Photometric Calibration

For the absolute flux calibration of the WFC3/UVIS detector, the aperture-corrected photometry of standard stars observed in staring mode is compared to the predicted photometry of simulated observations. Spatial scans offer greater precision than staring mode observations, but currently cannot be used directly for the absolute calibration of the instrument, as existing software used for generating synthetic observations lacks the capacity to model rectangular photometric apertures used for spatial scans. In this report, we introduce a novel method for calculating aperture corrections for spatial scans, and present the results of preliminary tests of this methodology. We find that ratios of observed-to-synthetic flux are constant over time, validating the implementation of current time-dependent zeropoints. However, the data exhibits a wavelength- and chip-dependent offset between observed and synthetic count rates. This offset may be due to underlying factors complicating the observed photometry, aperture corrections, or both. Until this discrepancy is resolved, spatial scans will not be directly used for the photometric calibration of the WFC3/UVIS instrument. Meanwhile, we provide calculated offset values for each chip and filter as evidence of our initial efforts. A future report will utilize deep exposures from an upcoming calibration program (Program 17271) to examine encircled energies at large radii in order to further refine the process of calculating aperture corrections for spatial scans.

The infrared channel of Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) is frequently used to obtain precision photometric measurements. We investigate the change in the sensitivity of the IR channel over time by comparing photometry of outer, less crowded regions of stellar cluster images at multiple epochs. The channel appears to be losing sensitivity at a rate of approximately 0.13±.02% per year with no apparent wavelength dependence, though the slope and observation dates vary from cluster to cluster. Staring mode observations of the standard stars appear to show more inconsistent results, even within the same filter, but this is likely due to the presence of systematics such as detector preconditioning from prior observations.

Using five years of observations from the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3), we assess the changing sensitivity rate of the two WFC3/UVIS charge-coupled devices (CCDs) and evaluate the photometric repeatability of the spatial scan observing mode in comparison to the standard staring mode. We perform aperture photometry on vertical, linear spatial scans of two white dwarf standard stars (GD153 and GRW+70D5824) taken on corner subarrays of the WFC3/UVIS detector, and compute the rate of photometric sensitivity decline from 2017 to 2021. To gauge the relative accuracy of scans, we compare sensitivity losses for staring mode observations over the same 5-year time scale and those acquired over longer time scales. After removing the time-dependence of the relative photometry, dispersion of the residuals is used as a proxy to measure repeatability of observing modes, and thus assess precision. We establish that spatial scans are more precise than staring mode observations. Scans with UVIS 1 show 2.4 times less residual noise than their staring mode counterparts; for UVIS 2, residual noise for scans is 2.5 times less than residual noise for staring mode. For scans, sensitivity losses are relatively flat independent of wavelength on both UVIS CCDs, with no evidence of contamination. UVIS 2 appears to have slightly higher losses (-0.17 +/- 0.01 %/yr) compared to UVIS 1 (-0.12 +/- 0.01 %/yr). When measured over the same time period, spatial scans and staring mode observations yield filter-dependent loss rates that agree well with each other in most filters within computed uncertainties.

We compute new UVIS encircled energy (EE) curves from images of HST standard stars observed from 2009-2020, after correcting for temporal changes in the sensitivity of each CCD and filter. The latest UVIS photometric calibration (Calamida et al. 2021) makes use of the updated EE values presented in this report for two filters (F275W and F814W). We extend this analysis to five additional filters (F336W, F200LP, F350LP, F775W, and F850LP) to investigate differences between the 2009 EE, derived just after WFC3’s installation, and the 2016 chip-dependent EE, computed by averaging inflight observations over ~6 years. To recompute the EE, we rescale the calibrated FLC science array values using the new time-dependent inverse sensitivity (PHOTFLAM) keyword values, align the images in detector coordinates, and combine all images in a given CCD and filter. This process allows for a more accurate measure of the PSF at large radii out to 6 arcsec. We compare the EE values in a 10-pixel radius aperture, EE(10), used for the photometric calibration, and we find that the values for the two CCDs now agree to ~0.1% for each filter, compared to the 2016 solutions which differed by up to 0.5%. At UV wavelengths, the EE(10) is now lower by ~1% for both CCDs, in closer agreement with the 2009 solution. For two ‘red’ filters (F775W and F850LP), we compare the EE curves for a white dwarf and a G-type star but find no significant differences due to the color of the source. The new EE(10) values will be applied in a future update to the UVIS photometric calibration.

We use spatial scanning observations of stars in the open cluster M35 to estimate the photometric repeatability of the Wide Field Camera 3 IR imaging mode and probe the time dependence of its sensitivity. With data taken using the WFC3 infrared detector and the F140W filter, we estimate the near-term 1σ photometric repeatability to be 0.65% and find a long-term sensitivity evolution at the rate of -0.024 ± 0.008% per year. Our observations with the F098M filter do not suggest any significant loss of sensitivity over time but additional data are required to reach more robust conclusions. We also investigate various possible systematics affecting these analyses.

We present new time-dependent WFC3 UVIS1 and UVIS2 inverse sensitivities for the 42 filters covering both detectors. The new values were calculated using photometry collected from 2009 to 2019 for five CALSPEC standards, the white dwarfs GRW+705824, GD153, GD71, G191B2B, and the G-type star P330E. Using these data, we compute sensitivity changes for each detector and filter and normalize the observed count rates of the standard stars to a reference time in 2009. The new set of inverse sensitivity values use new standard star models and an updated reference spectral energy distribution for Vega. By correcting for sensitivity changes with time, we derive improved detector sensitivity ratios and new encircled energy values for several filters. At the same time we update the inverse sensitivities for the 20 quad filters using the new models for the standard stars and Vega. However, for these filters no time-dependent sensitivity changes are calculated. The new inverse sensitivities provide a photometric internal precision better than 0.5% for wide-, medium-, and narrow-band filters, and 5\% for quad filters, a considerable improvement from the latest 2017 calibration. The new time-dependent inverse sensitivities are populated as photometric keywords in the image headers as of October 15, 2020.

New ‘pixel-to-pixel’ P-flats have been derived from deep images of the IR sky background, computed by stacking high signal-to-noise observations of sparse fields acquired over the lifetime of WFC3. The new sky flats correct for wavelength-dependent residuals of ± 0.5% in the central 800x800 pixel region of the detector and up to 2% near the detector edges. As of October 2020, these replace the prior 2011 set of P-flats, which were based on ground test data multiplied by a smoothed ‘grey’ (filter-independent) correction derived from sky flats using the first 18 months of in-flight data. An accompanying set of ‘delta’ D-flats now correct for 148 catalogued ‘blobs’ in six IR filters as a function of the epoch of observation. These were computed by stacking the same set of sky flat observations, but only after the appearance of each new blob.

We present the continued analysis of photometric measurements of the CALSPEC standard stars over the last 11 years in all of the 15 WFC3/IR filters. In general, the photometry (countrate) is consistent with the 2012 calibration to 1% or better. However, new models for the CALSPEC primary white dwarfs changed the HST photometric flux reference system, thus changing the inverse sensitivities and zeropoints. This change is less than 0.5% on average for the wide filters, but increases to just under 2% for the reddest medium and narrow filters. No discernible changes in sensitivity over time are detected in the measurements, but this is partially due to a lack of precision likely caused by persistence of previous observations, as well as other effects that are not currently well understood. The new zeropoint tables are presented on the website.