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Text revised from HST Astrometry Project Overview

Improved WFC3 and ACS Data Products

Improved astrometry for WFC3 and ACS imaging data is now available from MAST, which includes two new corrections to the header world coordinate system. First, an updated Hubble Guide Star Catalog (GSC 2.4) was released which updated the coordinates of the guide stars with the positions from Gaia DR1. The typical uncertainties in the positions of the guide stars is reduced to <~100 mas over the entire sky. Combining this new information with the knowledge of the instrument distortions, an a priori correction has been made for all WFC3 and ACS observations. When possible, an a posteriori correction has been applied to data by aligning sources in the images directly to the Gaia catalog. While some observing modes cannot be aligned to Gaia (e.g., grism and moving target observations), and the alignment can fail due to a lack of sources in either the HST data or the Gaia catalog, ~70% of the WFC3 and ACS data were aligned successfully. The typical uncertainty in the pointing is reduced to ~10 mas, although the uncertainties increase the farther in time from the Gaia epoch (2015.5)

Guide Star Catalogs

Historically, the accuracy of HST absolute astrometry has been limited primarily by uncertainties in the celestial coordinates of the guide stars. GSC1 had nominal rms errors per coordinate of ~0.5 arcsec, with errors as large as ~1‐3 arcsec reported near the plate edges. This accuracy improved substantially in October 2005 (during Cycle 15) with the introduction of GSC 2.3.2, which had rms errors per coordinate of ~0.3 arcsec over the whole sky.

As of November 2019, an updated version of the catalog (GSC 2.4) has now been released, improving the celestial coordinates with the positions from GAIA DR1 and reducing errors to < 30mas over the entire sky. Thus, after including uncertainties in the positions of the science Instruments in the alignment of the focal plane to the Fine Guidance Sensors (FGS), the total error in HST absolute astrometry is ~1 arcsec for observations made with GSC 1.1, ~0.3 arcsec for those made with GSC 2.3.2, and ~0.1 arcsec when using the new GSC 2.4.  A summary of the GSC properties and associated errors is given in Table 1. 


Table 1: GSC Properties 

Catalog

Release Date

Mean Epoch of catalog positions

Typical errors

Worst errors

Total Error (including SI to FGS alignment)

Comment

GSC 1.0

Jun 1989



1-2”


 GSC1 comparison

GSC 1.1

Aug 1992

1981.8

0.5”

~1”

~1”

First version published to the community

Used by HST operations prior to Cycle 15

(WFPC2 installed Dec 1993)

GSC 2.0

Jan 2000





Science target fields only

GSC2 comparison

GSC 2.2.0

Jun 2001





Public Release

(ACS installed Mar 2002)

GSC 2.3.2

Oct 2005

1992.5

0.3”

0.75”

0.3”

Public Release

Comparison between GSC1.1 and GSC2.3.2

GSC 2.3.3Oct 2009



(WFC3 installed May 2009)

GSC 2.3.4

 ??





Current version as of October 2019.  Citation?

GSC 2.4

Nov 2019

2015.0

0.03”


0.1”

GSC 2.3.4 aligned to Gaia DR1

**Please check cells highlighted in pink

HST Astrometry Project

The coordinates populated in the FITS headers of HST observations retrieved from DADS (the HST Data Archiving and Distribution Service) were derived based on the guide star coordinates in use at the time of the observation. As the accuracy in these catalogs were refined over time, the pointing accuracy of HST has also improved. Table 1 lists the catalog in use at the time of installation of the three main imaging cameras (WFPC2, ACS, and WFC3) and the typical errors at each epoch.

The goal of the HST Astrometry Project is to correct these inconsistencies in the archival data products as much as possible.  As observations are processed or reprocessed in the HST pipeline, their World Coordinate System (WCS) will be updated to use the most accurate solution available. There are 2 types of corrections that can be performed.

  • 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 and apply 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 a new WCS based on fitting x/y to RA/Dec

Note that the a priori corrections will still include any errors in the HST focal plane alignment and are only relevant for observations which executed prior to GSC 2.4. The a posteriori corrections are limited to imaging instruments for which there are enough sources to define a good reference catalog for matching. These solutions remove uncertainties in the focal plane and are expected to have the smallest absolute astrometric error.

Implementation

The key to implementing improvements to the astrometry is the use of headerlets, self-contained FITS extensions containing a WCS transformation which can be attached to a FITS file and applied to the primary WCS. An observation can have multiple headerlets, each of which may have astrometry derived by differing methods. As HST data is processed/reprocessed, all available headerlets will be present as FITS extensions in the archived image with the best solution applied to the primary WCS.  More details on how the WCS information is stored in headerlets may be found on the page Astrometry in Drizzled Products.

A database has been created to contain all the headerlets that is used by the xxxtask  to save and retrieve headerlets during pipeline processing. (Do we need this part?) This database is accessed using a restful web service as well as a simple web-form. Prior to enabling this database in the HST pipeline, it was pre-populated with headerlets derived from:

  • the original pipeline astrometry based on guide star positions at the time of observation
  • a priori corrected astrometry based on updating the guide star positions to the GAIA DR1 reference frame
  • a posteriori corrected astrometry based on sources in the Hubble Legacy Archive (HLA) images fitted to the Hubble Source Catalog (HSC). These positions are primarily based on the Pan-STARRS catalog, which is on the GAIA reference frame but with larger errors. These are only available for a subset of the ACS and WFC3 images that were public prior to October 2017.

Pipeline processing now includes a source finding step to create additional a posteroiri solutions via cross-identification with a limited set of external reference catalogs; namely, GAIA DR1 and DR2.  Alternate astrometric catalogs (like Pan-STARRS, SDSS, 2MASS, GSC 2.4 etc) may eventually be added to the pipeline in order to obtain the best a posteriori WCS, but only after they have been determined to provide high quality alignment.

WCS Naming Conventions

Successfully aligning an observation to GAIA using the a posteriori processing will result in an update of the 'active' WCS of the image with the new solution and the new headerlet extension. This headerlet not only includes the WCS keywords which define the transformation from pixels to GAIA-aligned positions on the sky, but it also contains information about how this solution was derived along with the errors to be expected based on the fit. 

The various WCS solutions are identified by the WCSNAME keyword found in each FITS headerlet and use the following naming convention:  wcsName = OriginalSolution - CorrectionType

where OriginalSolution may be either

  •    OPUS : initial ground system wcs
  •    IDC_xxxxxxxxx : initial distortion corrected wcs  (where xxxxxxxxx = geometric distortion model used, eg. the rootname of the IDCTAB reference file)

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) frame which is based on GAIA DR1
  •    FIT-IMG-ReferenceCatalog : a posteriori WCS derived from matching to a reference catalog, where 'IMG' implies individual image catalog fits
  •    FIT-REL-ReferenceCatalog : a posteriori WCS derived from matching to a reference catalog, where 'REL' implies images were aligned to each other before a catalog alignment

More details on interpreting the WCS names may be found on the Astrometry in Drizzled Products page. Several examples are listed below in Table 2.


Table 2: WCS keywords in the image header

WCSNAME

WCSTYPE

OPUS

‘distorted not aligned’

IDC_11d1433lj

‘undistorted not aligned'                                                                   

IDC_w3m18525i-HSC30

‘undistorted a priori solution based on HSC30’

IDC_q692007bj-GSC240

‘undistorted a priori solution based on GSC240'

IDC_0461802dj-FIT_REL_NONE

‘undistorted a posteriori solution relatively aligned to NONE’

IDC_w3m18525i-FIT_REL_GAIADR1

‘undistorted a posteriori solution relatively aligned to GAIADR1’

IDC_w3m18525i-FIT_REL_GAIADR2

‘undistorted a posteriori solution relatively aligned to GAIADR1’

IDC_0461802ej-FIT_IMG_NONE

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

IDC_0461802ej-FIT_IMG_GAIADR1

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

IDC_0461802ej-FIT_IMG_GAIADR2

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

Usage 

(Still needs revision)

Images downloaded from the HST archive after reprocessing with the new Enhanced Pipeline Products code will already have headerlets added as extra extensions to the FITS file. A new python notebook <insert LINK> has been developed to familiarize users with the structure of the new FITS images and to demonstrate how the primary WCS may be changed to any other preferred solution. Alternately, any of the available headerlets may be downloaded from MAST/STScI and applied to existing data.  For users who wish to manually reprocess existing data, the software will automatically connect to the database and retrieve/apply the headerlets.

Future Improvements

While the majority of calibrated data products are now aligned to a common absolute reference frame, there are still some improvements in the alignment which will be made in the next data release.  For example, exposures in different filters that 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. Additionally, grism images may be now offset from their direct image counterparts, where only the later of which may be aligned to an external reference catalog. These refinements in the alignment process will be coming soon in the upcoming release for Hubble Advanced Products: Single Visit Mosaics. These new products will likely be able to improve the alignment for exposures which were obtained in the same visit, but which had guide star lock issues. They will also correct for small errors in the pointing of the telescope when very large dithers are commanded withing a visit (eg. half the detector FOV).



____Some suggested very basic examples for the Jupyter notebook___:

1.) Show the extra extensions

from astropy.io import fits
fits.info('/internal/hladata/ENVS_OUTPUT/ALIGNDEV_12Oct19/popen-gw1/test_alignpipe_randomlist_J8C020/j8c041sdq_flc.fits')

No. Name Ver Type Cards Dimensions Format
0 PRIMARY 1 PrimaryHDU 279 ()
1 SCI 1 ImageHDU 253 (4096, 2048) float32
2 ERR 1 ImageHDU 57 (4096, 2048) float32
3 DQ 1 ImageHDU 49 (4096, 2048) int16
4 SCI 2 ImageHDU 249 (4096, 2048) float32
5 ERR 2 ImageHDU 57 (4096, 2048) float32
6 DQ 2 ImageHDU 49 (4096, 2048) int16
7 WCSCORR 1 BinTableHDU 59 14R x 24C [40A, I, A, 24A, 24A, 24A, 24A, D, D, D, D, D, D, D, D, 24A, 24A, D, D, D, D, J, 40A, 128A]
8 HDRLET 1 NonstandardExtHDU 22 (60480,)
9 HDRLET 2 NonstandardExtHDU 26 (112320,)
10 HDRLET 3 NonstandardExtHDU 26 (112320,)
11 HDRLET 4 NonstandardExtHDU 26 (112320,)
12 HDRLET 5 NonstandardExtHDU 26 (112320,)
13 HDRLET 6 NonstandardExtHDU 26 (112320,)
14 WCSDVARR 1 ImageHDU 15 (64, 32) float32
15 WCSDVARR 2 ImageHDU 15 (64, 32) float32
16 D2IMARR 1 ImageHDU 15 (64, 32) float32
17 D2IMARR 2 ImageHDU 15 (64, 32) float32
18 WCSDVARR 3 ImageHDU 15 (64, 32) float32
19 WCSDVARR 4 ImageHDU 15 (64, 32) float32
20 D2IMARR 3 ImageHDU 15 (64, 32) float32
21 D2IMARR 4 ImageHDU 15 (64, 32) float32
22 HDRLET 7 NonstandardExtHDU 26 (112320,)
23 HDRLET 8 NonstandardExtHDU 26 (112320,)

2.) Show the different headerlets and the corresponding WCSNAMEs.  Explain how to know which WCS is primary.

from stwcs.wcsutil.headerlet import headerlet_summary
headerlet_summary('/internal/hladata/ENVS_OUTPUT/ALIGNDEV_12Oct19/popen-gw1/test_alignpipe_randomlist_J8C020/j8c041sdq_flc.fits',columns='HDRNAME,WCSNAME')

EXTN              HDRNAME                                                   WCSNAME                           
8         j8c041sdq_flt_OPUS-hlet.fits                                 OPUS
9        OPUS2019-06-04                                                    IDC_0461802ej                    
10       j8c041sdq_flt_OPUS-GSC240-hlet.fits                 OPUS-GSC240                   
11        j8c041sdq_flt_IDC_0461802ej-GSC240-hlet.fits IDC_0461802ej-GSC240    
12       j8c041sdq_flt_OPUS-HSC30-hlet.fits                    OPUS-HSC30                     
13       j8c041sdq_flt_IDC_0461802ej-HSC30-hlet.fits    IDC_0461802ej-HSC30      
22      IDC_0461802ej                                                          IDC_0461802ej
23      IDC_0461802ej-FIT_REL_GAIADR2                          IDC_0461802ej-FIT_REL_GAIADR2

3.) Alternate way to get the keyword names. Is the last one always primary?

from stwcs.wcsutil import headerlet
headerlet.get_headerlet_kw_names('/internal/hladata/ENVS_OUTPUT/ALIGNDEV_12Oct19/popen-gw1/test_alignpipe_randomlist_J8C020/j8c041sdq_flc.fits',kw='WCSNAME')

['OPUS', 'IDC_0461802ej', 'OPUS-GSC240', 'IDC_0461802ej-GSC240', 'OPUS-HSC30', 'IDC_0461802ej-HSC30', 'IDC_0461802ej', 'IDC_0461802ej-FIT_REL_GAIADR2']

headerlet.get_headerlet_kw_names('/internal/hladata/ENVS_OUTPUT/ALIGNDEV_12Oct19/popen-gw1/test_alignpipe_randomlist_J8C020/j8c041051_drc.fits',kw='WCSNAME')

>This doesnt seem to work for the DRC image...





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