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WFC3 STAN Issue 50, October 2025

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October 17, 2025
WFC3 NEWSLETTERS

About This Article

1. Improvements to the WFC3/UVIS Saturation Map

M. Revalski, I. Rivera, V. Bajaj, F. Dauphin

The WFC3 team has released an updated full-well depth saturation map for the UVIS detector, which is compatible with calwf3 version 3.7.1 and newer. Previously, all pixels above a uniform threshold of 65,500 electrons were flagged as saturated in the Data Quality (DQ) arrays of WFC3/UVIS exposures. The team has now characterized the spatial variations in the full-well depth across the detector, and constructed a two-dimensional saturation map reference file. This file has been delivered to the Calibration Reference Data System (CRDS) and applied in the reprocessing of all UVIS observations in the archive. The updated map reveals variations of approximately 13% in the saturation limit across the detector, ranging from 63,465 to 72,356 electrons. The new saturation limits are higher across ~87% of the UVIS detector, resulting in fewer pixels being flagged as saturated. This results in the recovery of reliable photometry for pixels near many bright sources, and improves the quality of mosaics created with DrizzlePac. The techniques and results for this update are described in the team's recent report WFC3 ISR 2025-06, and additional information can be found on the full-well depth webpage.

Full-frame heatmap of the WFC3/UVIS detector showing spatially varying saturation limits in electrons (e⁻). Colors range from blue (lower saturation, ~64,000 e⁻) to red (higher saturation, ~72,000 e⁻). Prominent dark cross lines divide the four amplifier quadrants. The top of the detector shows higher saturation levels, while the bottom left has lower levels. Axes are labeled “Columns” (x-axis) and “Rows” (y-axis).
Figure 1: The CRDS-compliant saturation map that is now used with the calwf3 data calibration pipeline, shown in units of electrons. The revised map results in higher saturation limits across 87% of the detector as compared to the earlier constant threshold of 65,500 electrons. This version of the map has been bias-subtracted, converted to electrons by applying the gain (1.56), and includes overscan regions that are explicitly set to zero in order to effectively match the state of the data at which saturation flagging is performed in calwf3.

2. Improved UVIS Aperture Corrections Derived from Focus Diverse PSF Maps

K. Huynh, V. Bajaj, J. Mack, A. Calamida

For crowded fields, small-aperture photometry can reduce contamination from neighboring sources compared to larger apertures. However, the UVIS encircled energy (EE) varies with detector position and focus variations on orbital timescales for aperture radii less than ~10 pixels. To quantify these variations, Huynh et al. (2025) compute new 2D maps of the aperture correction between 5-10 pixels at different focus levels, based on empirical PSFs (Anderson 2018) for five UVIS filters: F275W, F336W, F438W, F606W, and F814W.  Using a large set of archival images of stellar fields, an empirical focus level is derived for each image, and stars are binned by detector position to compute maps of the aperture correction at different focus levels (see Figure 2).  

Over all focus levels, the mean correction varies by ~0.01 mag, with smaller values at nominal focus (see Figure 3). The upper-left and lower-right corners of the detector are more focus-sensitive than the rest of the field of view, where the mean correction is systematically ∼0.01 mag higher in the Amp A corner for bluer filters (F275W, F336W, F438W) and ∼0.01 mag higher in the Amp D corner for redder filters (F606W, F814W) at all focus levels. Aperture corrections derived from published EE tables are overplotted in blue for UVIS1 and in green for UVIS2. For F438W, these intersect with the empirical values for amps A and C where the EE is measured. In some cases, the UVIS2 aperture corrections are systematically larger than the empirical maps, especially in F275W and F814W.  

Users requiring photometric accuracy better than ∼1% for small apertures can use isolated stars in the individual FLT/FLC frames (or PSF cutouts at a similar detector position and focus level) to compute aperture corrections. For more details on our methodology and testing of our aperture correction maps, as well as our full recommendations to users, please see WFC3 ISR 2025-05 (Huynh et al. 2025).

Grid of ten square heatmaps titled “2D Aperture Correction Maps (Magnitude)” for filter F438W. Each subplot shows a small 2D spatial map labeled “Phylo Bin 1.0” through “Phylo Bin 10.0,” colored from light orange (higher correction) to dark orange (lower correction). The colorbar on the right ranges from 0.060 to 0.090 magnitudes, showing small variations across detector positions for each bin.
Figure 2: 2D spatial maps of the F438W aperture correction from 5-10 pixels (in magnitudes) derived from UVIS PSFs at different phylo (focus) levels, where one box represents 512x512 pixels. Matching exposures with the correct phylo bin allow the 5 pixel aperture photometry to be corrected for temporal variations due to telescope 'breathing' during HST's orbit. The large sample of PSFs at different regions of the detector provide sufficient resolution to map large-scale spatial variations. 
Scatter plot showing aperture correction (in magnitudes) versus phylo level for the F438W filter. Data points represent detector amplifiers A (blue triangles), B (orange triangles), C (green triangles), and D (red triangles), as well as the detector center (open black triangles) and overall mean (open black circles). Two horizontal dashed lines mark reference encircled energy (EE) corrections: UVIS 1 (blue, ~0.084 mag) and UVIS 2 (green, ~0.070 mag). Corrections decrease with phylo level before stabilizing near 0.07 mag.
Figure 3: F438W aperture correction (r = 5–10 pixels) versus phylo level for different regions of the UVIS detector. Colored triangles show a 512×512 pixel cutout at the corner of amplifiers A, B, C, and D, black triangles show the mean at the center of the UVIS FoV in a 1024×1024 pixel cutout, and black circles show the mean over the entire FoV. Aperture corrections are the smallest at nominal phylo levels (5-7), where they are roughly flat across the detector.  Dashed lines are plotted for reference only, to compare values derived from the published EE tables for UVIS1, Amp A at 0.085 mag (blue) and for UVIS2, Amp C at 0.070 mag (green). For F438W, the aperture correction for UVIS1 (UVIS2) is approximately equal to the mean of the blue (green) triangles over all phylo levels.  The upper-left corner of amp A is systematically offset from the rest of the FoV, so the UVIS1 EE tables should be used with caution for small apertures.

3. PSF quality under Single Star Guiding

J. Anderson, S. Baggett

HST was designed to use two guide stars and two fine guidance sensors (FGS) to maintain pointing and tracking during exposures. However, using a single guidestar (1GS) can provide flexibility in scheduling as well as minimize the observation failure rate and maximize the lifetime of the FGS. Under 1GS, the telescope roll is controlled by a gyro instead of a by second guidestar, which can cause a drift during the orbit. We have quantified the drift under 1GS using all available archival data along with imaging from a 1GS calibration proposal. We find that:

a) For exposures < 500 sec,the PSF quality under 1GS is indistinguishable from that under 2GS and,

b) Over the course of full orbits (~2500 sec), the drift in four of five cases was <0.2 pixel and ~0.4 pix in the other case. 

Drifts of ~0.2 pixel are marginally detectable in observations and should have a minimal impact on most science programs given that the variation in the PSF caused by drift is smaller than the PSF variations with focus and with location on the detector. In addition, for observers who wish to correct drift effects, there are post-acquisition analysis techniques that can remove essentially all astrometric residuals, even for drift levels up to 0.5 pix (see Figure 4), as well as most of the photometric residuals.  A WFC3 Instrument Science Report is available at WFC3 ISR 2025-07.

A figure with 8 subplots, 4 in the left column and 4 in the right column. The left column shows the 1-D jitter along x as for an unperturbed PSF in blue, and the right shows the same for perturbed PSF in green.
Figure 4: Astrometric residuals as a function of pixel phase for simulated trailed observations, with the effective 1-D jitter along x as labeled within each panel. Residuals measured after analysis with a nominal PSF and a perturbed PSF are shown at left and right, respectively. (Figure 9 in WFC3 ISR 2025-07.)

4. New Documentation

ISR 2025-03: WFC3/IR Starter Guide - P. R. McCullough, J. D. Green

ISR 2025-04: WFC3/IR Geometric Distortion - Time Evolution of Linear Terms w.r.t. Gaia DR3 - A. O'Connor, V. Bajaj

ISR 2025-05: Improved UVIS Aperture Corrections derived from Focus Diverse PSF Maps - K. Huynh, V. Bajaj, J. Mack, A. Calamida

ISR 2025-06: Updates to the WFC3/UVIS Saturation Map - M. Revalski, I. Rivera, V. Bajaj, F. Dauphin

ISR 2025-07: How Single-Star Guiding Affects HST's Pointing Stability - J. Anderson, S. Baggett

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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