Photometric Aperture Corrections for the ACS/SBC

We present aperture correction tables for the Advanced Camera for Surveys Solar Blind Channel (ACS/SBC). As part of a campaign to improve the instrument calibrations, we observed the white dwarf J132811.4+463050 using three filters (F125LP, F140LP, F150LP). The observed point spread functions (PSFs) contain more flux in the wings than Tiny Tim models, which can underestimate aperture corrections by as much as ∼ 9%, when compared with the observed fluxes. The updated aperture correction tables will be provided to the ReDCaT team so that they can be used in pysynphot and HST’s Exposure Time Calculator. Introduction Over the last decade, little work has been done on calibration for the Advanced Camera for Surveys (ACS) Solar Blind Channel (SBC). This report is the second in a series where various aspects of the SBC calibration will be inspected and updated if necessary. The documentation for radial profiles and encircled energy curves is spotty, unclear, and sometimes contradictory. A search of the literature did not uncover any documents providing aperture correction tables that users can refer to. The ACS Instrument Handbook (Avila et al., 2016) contains figures showing radial profiles and encircled energy curves for two filters, F125LP and F150LP (Figure 1). The provenance of these data is unclear, the most likely origin being ground testing (G. Hartig; priv. comm.). There are versions of these plots that Copyright c ©2016 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved. were presented by Tran et al. (2003) at the 2002 HST Calibration Workshop. Those plots come from work done during the service mission observatory verification (SMOV) campaign, where they used observations of the globular cluster NGC 6681 to characterize the on-orbit performance of the SBC. One key piece of information missing from this document is the size of the aperture used to normalize their encircled energy curves. Sirianni et al. (2003) presented work on the measurements of the on-orbit sensitivity of the camera, comparing observed to predicted count rates of stars in the same globular cluster as Tran et al. (2003). Sirianni et al. (2003) state that they used a 2.5′′ aperture to compare the observed and predicted count rates, although the absolute flux calibration was normalized using a 4′′ aperture. Finally, aperture corrections were provided by the ACS team for use in pysynphot (Lim et al., 2015) and the observatory’s Exposure Time Calculator (ETC), but it is also not clear how those were obtained. Figure 1: Radial profiles and encircled energy curves found in the ACS Instrument Handbook as of publication of this report. They will be replaced with the results from this work. Several users have also reported that both the encircled energy curves published in the instrument handbook and Tiny Tim models (Krist et al., 2011) do not accurately reproduce the observed wings of the PSF (M. Hayes; A. Bostroem, priv. comm.). In order to rectify these issues, the ACS Instrument Team decided that a calibration program should be carried out to observe a bright UV source from which aperture corrections could be established.