WFCAM Photometric Calibration

1
WFCAM Photometric Calibration

• Simon Hodgkin

http//www.ast.cam.ac.uk/vdfs/docs/photom.pdf
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)
• 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)

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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 (time variable?)

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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 )
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Scattered Light in INT ?

• Landolt field centred on each chip (chip4 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

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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

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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)
• WFCAM uses the same JHK filter system
• gt100 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

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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-?, CO) require
  observations of flux standards first

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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
• Choose fields
• Spaced every 2 hours in RA
• Equatorial (good for VISTA)
• ?20 degrees (X1.0)

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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

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Long Term Strategy

• Repeat measurements of secondary standard fields
  (gt3 measurements per field)
• Monitor spatial systematics (esp. as WFCAM comes
  off/on)
• Monitor colour equations

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Possible standard fields

• Around UKIRT standards
• 100s of stars, measured simul. with primary std
• Mostly red
• Near Galactic Plane (5h4518, 7h1500, 17h5000,
  20h3018)
• 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

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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
5h45m18d (l190, b-6)
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Galactic Plane Field
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The Globular NGC5053
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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 gt100
  stars per chip (to J16)
• 64 equatorial, 13 northern, 12 southern
• further refinement required

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WFCAM/VISTA Standard fields
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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?

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UFTI standards

• data span Jan 2000 to Oct 2003
• only selected nights with gt4 stds in each of
  J,H,K
• delta AM gt0.5

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Humidity
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How many standards?
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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/N100 J15 (5 min total)
• 30s S/N100 J16 (11min total)
• Frequency
• Hourly - this will not be under the observers
  control
• Extinction
• Do we measure it nightly/hourly/at all ?