The point-spread function (PSF) of the telescope modulates the scene that the telescope is able to deliver to the observer. For objects that are much larger than this fundamental resolution element, an intimate knowledge of the PSF is not necessary to do high-precision science. However, many astronomical studies can be pursued only when we have an accurate understanding of a detector’s point-spread function (PSF). For example, astrometry and photometry of point sources, bright and faint, cannot be done with high precision without PSF-fitting. In a similar vein, weak lensing and other studies of objects that are close to the resolution limit can be very dependent on the fidelity of the PSF model.
Unfortunately, even though accurate PSF models are critical to many astronomical studies, there are many reasons that very few published papers make use of good PSF models. For one thing, it is hard to construct good PSF models. The WFC3/UVIS detectors are mildly undersampled, which means that an accurate PSF can only be constructed from a dithered set of data, and one must take exquisite care to accurately represent the sub-sampled nature of the PSF. In addition, the PSF changes with position across the field, both due to variations in optical distortion and variations in the thickness of the detector (related to charge diffusion). Furthermore, the PSF also changes over time due to secular and breathing-related changes in instrument focus. All of these issues make it difficult to have perfect knowledge of the PSF in an image a priori.
Even when accurate PSF models are available, it is hard to use them to do science. PSF models are most accurate in the individual flat-fielded frames (the _flt images), since the pixel values in these images are the only true and direct constraints that we have on the astronomical scene. Even so, because of the undersampling and detector artifacts, a single exposure is not able to contain all the information the telescope can collect about the scene. It is necessary to dither the scene by whole pixels and fractional pixels in order to fully constrain the astronomical scene that has been delivered to the detector. Unfortunately, the large distortion that is present in HST’s detectors makes it difficult to interrelate the _flt pixels in different dithers. For this reason, many users make use of the Drizzle software suite, which is designed to combine the individual distorted and undersampled exposures into a single composite image that has better sampling and no distortion. This resampling process can be done in a way that preserves flux, but it is very hard to perform this operation without introducing irregularities in the sampling or introducing correlations among the output pixels. All this means that it is hard to do high-accuracy PSF analysis on the drizzle product, so all the PSFs provided here are in the _flt/_flc frame.
Empirically constructed WFC3 PSFs for the UVIS and IR detectors are provided below. To represent the spatial variation of the PSF across the detector, the UVIS PSFs are provided in a 7x4 array for each chip (7x8 overall). They are provided in two formats: a single 2-d image that shows the array of PSFs on the left and an array of residuals (relative to the average PSF) on the right, as seen in the image below for F606W.
Each of the 56 fiducial PSFs in this image is 101x101 pixels, supersampled by a factor of 4, representing the light distribution over a 25x25 pixel region centered on a point source. An additional format (the STDPSF format) puts all these PSFs into a single image cube that is 101x101x56, where the first 7 planes in the cube correspond to the bottom row, the next 7 planes the next row up, etc.
The WFC3/IR detector requires only a 3x3 array to describe the PSF's variation across the chip. We provide these PSFs only in STDPSF format. The STDPSF format is decscribed more fully in Section 6 of ISR 2016-12 by J. Anderson.
|Filter||PSF with residuals||PSF in Standard Format|
The following Instrument Science Reports are relevant to the WFC3 PSFs.
- ISR 2016-12: Empirical Models for the WFC3/IR PSF
J. Anderson 08 Aug 2016
- ISR 2015-08: A Study of the Time Variability of the PSF in F606W Images taken with WFC3/UVIS
J. Anderson, M. Bourque, K. Sahu, E. Sabbi, A. Viana 28 May 2015
- ISR 2015-02: Standard Astrometric Catalog and Stability of WFC3/UVIS Geometric Distortion
V. Kozhurina-Platais & J. Anderson 11 March 2015
- ISR 2014-24: Local Bundles: Bringing the Pixels to the People
Jay Anderson 18 Dec 2014
- ISR 2013-13: Evaluation and Comparison of Deep UVIS PSFs Observed at Three Epochs
L. Dressel 26 Jun 2013
- ISR 2013-11: UVIS PSF Spatial & Temporal Variations
E. Sabbi & A. Bellini 25 Jun 2013
- ISR 2012-14: Breathing, Position Drift, and PSF Variations on the UVIS Detector
L. Dressel 13 Jul 2012
- ISR 2009-38: WFC3 SMOV Programs 11436/8: UVIS On-orbit PSF Evaluation
G. F. Hartig 03 Dec 2009
- ISR 2009-37: WFC3 SMOV Programs 11437/9: IR On-orbit PSF Evaluation
G. F. Hartig 03 Dec 2009
- ISR 2009-20: WFC3 SMOV Program 11798: UVIS PSF Core Modulation
E. Sabbi 17 Nov 2009
- ISR 2008-41: WFC3 IR PSF Evaluation in Thermal-Vacuum Test #3
G. Hartig 16 Sep 2008
- ISR 2008-40: WFC3 UVIS PSF Evaluation in Thermal-Vacuum Test #3
G. Hartig 26 Aug 2008
- ISR 2005-10: WFC3 UVIS PSF Evaluation in Thermal-Vacuum Test #1
G. Harting 03 Mar 2005
- ISR 2004-08: Preliminary WFC3 UVIS PSF Evaluation
G. Hartig 24 May 2004
- ISR 2002-04: Using global PSF properties to probe the WFC3 UVIS alignment and focus
M. Stiavelli, C. Hanley 29 Apr 2002
- ISR 2001-12: Characterization of the UV PSF for WFC3
M. Stiavelli 27 Jun 2001
STDGDC Format: Detector-based Distortion Solutions
The distortion solutions described in the Instrument Handbook and provided on the Reference File page are designed to map the detector pixels into the V2-V3 telescope plane for the purposes of absolute astrometric calibration. To facilitate transformations among flt images, it is common to use more locally focused distortion solutions. These solutions map the distorted frame of the detector into the closest possible distortion-free frame. Typically, the correction is zero at the center of the detector and its scale and orientation match that of the y axis of that pixel. For all the other pixels in the detector, the correction simply tells us where the center of that pixel is located relative to the central pixel. The format of the following files is described HERE .
The WFC3/IR detector has a single STDGDC file that can be used for all filters: STDGDC_WFC3IR.fits
The WFC3/UVIS detector has a different solution for each filter, since the filters have been shown to introduce small perturbations ("fingerprint") on the distortion solution (see Kozhurina-Platais ISR 2015-02). The WFC3/UVIS solutions provided below come from Bellini, Anderson, & Bedin (2011 PASP 123 622).