Page History

...

Improved WFC3 and ACS Data Products

Improved Starting on xx November, 2019, improved astrometry for WFC3 and ACS imaging data is now 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 <~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 in order to lock all HST observations onto a common absolute reference frame. When possible, an additional correction has been applied by aligning sources in each HST image directly to the Gaia catalog, and 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 can may fail due to a lack of sources in either the HST data image or the Gaia catalog, approximately 70% of the WFC3 and ACS data frames have been aligned successfully. For these observationsdata products, the typical pointing uncertainty in the pointing 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). 

...

Historically, the accuracy of HST absolute astrometry has been limited primarily by uncertainties in the celestial coordinates of the guide stars. GSC 1.1 had nominal rms errors of ~0.5 arcsec per coordinate, 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, where rms errors per coordinate were reduced to ~0.3 arcsec over the whole sky.  An updated version of the catalog (GSC 2.4.0) was released in __xxx___MONTH/DATE______ 2019, 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 (SIs) 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 2.4.0.  A summary of the GSC catalogs and associated errors over the HST lifetime is given provided in Table 1. 

Table 1: Key Guide Star Catalog historyreleases and associated errors 

...

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.

...

•    OPUS : initial ground system wcs, no distortion correction
•    IDC_xxxxxxxxx : initial distortion corrected wcs  (where xxxxxxxxx = geometric distortion model used, eg. the rootname of the IDCTAB reference file)

...

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

Table 2: Sample active WCSNAME keyword values and the corresponding WCSTYPE description 

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 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. These instructions will also show how to back out the new WCS updates entirely if desired (see the section below on Future Improvements).

Alternatively, any of the new WCS solutions may be downloaded from MAST/STScI as separate headerlet files and applied to existing data. For users who wish to manually reprocess existing data, the software software linked above (??) will be able to automatically connect to the astrometry database to retrieve and apply the headerlets. Python functions for creating, updating, and applying headerlets to FITS images are described via the Headerlet User Interface.

...

Further refinements to the alignment will be available in the next release of Hubble Advanced Products, referred to as 'Single Visit Mosaics'.  REMOVE Should we keep this last sentence? These new products will correct the issues listed above, and they may further improve the relative alignment of exposures obtained in the same visit, for example for datasets with very large commanded dithers (eg. half the detector FOV) where small residual shifts and rotations are required to align frames.

____Some suggested very Suggested basic examples for the Jupyter notebook___:

...

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.

...

'IDC_0461802ej-FIT_REL_GAIADR2'

4.) Example for showing how to back out the primary WCS in order to restore alignment between grism observations and their direct image counterparts.

...