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Roeland van der Marel Jay Anderson, Colin Cox, Vera Kozhurina-Platais, Matt Lallo, Ed Nelan

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. 2 Translations (x, y) JWST: Guide stars + Target acquisitions

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Roeland van der Marel Jay Anderson, Colin Cox, Vera Kozhurina-Platais, Matt Lallo, Ed Nelan

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  1. Towards Creation of a JWST Astrometric Reference Field:Calibration of HST/ACS Absolute Scale and Rotation Roelandvan der Marel Jay Anderson, Colin Cox, Vera Kozhurina-Platais, Matt Lallo, Ed Nelan

  2. Astrometric Calibration • 2 Translations (x, y) • JWST: Guide stars + Target acquisitions • Scale • Rotation • 2 Skew terms (scale and rotation) • Higher-order distortions • JWST: Observations of Astrometric Reference Field 6 linearparameters

  3. JWST Astrometric Reference Field • Self-calibration of astrometry using JWST observations not feasible • 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 • In JWST Continuous Viewing Zone • Adequate stellar density • Low proper motions LargeMagellanicCloud (LMC)

  4. JWST Astrometric Requirements • Mission Requirement (MR-120) • 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 • Accurate higher-order distortion correction  LMC field observed with HST/ACS mid 2006 (Diaz-Miller et al.) • Calibration to same relative scale • Desire: also accurate absolute scale + rotation • 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)

  5. HST/ACS Astrometric Calibration • 2 Translations • Not relevant here • Scale • Rotation • 2 Skew terms • Higher-order distortions • Previously calibrated by Anderson • No known time-dependence Desired accuracy ~ 5 x 10-5 (corresponds to 5 mas over 1-2 arcmin FOV)

  6. Calibration of HST/ACSscale and rotation • Observations of a field with good astrometry M35 (HST/FGS calibration field) • Good relative astrometry (< 1 mas) • Good proper motions (< 0.2 mas/yr) • Poor absolute astrometry • Approach • Build M35 catalog with good absolute astrometry • Observe M35 stars in this catalog with HST/ACS

  7. Building an M35 Catalog • 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 • RMS 21 mas per coordinate • Fractional scale uncertainty 0.6 x 10-5 • Rotation uncertainty 0.6 x 10-5

  8. HST/ACS observations of M35 • 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 • (x,y) on Distortion-Corrected Frame (DCF)

  9. Scale and Rotation of DCF • Match stars using linear transformations with FGS/UCAC2 Catalog • RMS < 2 mas per coordinate • Fractional scale uncertainty 0.6 x 10-5 • Rotation uncertainty 0.6 x 10-5 • Some subtle corrections required • Differential velocity aberration affects scale • Sky looks distorted when projected on a plane • Orientation at ACS/WFC is not the same as at the V1 axis • Final results • s = 0.0497248 arcsec/pixel • b = 177.7612 degrees

  10. Time Variation of HST/ACS Linear Distortion Terms rotation scale • 5 years of repeated47 Tuc imaging (Anderson 2007) • Scale quite stable • Rotation shows ~0.003 deg non-repeatability • Skew varies linearly with time • Confirmed by M35 data M35 rotation scale

  11. Checks Performed to Assess Systematic Errors • 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)

  12. Final Result • Formalism to correct any ACS/WFC dataset to absolute coordinates (modulo translations) with [accuracy in units of 10-5] • 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

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