Download - 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
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
Roelandvan der Marel
Jay Anderson, Colin Cox, Vera Kozhurina-Platais, Matt Lallo, Ed Nelan
Astrometric CalibrationAstrometric 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
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
JWST Astrometric Reference FieldJWST 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
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)
JWST Astrometric RequirementsJWST 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)
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)
HST/ACS Astrometric CalibrationHST/ACS Astrometric Calibration 2 Translations
Not relevant here Scale Rotation 2 Skew terms Higher-order distortions
Previously calibrated by Anderson No known time-dependence
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)
Calibration of HST/ACSscale and rotation
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
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
Building an M35 CatalogBuilding 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
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
HST/ACS observations of M35HST/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)
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)
Scale and Rotation of DCFScale 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
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
Time Variation of HST/ACS Linear Distortion Terms
Time Variation of HST/ACS Linear Distortion Terms
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
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
scale rotation
rotation scale
M35
Checks Performed to Assess Systematic Errors
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)
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 ResultFinal 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
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