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January 2024:   For more information on Hubble Advanced Products (HAP), please see the following resources.

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NEWS:  September 9, 2022

In August 2022, a new ACS/WFC3

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instrument science report (ACS ISR 2022-03; WFC3 ISR 2022-06) titled 'Improved Absolute Astrometry for ACS and WFC3 Data Products'  was published. This ISR describes updated WCS solutions in MAST data as well two new types of Hubble Advanced Products (HAP). 

Abstract:
As of late-2019, MAST data products for ACS and WFC3 include improved absolute astrometry in the image header World Coordinate System (WCS). The updated WCS solutions are computed during pipeline processing by aligning sources in the HST images to a select set of reference catalogs (e.g. Gaia eDR3). We compute statistics on the alignment fraction for each detector and estimate the uncertainties in the WCS solutions when aligning to different reference catalogs. We describe two new types of Hubble Advanced Products (HAP), referred to as Single Visit Mosaics (SVMs) and Multi Visit Mosaics (MVM), which began production in MAST in late-2020 and mid-2022, respectively. The SVM products include an additional relative alignment across filters in a visit, and the drizzled images are used to generate point source and segment catalogs during pipeline processing. These catalogs supersede those produced by the Hubble Legacy Archive and will be the basis of the next version of the Hubble Source Catalog. The MVM data products combine all ACS/WFC, WFC3/UVIS, or WFC3/IR images falling within a pre-defined 0.2° x 0.2° 'sky cell' for each detector+filter, which are drizzled to a common all-sky pixel grid. When combining observations over a large date range, MVMs may have photometric errors of several percent or systematic alignment errors when combining visits with different catalog solutions. We therefore recommend these to be used as ‘discovery images’ for comparing observations in different detectors and passbands and not for precise photometry.

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PRIOR NEWS:

On December 3, 2019, the first set of improved astrometry data were released in MAST. The World Coordinate System (WCS) in the image header of all WFC3 and ACS datasets were updated and may include  one or more corrections. The first makes use of a new version of the 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 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). The software used to produce these drizzled products is described on the Pipeline Astrometric Calibration page.

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 17, 2020, the MAST began production of new ACS and WFC3 products in the HST data calibration pipeline (see the following MAST Newsletter article. These Hubble Legacy Archive (HLA)-style mosaics comprise the data from a single HST visit which are aligned to a common astrometric reference frame. These new 'Hubble Advanced Products' (HAP) are referred to as 'Single Visit Mosaics' (SVMs) and are described in a MAST Newsletter article from December 2020. The data products are all drizzled onto the same north-up pixel grid and may have include improved relative alignment across filters within a given for datasets acquired within the same visit, enabling easy comparison of the images across multiple wavelengths. The algorithm used to derive through multiple filters. When possible, sources in the images have been aligned directly to the Gaia catalog to improve the 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 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

On November 23, 2021, MAST began producing source catalogs as part of the SVM data products. BecauseSVMproductsincludeanadditionalrelativealignmentacrossfiltersina visit,the drizzled imagesmayusedto generatepoint sourceand segmentcatalogsduring pipeline processing. Thesecatalogssupersedethoseproducedby theHubbleLegacy Archiveand will be the basis ofthenext version of the Hubble Source Catalog.

On April 26, 2022, the HST data calibration and archive pipelines began producing a new Hubble Advanced Product (HAP) to be distributed through MAST. These are cross-visit, cross-proposal mosaics called Multi-Visit Mosaics (MVM), which combine public observations of fields observed multiple times by ACS and WFC3 into a set of products drizzled onto a common, pre-defined pixel grid. These new products were described in a MAST Newsletter article from May 2022 and complement the existing HAP Single Visit Mosaics (SVM) released in December 2020.

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').

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  • 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.

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 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.

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Table 2: Sample active WCSNAME keyword values and the corresponding WCSTYPE description 
relatively subsequently Gaia DR2 (including proper motion corrections to HST observation epoch) Gaia DR1 IDC_0461802ej-FIT_IMG_ an astrometric GAIADR* GAIADR an absolute astrometric frame with and drizzled onto a common output pixel grid

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 aligned as a set to Gaia DR1

IDC_0461802ej-FIT_REL_GAIADR2

catalog

‘undistorted a posteriori solution relatively aligned to GAIADR2’catalog’

Exposures

aligned to one another, and

then aligned as a set to

the reference  catalog

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_GAIADR1catalog

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

Exposures individually aligned to the reference catalog (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)

NONE

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

Exposures individually aligned to a reference catalog, but the quality of the fit is unverified and should be checked by the user
IDC_0461802ej-FIT_SVM_catalog‘undistorted a posteriori solution relatively aligned filter-by-filter to catalog*’**NEW** Exposures aligned to a reference catalog and include improved relative alignment across filters in a visit

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'.