WFCAM Photometric Calibration Simon Hodgkin ++ 11/6/2015 Simon Hodgkin CASU Overview • Conversion from WFCAM counts to Vega magnitudes at airmass unity in the MKO-NIR system • The goal is.
Download ReportTranscript WFCAM Photometric Calibration Simon Hodgkin ++ 11/6/2015 Simon Hodgkin CASU Overview • Conversion from WFCAM counts to Vega magnitudes at airmass unity in the MKO-NIR system • The goal is.
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)
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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)
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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: 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
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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
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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.
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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?
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