-step star field across array: Spatial systematics Begin programme to define secondary standards

4/30/2020 Simon Hodgkin CASU

Long Term Strategy

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Repeat measurements of secondary standard fields (>3 measurements per field)

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Monitor spatial systematics (esp. as WFCAM comes off/on)

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Monitor colour equations

4/30/2020 Simon Hodgkin CASU

Nightly Calibration

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Overheads

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E.g. ( (3x5s) +20s ) x 5 filters + slew + acq

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5s S/N=100 J=15 (5 min total) 30s S/N=100 J=16 (11min total) Frequency

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Depends on stability of a photometric night: hourly ? 2-hourly ?

Extinction

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Do we measure it – nightly/hourly/at all ?

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Or do we observe standards close to targets

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1.

2.

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4.

Possible standard fields

Around UKIRT standards

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100s of stars, measured simul. with primary std Mostly red Near Galactic Plane (5h45+18, 7h15+00, 17h50+00, 20h30+18)

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1000s of stars, avoid worst crowding Mostly red Globular clusters (NGC5053, M3)

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Horizontal branch for blue stars Small, dense cores Open clusters (Pleiades, Praesepe)

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Numerous, large areal coverage Not many stars

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NGC5053: 2MASS J

8 arcminutes

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NGC5053 source counts

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The Globular M3

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M3 Source Counts

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Gal Plane: 1000 (red) stars

5h45m+18d (l=190, b=-6)

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Galactic Plane Field

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Pre-WFCAM observations?

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How photometrically stable is MKO, e.g. with humidity?

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How does extinction vary with time on a wet vs dry night?

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Enough data to investigate this?

4/30/2020 Simon Hodgkin CASU