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WFC3 PSFs: Introductory Remarks

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 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 frame.

WFC3 PSF Information & Downloads

Representative WFC3 PSFs for the UVIS and IR detectors are provided below. To represent the spatial variation of the PSF, a grid of 7x8 PSFs for the UVIS detector, and a grid of 4x4 PSFs for the IR detector are presented here. In addition, the large files (PSFEFF_*.fits) have the residuals at the right, which represent the difference between the individual PSFs and the average of all the PSFs. An example PSF image is displayed below.

The PSFs are also presented in form of a data cube (PSFSTD_*.fits) where the first 7 planes in the cube correspond to PSFs 1 through 7 in the first (bottom) row of the corresponding PSFEFF image, etc. [Note to IRAF/PyRAF users: the first plane in the cube can be displayed by the command display PSFSTD_WFC3UV_F275W.fits[*,*,1], etc.]

Effective PSF and residuals for F225W.

We note that, in addition to the temporal variations caused by focus change and breathing, the PSFs have spatial variations within the detector. The PSFs provided here were determined from observations of a globular cluster containing a large number of stars at every part of the detector. So these PSFs correspond to a only specific focus value, which needs to be taken into account.

The standard PSF format is described in Section 6 of WFC3 ISR 2016-12 by J. Anderson

WFC3/UVIS PSF Downloads

Filter PSF with residuals PSF in Standard Format
F225W PSFEFF_WFC3UV_F225W_C0 Currently unavailable
F390W PSFEFF_WFC3UV_F390W_C0 Currently unavailable
F467M PSFEFF_WFC3UV_F467M_C0 Currently unavailable
F555W PSFEFF_WFC3UV_F555W_C0 Currently unavailable
F775W PSFEFF_WFC3UV_F775W_C0 Currently unavailable

WFC3/IR PSF Downloads

Filter PSFs

PSF Instrument Science Reports

The following Instrument Science Reports are relevant to the WFC3 PSFs.


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 WFC3/ISR 2015-02). The WFC3/UVIS solutions provided below comes from Bellini, Anderson, & Bedin (2011 PASP 123 622).

Filter Distortion solutions
F225W STDGDC_WFC3UV_F225W.fits
F275M STDGDC_WFC3UV_F275M.fits
F336W STDGDC_WFC3UV_F336W.fits
F390W STDGDC_WFC3UV_F390W.fits
F438W STDGDC_WFC3UV_F438W.fits
F467M STDGDC_WFC3UV_F467M.fits
F555W STDGDC_WFC3UV_F555W.fits
F606W STDGDC_WFC3UV_F606W.fits
F775W STDGDC_WFC3UV_F775W.fits
F814W STDGDC_WFC3UV_F814W.fits
F850L STDGDC_WFC3UV_F850L.fits


The PSFs provided here are stored in STDPSF format. This format provides a flexible way of storing an array of empirical PSFs to describe the variation of the point-spread function across the field of a detector. The format is described HERE.