Towards Creation of a JWST
Astrometric Reference Field:
Calibration of HST/ACS
Absolute Scale and Rotation
Roeland
van der Marel
Jay Anderson, Colin Cox, Vera Kozhurina-Platais, Matt Lallo, Ed Nelan
Astrometric Calibration
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2 Translations (x, y)
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JWST: Guide stars + Target acquisitions
Scale
Rotation
2 Skew terms (scale and rotation)
Higher-order distortions
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6 linear
parameters
JWST: Observations of Astrometric Reference Field
JWST Astrometric Reference Field
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Self-calibration of astrometry using JWST
observations not feasible
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Requires observations at multiple ORIENTs (interval
of months)
Observatory characteristics might change between
such observations (WFS&C every 2 weeks)
Alternative: observe astrometric reference field
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In JWST Continuous Viewing Zone
Adequate stellar density
Low proper motions
Large
Magellanic
Cloud (LMC)
JWST Astrometric Requirements
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Mission Requirement (MR-120)
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After calibration, the field distortion uncertainty … shall
not exceed 5 mas, 1-sigma per axis
Motivation: need to prepare NIRSpec observations
based on NIRCam images
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Accurate higher-order distortion correction
 LMC field observed with HST/ACS mid 2006 (Diaz-Miller et al.)
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Calibration to same relative scale
Desire: also accurate absolute scale + rotation
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Use of coordinates from other observatories
Accurate calibration of focal plane orientation of
instruments (w.r.t. star trackers)
 Calibrate absolute scale and rotation of HST/ACS (this talk)
HST/ACS Astrometric Calibration
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2 Translations
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Not relevant here
Scale
Desired accuracy ~ 5 x 10-5
(corresponds to 5 mas
Rotation
over 1-2 arcmin FOV)
2 Skew terms
Higher-order distortions
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Previously calibrated by Anderson
No known time-dependence
Calibration of HST/ACS
scale and rotation
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Observations of a field with good
astrometry  M35 (HST/FGS calibration field)
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Good relative astrometry (< 1 mas)
Good proper motions (< 0.2 mas/yr)
Poor absolute astrometry
Approach
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Build M35 catalog with good absolute astrometry
Observe M35 stars in this catalog with HST/ACS
Building an M35 Catalog
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Start with M35 FGS catalog in V2-V3 HST focal
plane (92 stars with proper motions from FGS
Science Team)
Cross-identify these stars with USNO UCAC2
on ICRS
Match stars using linear transformations
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RMS 21 mas per coordinate
Fractional scale uncertainty 0.6 x 10-5
Rotation uncertainty 0.6 x 10-5
HST/ACS observations of M35
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10 stars
V magnitude = 8 - 13
WFC , F658N
4 dither positions
2 sec exposures
Dec 2006
Measure positions and correct for higher-order
distortions using Anderson software
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(x,y) on Distortion-Corrected Frame (DCF)
Scale and Rotation of DCF
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Match stars using linear transformations with
FGS/UCAC2 Catalog
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RMS < 2 mas per coordinate
Fractional scale uncertainty 0.6 x 10-5
Rotation uncertainty 0.6 x 10-5
Some subtle corrections required
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Differential velocity aberration affects scale
Sky looks distorted when projected on a plane
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Orientation at ACS/WFC is not the same as at the V1 axis
Final results
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s = 0.0497248 arcsec/pixel
b = 177.7612 degrees
Time Variation of HST/ACS
Linear Distortion Terms
rotation
scale
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5 years of repeated
47 Tuc imaging
(Anderson 2007)
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Scale quite stable
Rotation shows ~0.003
deg non-repeatability
Skew varies linearly
with time
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M35
Confirmed by M35 data
rotation
scale
Checks Performed to Assess
Systematic Errors
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Consistency between UCAC2 to GSC2
Residual skew terms between catalogs
Influence of CTE on ACS results
Consistency between different ACS filters
Consistency with past calibrations and
Multidrizzle output
Consistency with results implied by analysis of
telescope slews (POSTARGs)
Final Result
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Formalism to correct any ACS/WFC
dataset to absolute coordinates (modulo
translations) with [accuracy in units of 10-5]
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Known Scale to 1.1 (random) and 0.6 (syst)
Known Rotation to 4.8 (random) and 3.9 (syst)
No residual skew to 0.3 (random) and 1.3 (syst)
Should be sufficient for JWST purposes