ACS STAN October 2019
1. SBC Absolute Flux Calibration Revised by ~30%
Roberto J. Avila, Ralph Bohlin, Marco Chiaberge, Nimish Hathi, Sean Lockwood, Diana Kossakowski, Jesse Averbukh, Pey Lian Lim, Michele De la Peña
The ACS Instrument Team has delivered new files that significantly revise the absolute flux calibration of the Solar Blind Channel (SBC) imaging modes. The throughput curves for the imaging modes have been updated to correct a long-standing 15% – 30% error in the absolute flux calibration. The offset is removed by adjusting throughput curves of various components of the different observing modes, bringing synthetic photometry in agreement with observed photometry. The resulting curves show that the detector is more sensitive that previously estimated. The practical result of these changes is that the new zeropoints are fainter than before. In other words, until now, the astrophysical fluxes of sources observed with the SBC have been overestimated.
The time-dependent (TDS) and spatial sensitivities of the SBC were measured using observations of the calibration star cluster NGC6681. The sensitivity of the detector declined by up to ∼ 9% since launch, with a rate of ∼ 0.5% per year since 2007. New low-frequency L-flats were derived by directly fitting 2D polynomial surfaces to spatial sensitivity data. The resulting products are smoother than the previous versions, due to the different method of deriving the flats. The overall photometric accuracy is 2.5% (except for F165LP which is 3.3%), after combining the low-frequency L-flats with the high-frequency P-flats to make new LP-flats, and applying the new TDS corrections.
Both the TDS and corrected throughput curves are now included in pysynphot models, the ACS exposure time calculator, and the ACS zeropoint calculator. ACS SBC observation files downloaded from MAST after October 23, 2019 will contain the updated photometric calibration header information. Users who wish to recalibrate raw SBC files will need to update their CALACS to the latest version to support these updates, as well as retrieve the latest reference calibration data from CRDS.
2. AstroDrizzle Bug Fix
Samantha L. Hoffmann & Jennifer Mack
Recently a bug was discovered and fixed in AstroDrizzle, caused by a change in a dependency: numpy version 1.12 or higher released on January 15, 2017. When drizzling images with final_wht_type= ‘IVM’, inverse variance weights should be automatically computed by AstroDrizzle using the flat-field reference file and the dark value from the image header. Instead, this bug resulted in IVM maps where all pixel values were set to 1.0. The bug did not cause AstroDrizzle to crash, but a warning wrote out to the log file, so those who used the AstroDrizzle calculated IVM maps starting in 2017 can check to see if their drizzled images were affected by the bug.
For more info about this fix and the other changes made, please see the 3.0.2 DrizzlePac release notes from July 15, 2019 as well as the GitHub issue documenting this bug. Please contact the HST DrizzlePac help desk via the HST Help Desk website with any questions.
3. Recent Instrument Science Reports
For a complete list with abstracts, please visit the ACS ISR webpage.
- ISR 2019-01: The ACS/WFC G800L Grism: I. Long-term Stability (Hathi et al.)
- ISR 2019-02: Post-SM4 ACS/WFC Bias I: The Read Noise History (Desjardins)
- ISR 2019-03: Assessing the Accuracy of Relative Photometry on Saturated Sources with ACS/WFC (Olaes, Hoffmann, & Bellini)
- ISR 2019-04: SBC Time-Dependent Sensitivity and L-flats (Avila et al.)
- ISR 2019-05: SBC Absolute Flux Calibration (Avila et al.)
- ISR 2019-06: Post-SM4 ACS/WFC Bias II: Temporal Structure in the Prescan Bias Level (Desjardins & Khandrika)
- ISR 2019-08: Temporal Stability of the ACS/WFC 0D-800W LED (Miles & Grogin)