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Results: 168

Post-SM4 ACS/WFC Bias I: The Read Noise History (ACS ISR 2019-02)

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.

The ACS/WFC G800L Grism: I. Long-term Stability (ACS ISR 2019-01)

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/WFC Parallel CTE from EPER Tests (ACS ISR 2018-09)

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.

Focus-diverse, empirical PSF models for the ACS/WFC (ACS ISR 2018-08)

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.

Mitigating Elevated Dark Rates in SBC Imaging (ACS ISR 2018-07)

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.

Remeasuring the ACS/WFC Absolute Gains (ACS ISR 2018-06)

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.

Updates to the CALACS Cosmic Ray Rejection Routine: ACSREJ (ACS ISR 2018-05)

September 28, 2018N.D. Miles et al.
This report presents an analysis of the updated version of the ACSREJ contained in the current release of hstcal and available for download via AstroConda. The updated ACSREJ algorithm remedies a bug that caused the ERR extensions to be underestimated by a factor of 1/√g, where g is the CCD gain. This fix triggered a reprocessing of all ACS/WFC superdarks and consequentially affects all ACS/WFC observations. The effect of the increased error contributed from the superdarks is most pronounced for observations with extremely low background (≤ 2e−). Typical backgrounds in ACS/WFC observations are well above this limit and so for most cases the total noise is still dominated by the amplifier read noise and the sky background. Next, the core algorithm has been updated to use the ERR extensions when performing statistical rejection of cosmic rays, as opposed to an estimate derived from the comparison image. The updates to the rejection algorithm required modifications to be made to the cosmic ray rejection table, CRREJTAB. These updates only affect CR-SPLIT observations and a photometric analysis concludes the changes have no affect on actual sources. Lastly, in order to make the effects of newbias keyword more explicit, it was changed to be readnoise only. This keyword is only used to combine images with EXPTIME = 0 (i.e. bias frames) and as such only affects the generation of the ACS/WFC superbias reference files.

Improving the Pixel-Based CTE-correction Model for ACS/WFC (ACS ISR 2018-04)

August 29, 2018J. Anderson and J.E. Ryon
The pixel-based CTE correction was last constructed for ACS/WFC in 2010 and for WFC3/UVIS in 2013. Each of these instruments has now been in orbit for about twice as long as when the model was last constructed. Since the strength of CTE generally increases linearly with time, similar to the population of warm pixels, it makes sense to revisit the correction now that the effect is twice as strong and there exist twice as many pixels to measure it with. This ISR will demonstrate how we constructed a new model for ACS/WFC and will evaluate the model with on-sky data.

A Minor Contamination Event in May 2017 Affecting the ACS/WFC CCDs (ACS ISR 2018-03)

May 25, 2018S.L. Hoffmann et al
Here we present our investigation of three image artifacts that recently appeared in images from Hubble's Advanced Camera for Surveys (ACS) Wide Field Channel (WFC). We discovered one of the artifacts through visual inspection of a WFC image during an unrelated task. A search of routine calibration data revealed that it, and two additional artifacts, appeared on May 5th, 2017. We named the three image artifacts "flecks" because they looked to be small flecks of material sitting on the surface of the WFC detector in our initial examination. We characterized the flecks by analyzing them in routine calibration images. Due to their small size, we do not believe the flecks will have an impact on science use of ACS/WFC. Finally, while we believe the appearance of the flecks to be an isolated incident, we will continue to monitor ACS/WFC images for new flecks in the future.

ACS/WFC Sky Flats from Frontier Fields Imaging (ACS ISR 2017-09)

April 17, 2018J. Mack, R.A. Lucas, et al
Parallel imaging data from the HST Frontier Fields campaign (Lotz et al. 2017) have been used to compute sky flats for the ACS/WFC detector in order to verify the accuracy of the current set of flat field reference files. By masking sources and then co-adding many deep frames, the F606W and F814W filters have enough combined background signal that errors from Poisson statistics are <1% per pixel. In these two filters, the sky flats show spatial residuals ~1% or less. These residuals are similar in shape to the WFC flat field ‘donut’ pattern, in which the detector quantum efficiency tracks the thickness of the two WFC chips. Observations of blue and red calibration standards measured at various positions on the detector (Bohlin et al. 2017) confirm the fidelity of the F814W flat, with aperture photometry consistent to ~1% across the FOV, regardless of spectral type. At bluer wavelengths, the total sky background is substantially lower, and the F435W sky flat shows a combination of both flat errors and detector artifacts. Aperture photometry of the red standard star shows a maximum deviation of 1.4% across the array in this filter. Larger residuals up to 2.5% are found for the blue standard, suggesting that the spatial sensitivity in F435W depends on spectral type.

Updates to Post-Flash Calibration for the Advanced Camera for Surveys Wide Field Channel (ACS ISR 2018-02)

March 29, 2018N.D. Miles et al.
This report presents a new technique for generating the post-flash calibration reference file for the Advanced Camera for Surveys (ACS) Wide Field Channel (WFC). The new method substantially reduces, if not, eliminates all together the presence of dark current artifacts arising from improper dark subtraction, while simultaneously preserving flat-field artifacts. The stability of the post-flash calibration reference file over time is measured using data taken yearly since 2012 and no statistically significant deviations are found. An analysis of all short-flashed darks taken every two days since January 2015 reveals a periodic modulation of the LED intensity on timescales of about one year. This effect is most readily explained by changes to the local temperature in the area surrounding the LED. However, a slight offset between the periods of the temperature and LED modulations lends to the possibility that the effect is a chance observation of the two sinusoids at an unfortunate point in their beat cycle.

Comparing the ACS/WFC and WFC3/UVIS Calibration and Photometry (ACS ISR 2017-10)

March 12, 2018S.E. Deustua and J Mack
A study was undertaken using synthetic photometry of CALSPEC stars to compare the ACS Wide Field Channel (WFC) photometry to the WFC3 UVIS imaging channel in eight similarly named passbands corresponding to the broadband filters F435W (ACS/WFC) F438W (WFC3/UVIS) and F475W, F555W, F606W, F625W, F775W, F814W and F850LP (both ACS/WFC and WFC3/UVIS). The uncertainty of the photometric calibration of ACS/WFC and WFC3/UVIS with respect to the white dwarf standard stars is within +/- 0.5% for F814W, F775W, F606W and F475W, and within +/-1% for F625W and F850LP. For F555W the apparent difference in the calibration is 2% for F555W and 6% for UVIS/F438W and ACS/F435W due to inherent differences in the filter passbands. Comparing the ACS/WFC to WFC3/UVIS mean flux for stars having a range of spectral types shows a color dependence. The WFC to UVIS F814W color dependence is +/- 0.02 mags for F814W, F775W, F475W and F606W. For the other filters the range is -0.06 to +0.02 mags. Aperture photometry of the 47 Tucanae cluster confirm the results from using synthetic photometry of CALSPEC stars.

Accuracy of the HST Standard Astrometric Catalogs w.r.t Gaia (ACS ISR 2018-01)

February 16, 2018V. Kozhurina-Platais et al.
The goal of astrometric calibration of the HST ACS/WFC and WFC3/UVIS imaging instruments is to provide a coordinate system free of distortion to the precision level of 0.1 pixel ( 4-5mas) or better. This astrometric calibration is based on two HST astrometricstandard elds in the vicinity of the globular clusters, 47 Tuc and ω Cen, respectively. The derived calibration of the geometric distortion is assumed to be accurate down to 2-3 mas. Is this accuracy in agreement with the true value? Now, with the access to globally accurate positions from the rst Gaia data release (DR1), we found that there are measurable o sets, rotation, scale and other deviations of distortion parameters in two HST standard astrometric catalogs. These deviations from the distortion-free and properly aligned coordinate system should be accounted and corrected for, so that the high precision HST positions are free of any systematic errors. We also found that the precision of the HST pixel coordinates is substantially better than the accuracy listed in the Gaia DR1. Therefore, in order to nalize the components of distortion in the HST standard catalogs, the next release of Gaia data is needed.

Accounting for Readout Dark in ACS/WFC Superbiases (ACS ISR 2017-13)

December 22, 2017J.E. Ryon et al.
We investigate the properties of excess dark current accumulated during the 100-second full-frame readout of the Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) detectors. This excess dark current, called “readout dark”, gives rise to ambient background gradients and hot columns in each ACS/WFC image. We present an analysis of simulated readout dark images developed in ACS ISR 2014-02, and find that the results from the simulated data cannot be fully reconciled with the results from observed bias images. We develop a new method to estimate the readout dark noise properties in ACS/WFC observations instead of using simulated images. While readout dark signal is removed from science images during the bias correction step in CALACS, the additional noise from the readout dark is currently not taken into account. We will update the ERR extensions of superbias images to include the appropriate noise from the ambient readout dark gradient and stable hot columns. We will also flag unstable hot columns in the DQ extensions of the superbiases. A new reference file pipeline for ACS/WFC that will implement these changes is currently under construction.

A Study of PSF Models for ACS/WFC (ACS ISR 2017-08)

October 26, 2017S.L Hoffmann and J. Anderson,
This study compares PSFs generated with Tiny Tim against an empirically-derived, effective PSF (Anderson & King, 2006) for the Hubble Space Telescope Advanced Camera for Surveys/Wide Field Channel imaging. We manipulate the Tiny Tim PSF FITS files into a format that can be utilized by the effective PSF FORTRAN photometry code. Then we perform PSF photometry on globular cluster NGC 6397 and analyze the photometry and astrometry results. We measure a value of 0.227 ± 0.032 for a quality-of-fit metric of the Tiny Tim PSF and a corresponding 0.117 ± 0.021 for the effective PSFs, an improvement of a factor of approximately two. We find that the effective PSF models outperform the Tiny Tim PSFs in every measurement of stellar sources in this field.
LAST UPDATED: 06/13/2019

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