About This Article
The WFC3 and ACS teams are happy to announce the release of a software routine,
hst1pass, that allows users to do high-precision PSF fitting to stars in the unresampled flat-fielded
_flc images. The routine and how to use it are described in WFC3 ISR 2022-05 and ACS ISR 2022-02.
The routine represents a consolidation of instrument-specific photometry routines that have been developed over the years to do science. A single generalized routine is now able to provide measured positions, fluxes, and associated parameters for all of HST's main imagers (WFPC2, ACS/HRC, ACS/WFC, WFC3/UVIS, and WFC3/IR). Empirical PSFs (point-spread functions) and GDCs (geometric-distortion corrections) are provided in associated library files that enable users to model stars accurately and map results from individual flt/flc frames into the associated drizzle frame for identification and collation.
As the routine's name suggests, its focus is on "one pass" photometry. Other routines (such as DAOPHOT or DOLPHOT) are needed for multi-pass applications, but thanks to HST's tight PSF, most photometry/astrometry can safely measure stars in small 5x5-pixel fitting boxes and treat them as isolated in this region. In a typical run, the user specifies some simple finding parameters, and the routine reads in (1) an HST image (_flt or _flc), (2) a PSF, and (3) a distortion solution. The routine then goes through the image pixel-by-pixel and returns a list of stars found and measured in the image. This single-exposure star list can then be collated with similar lists, such as from a set of dithered exposures from the same program. The collation process will be described in more detail in a future ISR, but a simplified version of the
hst2collate collation software is provided along with
hst1pass to facilitate preliminary analysis.
Along with providing high-precision photometry and astrometry, one key benefit of PSF fitting to un-resampled images is that it is much easier to identify stellar and non-stellar sources. Figures 1 and 2 below show the quality-of-fit metric for sources found in two long exposures, the first the UDF and the second a UVIS image with thousands of times more cosmic rays than sources. The PSF-fitting shows very clearly which sources are stars and which are not. These stars can then be used for science or for tweak-reg-type image alignment.
J. Mack, W. Hack, M. Burger, R. L. White, V. Bajaj, R. J. Avila, G. S. Anand, & M. de la Pena
MAST began production of two new types of advanced products, referred to as Single Visit Mosaics (SVMs) and Multi Visit Mosaics (MVM), in late-2020 and mid-2022, respectively. These new products include improved absolute astrometry in the image header World Coordinate System (WCS) for both WFC3 and ACS, derived from cross-matching sources in the HST images to a set of external reference catalogs (see WFC3 ISR 2022-06; ACS ISR 2022-03).
- SVM data products (hst_prop_visit_*.fits) improve on standard HST data products (ippsssoot_*.fits), with an additional relative alignment performed for all filters in a given visit. The drizzled images are used to generate both point source and segment catalogs during MAST pipeline processing. These new catalogs supersede those produced by the Hubble Legacy Archive and will be the basis of the next version of the Hubble Source Catalog.
- MVM data products (hst_skycell-_*.fits) combine all WFC3/UVIS, WFC3/IR, and ACS/WFC images falling within a pre-defined 0.2° x 0.2° "sky cell" for a given filter, drizzled to a common all-sky pixel grid. When combining observations spanning a large date range, MVMs may have photometric errors of several percent or systematic alignment errors when combining visits with different catalog solutions. These are therefore recommended as ‘discovery images’ for comparing observations in different detectors and passbands and not for precise photometry.
Reprocessing of archival WFPC2 data is currently underway and will include updates to the image header WCS, as well as the production of advanced data products for easy comparison with WFC3 and ACS.
ISR 2022-04: Monitoring WFC3/UVIS Photometric Sensitivity with Spatial Scans - M. Marinelli, V. Bajaj, A. Calamida, H. Khandrika, J. Mack, A. Pidgeon, C. Shanahan, D. Som
ISR 2022-05: One-Pass HST Photometry with hst1pass - J. Anderson
ISR 2022-06: Improved Absolute Astrometry for ACS and WFC3 Data Products - J. Mack, W. Hack, M. Burger, R. L. White, V. Bajaj, R. J. Avila, G. S. Anand, M. de la Pena
ISR 2022-07: WFC3/IR Photometric Stability Stellar Cluster Study - V. Bajaj, A. Calamida, J. Mack, D. Som
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