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