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November 2, 2022
WFC3 NEWSLETTERS

WFC3 STAN Issue 40, November 2022

1. One-Pass Photometry with hst1pass

J. Anderson

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 _flt and _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.

An image of two plots. On the left is a star field in grayscale (bright sources are black). At right is a plot of instrumental magnitude as a function of the quality of fit. The origin represents saturated stars. You can see that the stars have a similar value of q regardless of magnitude, while other resolved sources have worse quality of fit.
Figure 1:  At left is a single F606W ACS/WFC exposure of the Ultra Deep Field. On the right is the plot of q against instrumental magnitude for the sources identified. Note that below m ~ -10, stars have about the same value of q, but above this, the increasing S/N in the star affects the fit. The detections at the top with poor fits are either galaxies, cosmic rays (CRs) or unsubtracted hot pixels.
An image of two plots. On the left is a star field in grayscale. On the right is a plot of instrumental magnitude as a function of the quality of fit (q). Most sources are cosmic rays or resolved, and they have a higher q and increased magnitude. Stars are more sparsely located at a lower q across a wider range of magnitude. The origin represents saturated stars.
Figure 2: At left is a single half-orbit exposure (ieou07kuq) through F438W with WFC3/UVIS. The vast majority of apparent sources are cosmic rays. At right, the q parameter clearly identifies the 9 stars out of the more than 50,000 non-stellar sources. You can see one star with S/N ~100 at the left of this 400´200-pixel image. Identification of stars this way makes it easier to get accurate inputs for tweak-reg in the AstroDrizzle package to accomplish good alignment.

 

2. Hubble Advanced Products in MAST

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.

MVM color composite showing a 3x3 mosaic of the Horsehead Nebula in WFC3/IR. The F110W layer is shown in blue and the F160W layer in red.
Figure 3: MVM color composite of hst_skycell-p1253x06y09 showing a 3x3 mosaic of the Horsehead Nebula in WFC3/IR. The F110W layer is shown in blue and the F160W layer in red.
MVM color composite for a large region surrounding M51, a spiral galaxy. The ACS/WFC F435W layer is shown in blue, the ACS/WFC F555W layer in green, and the WFC3/IR F110W layer in red. A large number of parallel WFC3/IR observations sweep out a circle around the target and correspond to a range of dates from Oct 2016 – Sep 2017 when observing with ACS/WFC as the prime instrument. (Not all regions of the field were observed in each of the three filters in this composite image.)
Figure 4: MVM color composite of hst_skycell-p2261x11y16 for a large region surrounding M51. The ACS/WFC F435W layer is shown in blue, the ACS/WFC F555W layer in green, and the WFC3/IR F110W layer in red. A large number of parallel WFC3/IR observations sweep out a circle around the target and correspond to a range of dates from Oct 2016 – Sep 2017 when observing with ACS/WFC as the prime instrument. (Not all regions of the field were observed in each of the three filters in this composite image.)

 

3. New Documentation

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

The complete WFC3 ISR archive is available here. Additional information about WFC3 calibration, performance, data analysis, software tools, and more can be found online.

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