March 06, 2020Y. Cohen et al.
We document a recently noticed phenomenon in which post-SM4 ACS/WFC raw images appear to have a lower pixel value ceiling than expected. During readout of an exposure, if a given pixel would have a value in excess of 2^16 - 1 = 65,535 DN after converting to DN from electrons given the gain setting, the A-to-D converter should simply record a value of 65,535 for that pixel. This is the case for pre-SM4 images, but for post-SM4 images, there appears to be a bug which causes the ceiling value to be slightly lower, and somewhat variable, typically in the range of 64,500 +/- 1,000 DN. The cause of this issue is presently unknown, but it affects only a small number (~100) of archival images and is not expected to affect any future ACS observing programs. The ACS team has now rectified the data quality flagging to catch such A-to-D saturated pixels, which should be treated as unusable.
(ACS ISR 2020-02) New and Improved Saturated Pixel Flagging for the ACS/WFC
January 06, 2020Y. Cohen et al.
Accurate characterization of the saturation level of the ACS/WFC CCD is crucial for proper flagging of affected pixels, which users and calibration routines require knowledge of. In this work, we present a new analysis of the saturation level that offers significant improvements and advantages over previously used methods. Unlike previous work, we measure the onset of saturation directly by identifying the precise charge level at which the brightest pixel of point sources begins to spill charge into neighboring pixels. This results in a sharp decrease in the fraction of charge contained in the central pixel, coupled with a sharp increase in the fraction of charge contained in neighboring pixels. Through this analysis, we find that the saturation level has a strong spatial dependence over the detector area and exhibits significant (+/- ~6% about the mean) variations, in agreement with previous work. Despite this qualitative agreement, we find that the saturation level currently used in the CALACS calibration pipeline to flag affected pixels is much too high, causing it to routinely miss many clearly saturated pixels. When using our new saturation map to perform the flagging, we find visually superior results and as many as ~15% more pixels being flagged as saturated in any given frame. We announce plans to implement our new saturation map into CALACS, and discuss extensions of this work.
(ACS ISR 2020-01) Post-SM4 ACS/WFC L-Flats and Photometric Errors from Observations of Stellar Fields
January 06, 2020Y. Cohen et al.
We present the first comprehensive post-SM4 study of the low-frequency correction to the ACS/WFC flat-field (L-flat). Using a large sample of constant brightness sources from archival image data, we map out variations in their brightness from multiple dithers over the WFC CCD area, and fit a low-order flat-field model that minimizes those variations. We find strong similarity between our resulting model and the current database L-flat, with <~ 1% differences everywhere, indicating that the L-flat has remained quite stable over time. However, even after correcting for the flat-field, we find that the photometric scatter of dithered point sources is larger than expected from current error models, ranging between 0.5% to 3% depending on instrumental magnitude and exposure time. This suggests that one or more of the terms in the current error model, which includes contributions from the various reference files (e.g. flats, darks, biases) and CTE losses, are underestimated. The team is now considering revising the error arrays and expanding the scope of the photometric error analysis.
(ACS ISR 2019-10) Bright Object Magnitude Limits for ACS/SBC and Color Corrections for All Three Channels
December 06, 2019J. E. Ryon et al
Recent updates to the ACS/SBC throughput curves prompted a review of the bright object limits and color correction tables in the ACS Instrument Handbook. We recalculate the Johnson V magnitudes that correspond to the local absolute count rate limit of the SBC, 50~counts/sec/pixel, for a range of stellar models and real stars. We also recalculate the color corrections between Johnson V and ACS magnitudes for all three ACS channels, considering a variety of target spectra. Two new sets of color corrections are determined for GALEX-to-SBC conversions. The updated bright object limits and color corrections can be found in the ACS Instrument Handbook for Cycle 28 and the Appendix of this report.
(ACS ISR 2019-08) Temporal Stability of the ACS/WFC OD-800W LED
September 16, 2019N.D. Miles and N. A. Grogin
This report summarizes an analysis of the ACS/WFC post-flash LED stability over a ~4.5 year period. We analyze 1,294 post-flash calibration darks generated from January 2015 to July 2019. We find the observed intensity of the LED, relative to the intensity in Jannuary of 2015, to be declining at a rate of ~0.2% per year. We use the resulting fit to the LED signal over time to compute a time-dependent normalization factor for each annual post-flash reference file. This normalization reduces the observed scatter between the post-flash reference files caused by short-term fluctuations in the intensity of the LED. The normalized post-flash reference files are currently available in the HST Calibration Reference Data System (CRDS).
(ACS ISR 2019-06) Post-SM4 ACS/WFC Bias II: Temporal Structure in the Prescan Bias Level
July 16, 2019T.D. Desjardins and H.G. Khandrika
We present a study of the temporal variations in the ACS/WFC bias prescan level following SM4. We found two distinct properties of the bias level over time: 1) a nearly 50 electron loss in bias level in the 3 years immediately following SM4; and 2) periodic behavior in the bias level. A Lomb-Scargle periodogram analysis yielded four distinct periods in the bias level of 364.60, 54.72, 42.19, and 23.84 days. We found similar periods in the temperature of the CCD electronics box replacement (CEB-R), which suggested that small, ambient thermal variations near the electronics were manifesting as fluctuations in observable properties of the WFC CCDs. We connected the observed periods, in decreasing order of period duration, to the orbit of Earth around the Sun, the precession of the ascending node of the HST orbit, and the angle of the telescope. We were not able to explain the origin of the 23.84 day period. Regarding the loss of 50 electrons of bias level, we conjectured that this behavior was the result of changes in the long-term performance of one of the CEB-R components such as the ASIC.
(ACS ISR 2019-05) SBC Absolute Flux Calibration
October 17, 2019R. J. Avila et al.
The throughput curves for the imaging modes of the Advanced Camera for Surveys Solar Blind Channel (SBC) have been updated to correct for a 15% -- 30% error in the absolute flux calibration. The offset is removed by adjusting throughput curves of various components of the different observing modes, and bringing synthetic photometry into agreement with observed photometry. The resulting curves show that the detector is more sensitive than previously estimated. The practical result of these changes is that the new zeropoints are fainter than before. In other words, until now, the observed astrophysical fluxes of sources have been overestimated. Updated zeropoints for F122M and F165LP have accuracies of ~4.5%, while the other filters have accuracies better than ~1.7%. New throughput curves and other necessary support files have been delivered to the calibration pipeline so that, from now on, SBC images downloaded from MAST contain the appropriate zeropoints.
(ACS ISR 2019-04) SBC Time-Dependent Sensitivity and L-flats
September 16, 2019R. J. Avila et al.
The time-dependent and spatial sensitivities of the SBC detector on ACS 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%/year since 2007. New calibration files (IMPHTTAB) were produced, and will be included in the calibration pipeline in order to properly update the photometric zeropoints of every FLT image. The low-frequency L-flats were derived by directly fitting 2D polynomial surfaces to the spatial sensitivity data. The resulting products are smoother than the previous versions, due to the diffrent method of deriving the flats. The corrections in the flats are on the order of ~8%. 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.
(ACS ISR 2019-03) Assessing the Accuracy of Relative Photometry on Saturated Sources with ACS/WFC
July 30, 2019M. Olaes, S. Hoffman, and A. Bellini
Upon full-well saturation, the pixels on the ACS/WFC CCDs bleed excess charge onto adjacent pixels along their column. For these saturated sources, aperture photometry may report a lower flux than expected. However, this effect can be mitigated by defining an aperture which encompasses all of the pixels which contain the full-well bleed. Here we present an assessment of relative photometry of saturated sources from observations of the globular cluster 47 Tuc. We demonstrate an alternate aperture photometry method that defines a custom aperture for each source by identifying pixels which contain the lost flux. This "aperture+" photometry method obtains >90% accurate photometry of saturated stars out to 3.34+/-0.015 magnitudes brighter than 0.5" circular aperture photometry.
(ACS ISR 2019-02) Post-SM4 ACS/WFC Bias I: The Read Noise History
March 28, 2019T. D. Desjardins
We report on the read noise history of the ACS/WFC readout amplifiers since the repair of the instrument during Servicing Mission 4 in May 2009. We find that readout amplifiers B and C remain well-behaved with a slow increase in the read noise of approximately 0.0035–0.0048 electrons per year. Amplifiers A and D (since its read noise anomaly in January 2013) exhibited periods of instability in read noise with infrequent jumps of several hundredths of an electron, faster than typical increases, and occasional decreases in noise over prolonged periods. We also investigate for the first time the read noise of the ACS/WFC subarray modes both before and after the change to the subarray format in Cycle 24. We find that the subarray modes prior to Cycle 24 had systematically higher read noise values, and the read noise was inversely proportionate to the size of the subarray, i.e., smaller subarrays had higher read noise. After the changes to the subarray readout patterns in Cycle 24, the read noise values in subarray readouts match the full-frame.
(ACS ISR 2019-01) The ACS/WFC G800L Grism: I. Long-term Stability
February 28, 2019N. Hathi et al.
We have obtained new ACS/WFC G800L grism observations of the Wolf-Rayet star WR96, a wavelength calibration target, in HST Cycle 25 (PID: 15401) to evaluate differences, if any, in the basic grism properties compared to the previous calibration data. The past calibration efforts for the ACS/WFC G800L grism were based on observations from 2003. In this ISR, we compare these new observations with the previous (pre-SM4) results to validate various basic grism properties: the length and separation of different grism orders, the X/Y shift between the object position in the direct image and the position of the grism 0th order, the spectral tilt, and the wavelength calibration. Our results qualitatively agree with the previous measurements, and confirm that the wavelength calibration of the ACS/WFC G800L grism is consistent within 1 pixel (∼40 ̊A). In an upcoming ISR, we will use all the existing WR96 ACS/WFC grism data along with a new and improved data analysis technique to refine the wavelength calibration of the ACS/WFC G800L grism.
(ACS ISR 2018-09) ACS/WFC Parallel CTE from EPER Tests
December 20, 2018J. Ryon et al.
We present a new analysis of parallel charge transfer efficiency (CTE) in ACS/WFC over its operational lifetime. We utilize extended pixel edge response (EPER) data to monitor the signal and time dependence of CTE in the WFC CCDs, taking a similar approach to Mutchler & Sirianni (2005). We find that CTE has a power law dependence on signal level,such that CTE is worst for low signal levels and best for high signal levels. We also find that CTE decreases linearly with time. The rate of decrease is higher for low signal levels, but may be flattening in recent data at higher signal levels. Monitoring and comparison to other CTE studies will continue for the rest of ACS’s lifetime.
(ACS ISR 2018-08) Focus-diverse, Empirical PSF Models for the ACS/WFC
November 26, 2018A. Bellini et al.
Focus variations, primarily due to uneven Sun heating of the telescope tube, have a significant impact on the shape of the ACS/WFC point-spread function (PSF). These variations can be properly accounted for on an image-by-image basis by perturbing the library PSF models (Anderson & King 2006) when many bright, relatively isolated stars are present: a luxury that many HST users do not enjoy. This report presents an exploratory analysis of these focus variations and describes the procedures to obtain focus-diverse, spatially-variable PSF models from flc ACS/WFC images taken with the two most commonly used filters: F606W and F814W. The new PSF models are shown to be superior to the library PSF models, particularly when the focus level is extreme, and provide results comparable to those obtained by PSF-perturbation techniques without the need for populated stellar fields in an image. Future analyses will comprise the construction of focus-diverse PSF models for the several other commonly used filters of the ACS/WFC and their implementation in the hst1pass reduction package.
(ACS ISR 2018-07) Mitigating Elevated Dark Rates in SBC Imaging
October 26, 2018R.J. Avila et al.
We present a new aperture that can be used to mitigate elevated dark rates in SBC imaging modes. The reference pixel of this new aperture is located at (175,185) on the detector. At this location the dark rate remains constant at all temperatures. This aperture is limited to observations of small targets, but visits can span an extended number of continuous orbits. We also present results on the heating and cooling rates of the detector. The length of time that the SBC is enabled affects how long it takes to cool back down to its initial temperature. It takes ~2 hours for the detector to reach a temperature at which the dark rate becomes elevated. Once that threshold is reached, it takes ~6 hours after the detector is turned off for the temperature to go back down to acceptable levels.
(ACS ISR 2018-06) Remeasuring the ACS/WFC Absolute Gains
October 22, 2018T.D. Desjardins and N.A. Grogin
We measure the absolute gains of the ACS/WFC readout amplifiers for the first time since Servicing Mission 4 (SM4) in 2009. Due to effects now known to be present in post-SM4 ACS observations, but which were either unknown or not well-calibrated at the time, we also recalculate the absolute gains from the Servicing Mission Observatory Verification (SMOV) period immediately after SM4 using a subset of the original data. At the 95% confidence level, we find that the gains measured from data obtained in 2017 match those from SMOV data within the uncertainties.