October 15, 2020Y. Cohen and N. A. Grogin
When CCD pixels become saturated during an integration, excess charge may begin to spill over into neighboring pixels.
This effect is commonly referred to as `blooming', and the architecture of the CCD detector makes it such that this blooming is expected to occur only in the same column as the saturated pixel (vertically).
However, for the ACS/WFC CCD, we recently noticed a previously-undocumented effect whereby a relatively small fraction (< 10%) of excess charge from saturated pixels spills over into neighboring pixels in the same row (horizontally).
In this report, we present the characteristics of this horizontal spilling of charge, or `x-bloom', and a resulting pipeline to automatically identify and flag affected pixels.
We demonstrate that proper accounting for the charge displaced by x-bloom, in addition to that lost to traditional vertical bloom, enables high accuracy photometry of arbitrarily saturated sources.
This result can significantly increase the science value of archival WFC data, since a large fraction of archival images contain saturated sources which are subject to x-bloom.
August 05, 2020M. C. McDonald, N. A. Grogin
Since installation aboard the Hubble Space Telescope (HST) in 2002, the Advanced Camera for Surveys (ACS) undergoes a monthly annealing process that is effective in reducing dark current in the Wide Field Channel (WFC) pair of CCD detectors. After failing to return to operate mode following two completed anneals in early 2019, ACS suffered back to back safing events. As a result, a new Reduced Operate Anneal (ROA) mode was implemented in October 2019 to avoid future reboot failures. In this report, we investigate the effects of the newly instated ROA mode through an analysis of two different sets of pre- and post-anneal dark frames: those from the CCD Hot Pixel Annealing and Daily Monitor calibration programs. We conclude that ROA mode is similar or more effective than the previous anneal mode in reducing the global dark current and the percent CCD coverage of hot and warm pixels in the ACS/WFC.
April 30, 2020M. C. McDonald et al.
The population of hot pixels in the Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) has been shown to be temporarily reduced by a monthly annealing process. In this report, we investigate the incidence and frequency of hot pixels, from the installation of ACS on the Hubble Space Telescope in 2002 to mid 2019, to assess the efficacy of anneals in reducing the overall dark current and total number of hot pixels in the WFC CCD detectors. An inspection of calibrated and combined raw dark frames taken directly before and after each anneal imply less effective hot pixel healing in recent years, with several post-anneal darks having a higher dark current rate and hot pixel count than their pre-anneal counterparts. The percent hot pixel coverage is tracked using individual calibrated dark frames over a four-year period from 2015 to 2019. We find a characteristic zig-zag shape, indicative of the periodic rise and fall of hot pixel coverage over several anneal cycles, but with lots of scatter between anneal dates. Finally, the dark current of a single, stable, frequently hot pixel is mapped to reveal significant variations, up to 2.0 e-/s, over the span of several years.
April 15, 2020J. E. Ryon, N. A. Grogin, T. D. Desjardins
We describe our approach to accounting for readout dark current in subarray superbiases. Originally, the subarray modes available for ACS/WFC used different timing patterns during readout than full-frame images, resulting in different amounts of readout dark as compared to the equivalent regions of the full-frame. These subarray modes were replaced at the start of Cycle 24 (October 2016) with the current modes, which use the same timing patterns as full-frame images. There are therefore three subarray cases that must be dealt with separately: (1) pre-SM4, (2) post-SM4 prior to Cycle 24, and (3) post-SM4 after Cycle 24 to present. The first two regimes will be handled by creating superbiases for the original subarray modes that account for readout dark. The third regime is handled internally in CALACS. We also discuss an update to the calculation of ambient readout dark, which applies to both full-frame and subarray superbiases.
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.
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.
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.
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.
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).
June 26, 2020R. A. Lucas et al.
When planning Advanced Camera for Surveys (ACS) observations, there are a number of things one should always check and consider, both in Phase I proposing and later in Phase II observing program development and design. This ISR lists many of these topics here in one place, together with references, and introduces up-to-date information via this report and a new web page regarding current best practices for proposing and observing with the ACS on the Hubble Space Telescope.
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.
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.
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.
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.
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.