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Improved WFC3 and ACS Data Products

Beginning 3 December 2019, improved astrometry for WFC3 and ACS imaging data is available from MAST and includes two new corrections to the header world coordinate system (WCS). The first includes an updated On December 17, 2020MAST began production of new ACS and WFC3 products in the HST data calibration pipeline. These Hubble Legacy Archive (HLA)-style mosaics comprise the data from a single HST visit and are aligned to a common astrometric reference frame. These mosaics will be available as they are produced in the pipeline; it will take some roughly 1-2 months before all archival data have been processed.

These new 'Hubble Advanced Products' (HAP) are referred to as 'Single Visit Mosaics' (SVMs). They are all drizzled onto the same north-up pixel grid and may have improved relative alignment across filters for images acquired within the same visit, enabling easy comparison of the images through multiple filters. When possible, sources in the images have been aligned directly to the Gaia source catalog to improve the image world coordinate system (WCS). SVM data products with both relative alignment (by filter) and absolute alignment to Gaia will contain the string 'FIT_SVM_GAIA' in the 'WCSNAME' keyword in the science extension of the image header. The software used to compute these new data products is described in the DrizzlePac documentation for Single Visit Mosaic Processing.


On December 3, 2019, the first set of improved astrometry data were released in MAST. All WFC3 and ACS imaging filters were aligned to a common astrometric reference frame and include two corrections to the header WCS. The first makes use of a new version of the Hubble Guide Star Catalog (GSC version 2.4.0) which updates the coordinates of the guide stars with the positions from Gaia DR1. This reduces the typical uncertainties in the positions of the guide stars to ~200 mas over the entire sky. Combining this new information with the with knowledge of the instrument distortions, an a priori correction has been made for all WFC3 and ACS observations in order to lock all HST observations onto a common absolute reference framewas made. When possible, an additional correction has been was applied by aligning sources in each HST image directly to the Gaia catalog; this fit is , referred to as an a posteriori correction. While some observing modes cannot be aligned to Gaia (e.g. grism and moving target observations) or the alignment may fail due to a lack of sources in either the HST image or the Gaia catalog, approximately 80% of ACS/WFC and 50% of WFC3/IR frames have been directly aligned. For these data products, the typical pointing uncertainty is reduced to ~10 mas, although the uncertainties increase for observations further in time from the Gaia reference epoch (2015.0 for DR1, 2015.5 for DR2). On 10 December 2020, further improvements to drizzled data products have been implemented as new Hubble Advanced Products referred to as Single Visit Mosaics (SVMs). These data are all drizzled onto the same north-up pixel grid and may have improved relative alignment across filters within a given visit, enabling easy comparison of the images across multiple wavelengths. The algorithm used to derive these new data products is described in the drizzlepac documentation for Single Visit Mosaic Processing. For comparison, the algorithm used to produce standard drizzled data products is described in the documentation for Pipeline Astrometric Calibration. The software used to produce these drizzled products is described on the Pipeline Astrometric Calibration page.

Usage 

Images downloaded from the archive after reprocessing with the new Enhanced Pipeline Products code will have headerlets added as extra extensions to the FITS file. A new python notebook, 'Using updated astrometry solutions', will familiarize users with the structure of the new FITS images and demonstrate how the primary WCS may be changed to any other preferred solution. These instructions will also show how to back out the new WCS updates entirely if desired (see the section below on 'Caveats').

...

  • a priori         : correct the coordinates of the guide stars in use at the time of observation to the coordinates of those guide stars as determined by Gaia by , applying a global offset to the WCS
  • a posteriori  : identify sources in the HST image and cross-match with positions from an external reference catalog (such as Gaia) to derive an improved WCS based on improve the WCS (fitting x/y to RA/Dec)

Note that a priori corrections are only relevant for observations which executed prior October 2017 (eg. prior to the release of GSC 2.4.0), and these will still include small errors in the alignment of the science instruments to the HST focal plane. The a posteriori corrections are limited to imaging instruments for which there are an adequate number sources to define a reference catalog for matching. These solutions remove uncertainties in the focal plane and are expected to have the smallest absolute astrometric error.

...

 and CorrectionType may have several forms

  •    GSC240 : 'a priori' WCS where guide star coordinates are corrected from the original reference frame (e.g. GSC1.1 or GSC2.3) to the Gaia DR1-based GSC2.4.0
  •    HSC30 :   'a priori' WCS corrected from the original reference frame to the Hubble Source Catalog (HSC v3.0) frame, which is based on Gaia DR1
  •    FIT-IMG-ReferenceCatalog RefCat  : 'a posteriori' WCS derived from matching matched to a reference catalog, where 'IMG' implies individual fits for each image each FLT is separately aligned to the reference catalog
  •    FIT-REL-ReferenceCatalog RefCat   : 'a posteriori' WCS derived from matching matched to a reference catalog, where 'REL' implies images within a given filter were aligned to one other that FLTs within the same filter within the same visit are aligned before a global catalog alignment
  •    FIT-SVM-ReferenceCatalog RefCat : 'a posteriori' WCS derived from matching matched to a reference catalog, where 'SVM' implies improved relative alignment across that FLTs in multiple filters within the same visit are aligned before a global catalog alignment

More details on interpreting the WCS names may be found on the Astrometry in Drizzled Products page. A list of possible 'active' WCSNAME values populated in the image headers is provided in Table 2.

...

WCSNAME

WCSTYPE

Comment

OPUS

‘distorted not aligned’

No distortion correction has been applied; analysis of these FLT/FLC files may only be performed if corrected by the instrument-specific pixel area map

IDC_0461802ej

‘undistorted not aligned'                                                                   

Distortion-corrected using the IDCTAB reference file '0461802ej_idc.fits', but not aligned to an external catalog

IDC_0461802ej-GSC240

‘undistorted a priori solution based on GSC240'

Alignment based on Guide Star Catalog v2.4.0 (GSC240).  Absolute errors ~0.1"

IDC_0461802ej-HSC30

‘undistorted a priori solution based on HSC30’

Alignment based on Hubble Source Catalog v3.0.  HSC30 errors are typically smaller than GSC240. If both corrections are available, HSC takes precedence.

IDC_0461802ej-FIT_REL_GAIADR1

‘undistorted a posteriori solution relatively aligned to GAIADR1’

Exposures relatively aligned to one another, and subsequently then aligned as a set to Gaia DR1

IDC_0461802ej-FIT_REL_GAIADR2

‘undistorted a posteriori solution relatively aligned to GAIADR2’

Exposures

relatively

aligned to one another, and

subsequently

then aligned as a set to Gaia DR2

(

, including proper motion corrections to HST observation epoch

)

IDC_0461802ej-FIT_REL_NONE

‘undistorted a posteriori solution relatively aligned to NONE’

Exposures relatively aligned to one another, but the quality of the fit to an absolute reference catalog is unverified and should be checked by the user

IDC_0461802ej-FIT_IMG_GAIADR1GAIADR*

‘undistorted a posteriori solution aligned image-by-image to GAIADR1’GAIADR*’

Exposures individually aligned to Gaia DR1 or DR2 (not as a set)

IDC_0461802ej-FIT_IMG_GAIADR2

‘undistorted a posteriori solution aligned image-by-image to GAIADR2’

Exposures individually aligned to Gaia DR2 (including proper motion corrections to the HST observation epoch)

IDC_0461802ej-FIT_IMG_NONE

‘undistorted a posteriori solution aligned image-by-image to NONE’

Exposures individually aligned to an astrometric a reference catalog, but the quality of the fit is unverified and should be checked by the user
IDC_0461802ej-FIT_SVM_GAIADR*‘undistorted a posteriori solution relatively aligned filter-by-filter to GAIADR*’**NEW** Exposures aligned to an absolute astrometric frame to to Gaia DR1 or DR2, with improved relative alignment across filters in a visit and then drizzled onto a common output pixel grid

Caveats

While the majority of calibrated HST data products are now aligned to a common absolute reference frame, further improvements may be possible via manual realignment using the drizzlepac tools.  This is particularly true for exposures acquired in the same visit where the WCSNAMEs does not contain the string 'FIT_SVM_GAIA'.  For standard drizzled data products:

  • Short and long exposures obtained in the same visit may no longer be aligned due to potentially different number of Gaia matches.
  • Exposures in different filters (eg. narrowband vs broadband) which were obtained in the same visit may no longer be aligned to one another, for example, if each filter had a different number of matches to Gaia.

Furthermore, grism images will now be offset from their direct image counterparts, where only the later of which may be aligned to an external reference catalog. In order to preserve relative alignment between grism and direct images, users may wish to back out the updated WCS solutions entirely, as described in Section 5 of the python notebook, 'Using updated astrometry solutions'.

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