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The 2005 HST Calibration Workshop Space Telescope Science Institute, 2005 A. M. Koekemoer, P. Goudfrooij, and L. L. Dressel, eds.

Calibration Status of the Advanced Camera for Surveys
K. Sembach Space Telescope Science Institute, Baltimore, MD 21218 Abstract. The Advanced Camera for Surveys is producing sp ectacular science results that require careful instrument calibration. This article provides an overview of recent calibration highlights and calibration plans for Cycle 14.

1.

Introduction

The Advanced Camera for Surveys (ACS) is op erating nominally at the time this review is b eing written. ACS instrument p erformance with HST in two-gyro mode is indistinguishable from its p erformance in three-gyro mode (see Sembach et al., this volume and references therein). The ACS Calibration Team at STScI and ST-ECF continually monitors instrument p erformance through its ongoing instrument calibration activities, and up dates the ACS Instrument Handbook (Gonzaga et al. 2005) released with the HST Cal l for Proposals at the b eginning of each observing cycle. Cycle 14 brings some new challenges for the calibration of the instrument, as there is somewhat increased demand for Solar-Blind Channel (SBC) observations and prism/grism observations now that STIS is no longer op erating. On b ehalf of all the p eople involved in ACS calibration activities, this article contains a brief overview of the instrument status and calibration program for Cycle 14. More information ab out sp ecific calibrations can b e found in accompanying articles by ACS Calibration Team memb ers and Guest Observers in the astronomical community. Instrument Science Rep orts (ISRs) that provide detailed descriptions of the calibration procedures and analyses can b e found on the ACS ISR web page: http://www.stsci.edu/hst/acs/documents/isrs Information ab out calibration activities in previous cycles, including the titles and program numb ers of calibration programs, can b e found at http://www.stsci.edu/hst/acs/analysis/calib plan In preparation for Cycle 14, the ACS Calibration Team reviewed previous Cycle 13 calibration programs and identified activities that needed to b e continued into Cycle 14. It also identified new calibration needs that were not covered by the existing programs. As part of this exercise, the team trimmed the sizes of some routine calibration programs by consolidating and reducing the cadence of observations whenever p ossible. Table 1 contains a summary of the numb er of ACS calibration orbits allotted for Cycle 14. With contingency, the total orbit request for the primary calibration program is 1320 orbits (67 external, 1253 internal). For comparison, the Cycle 13 ACS calibration program required 70 external orbits and 1138 internal orbits. Thus, the numb er of orbits required for the Cycle 14 calibration program is comparable to that in Cycle 13. Routine monitoring programs take less time than b efore, but sp ecial calibrations added in this cycle require more orbits than the sp ecial programs in the previous cycle (see Tables 2-4 in the following sections). A small amount of 3 c Copyright 2005 Space Telescop e Science Institute. All rights reserved.


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Sembach Table 1: Cycle 14 ACS Calibration Orbit Summary Cycle 14 Total Time (routine + sp ecial programs) With Contingency External 62 67 Orbits Internal Outsourced 1142 11 1253 11

a

a

Two programs (PI = Dolphin): ACS Zero-Point Verification (#10621) and ACS Photometric Calibration from Faint Standards (#10622).

contingency time (8-10%) is included in the calibration program for each cycle to account for unforeseen calibration needs. The 11 outsourced orbits awarded in Cycle 14 by the HST Time Allocation Committee are included in Table 1 for completeness. Calibration outsourcing is an imp ortant comp onent of the calibration program since it allows Guest Observers to obtain calibrations that are not otherwise covered by the primary calibration plan defined by the STScI ACS Team. We encourage Guest Observers to prop ose calibration GO programs when they need more precise calibration data than the standard instrument calibration activities provide, or when they need calibrations for an uncalibrated mode of op eration. Requests and suggestions for calibrations to include in the standard ACS calibration plan are also welcomed and will b e given full consideration within the context of the overall calibration plan for the subsequent cycle of observations. 2. Cycle 14 Routine Monitoring Programs

Various routine monitoring and maintenance programs are required to track the p erformance of the instrument and ensure that observers have the most up-to-date calibrations available to reduce their data. Examples of routine calibrations for Cycle 14 include maintaining accurate flat field, dark, and bias frames, monitoring photometric throughput and stability, and tracking CCD charge transfer efficiency (CTE). A summary of the routine calibration programs for Cycle 14 is given in Table 2, which lists the prop osal numb er, principal investigator, program title, frequency of observations, and the numb er of external and internal orbits required during the cycle. These programs are describ ed briefly b elow. Table 2: Cycle 14 Routine Monitoring Programs
Program ID 10729 10730 10732 10733 10734 10735 10736 10737 10738 10739 10740 Total (Cycl PI Sirianni Chiaberge Mutchler Cox Cox Cox Maiz Mack Mack Bohlin Bohlin e 14 routine Title CCD Daily Monitor External CTE Monitor Internal CTE Monitor CCD Hot Pixel Annealing CCD Post-Flash Verification SBC MAMA Recovery UV Contamination Monitor CCD Stability Monitor Earth Flats Internal Flat Fields Photo-Spectrophot. Abs. Cal. monitoring programs) Frequency 4/week 6 months yearly monthly yearly as needed 6 months 3 months weekly 4 months yearly Orbits Ext. Int. 0 840 9 0 0 35 0 143 0 4 0 4 4 2 13 0 0 52 0 44 7 0 33 1124 Note Dark, bias creation CTE loss calibration Check against ground Includes monthly CTE Tracks capability only After irregular safing SBC, HRC tracking L-flat, distortion, photometry Tracks coronagraphic spot SBC components once Filter throughputs, QE


ACS Calibration Status 2.1. Darks and Biases

5

The CCD Daily Monitor program (#10729) measures the read-noise and dark current in the ACS CCDs as a function of time. It also tracks the growth of hot pixels. CALACS uses the reference files generated by this calibration program for every CCD science exp osure it processes, so it is essential that the prop erties of the darks and biases b e up dated frequently. As a result, the numb er of orbits required for this program accounts for the vast ma jority of ACS internal calibration orbits. For Cycle 14, the default gain setting for the Wide-Field Channel (WFC) has b een changed to GAIN=2 e- /ADU, so we will b e obtaining dark and bias data to supp ort this new gain setting as well as other supp orted gain settings. See Lucas et al. (this volume) and Mutchler et al. (this volume) for more information on the automated procedures used to process ACS dark and bias frames for use in CALACS. Recent relevant ISRs include: § SBC Dark and Cumulative Images, ACS ISR 2004-14, by C. Cox § Bias and Dark Calibration of ACS Data, ACS ISR 2004-07, by M. Mutchler et al. 2.2. CCD Hot Pixel Annealing

The CCD Hot Pixel Annealing program (#10733) reduces the numb er of non-p ermanent hot pixels on the ACS CCDs by warming the detectors to temp eratures of +20 C for 6 hours b efore returning to the normal op erating temp eratures (-77 C for the WFC and -80 C for the HRC). These anneal times are a factor of two shorter than in previous cycles; program #10453 in Cycle 13 demonstrated the effectiveness of reducing the anneal time from 12 hours to 6 hours. Reduced anneal times make scheduling easier. At the current hot pixel threshold of 0.08 e- pix-1 sec-1 , the anneals typically eliminate 82% of the non-p ermanent hot pixels on the WFC and 87% on the HRC (see Sirianni et al., this volume, for more information). Data from this program are also used in the monitoring of CTE (internal) and the tracking of dark levels. 2.3. Flat Fields

Several routine monitoring programs provide information ab out the flat field prop erties of the ACS CCDs as measured using b oth external observations (stellar photometry and bright Earth illumination) and internal lamp exp osures. The CCD Stability Monitor program (#10737) is the primary source of low-frequency flat fields (L-flats). Regular observations of the same star field within the globular cluster 47 Tuc provide L-flats that have an accuracy of 1% over the full fields of view of the HRC and WFC. The observations track changes in relative sensitivity with an accuracy of 0.1% p er year. This program also monitors variations in the CCD geometric distortion corrections with time. The Earth Flats program (#10738) cross-checks the L-flats created by the CCD Stability Monitor and tracks any changes on time scales shorter than the three month interval of the CCD Stability Monitor by obtaining exp osures of the bright Earth during occultations. This program also determines the coronagraphic sp ot p osition to b etter than 1 pixel accuracy. The Internal Flat Field (#10739) program uses the internal tungsten lamp to assess the stability of the pixel-to-pixel flat fields (P-flats) in several HRC/WFC filters (F435W, F625W, F814W). In Cycle 14, new P-flats will b e obtained for the six SBC filters and the SBC prisms (PR110L and PR130L). In all cases, the goal is to produce P-flats accurate to b etter than 1%. We comment on flats for p olarization and ramp filter observations b elow in зз5 and 6. Further information ab out flat fields can b e found in the article by Mack et al. (this volume). Recent ISRs related to routine flat field creation and monitoring include:


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Sembach § SBC L-Flat Corrections and Time-Dependent Sensitivity, ACS ISR 2005-13, by J. Mack et al. § Earth Flats, ACS ISR 2005-12, by R. Bohlin et al. § The Internal CCD Flat Fields, ACS ISR 2005-09, by R. Bohlin & J. Mack § Flats: SBC Internal Lamp P-Flat, ACS ISR 2005-04, by R. Bohlin & J. Mack § ACS Coronagraphic Flat Fields, ACS ISR 2004-16, by J. Krist

2.4.

Charge Transfer Efficiency

The ACS CCD detectors degrade with time due to radiation damage. One effect of this degradation is a decrease in CTE. Several monitoring programs track the level of CTE and its change with time. The External CTE Monitor (#10730) obtains images of 47 Tuc with half-field of view dithers to estimate photometric losses due to CTE as a function of time and p osition on the CCDs. The goal of this program is to provide corrections for photometric measurements to an accuracy of 1-2%. An indep endent check on the CTE is provided by data obtained as part of the CCD Stability Monitor program. The ACS Team is currently investigating procedures for mitigating the effects of CTE in the future, including changing the temp erature at which the CCDs op erate (Cycle 14 sp ecial program #10771). The Internal CTE Monitor (#10732) uses internal tungsten lamp data to trend the overall CCD radiation damage. It tracks b oth parallel and serial CTE p erformance. The data for this program are not used directly in the calibration of ACS observations. The CCD Post-Flash Verification program (#10734) occasionally tests the ability to illuminate the ACS CCDs with a light emitting diode in a rep eatable fashion. This capability may b e needed in the future to improve CTE for some exp osures (at the exp ense of adding noise to the data). The p ost-flash is not yet necessary or available for science observations. A description of the results from the ACS CTE monitoring programs is given by Chiab erge et al. (this volume). Recent ISRs related to CTE include the following: § Internal Monitoring of ACS Charge Transfer Efficiency, ACS ISR 2005-03, by M. Mutchler § Time Dependence of ACS WFC CTE Corrections for Photometry and Future Predictions, ACS ISR 2004-06, by A. Riess & J. Mack § Elevated Temperature Measurements of ACS Charge Transfer Efficiency, ACS ISR 2004-04, by M. Mutchler & A. Riess 2.5. Photometric/Sp ectrophotometric Throughput and Contamination Monitoring

The absolute sensitivity and rep eatability of ACS photometric and sp ectrophotometric observations are calibrations that require monitoring on a yearly timescale. The PhotoSp ectrophotometric Absolute Calibration monitor (program #10740) establishes the relative magnitudes of three primary white dwarf calibrators to 0.1% accuracy and checks rep eatability of the WFC and HRC filter throughputs to 0.2% accuracy using observations of single-star flux standards. These measurements are needed to refine the filter bandpasses. A p ortion of the time in this program is also b eing used to cross-calibrate the ACS prism, grism, and F850LP filter with NICMOS and STIS. Changes in sensitivity and the CCD quantum efficiency as a function of time are are also tracked by the CCD Stability Monitor (program #10737), which observes large numb ers of stars in 47 Tuc. A comprehensive pap er describing the ACS photometric calibration and the photometric transformations to other photometric systems has b een completed recently by Sirianni et


ACS Calibration Status

7

al. (2005). The pap er includes transformation coefficients for converting ACS HRC/WFC photometry to WFPC2 and the Landolt UBVRI photometric systems. It also contains information ab out ACS ap erture corrections for p oint source photometry. The UV Contamination Monitor program (#10736) tracks the throughputs in the six SBC filters (F115LP, F122M, F125LP, F140LP, F150LP, F165LP), the SBC prisms (PR110L, PR130L), three HRC UV filters (F220W, F250W, F330W), and HRC PR200L. These results are cross-referenced to previous STIS observations of the same cluster (NGC 6681). The goal is to track the UV photometry to 1% accuracy to monitor small changes in the throughput on timescales of 6 months. Recent ISRs related to ACS sensitivity and UV contamination monitoring include: § SBC L-Flat Corrections and Time-Dependent Sensitivity, ACS ISR 2005-13, by J. Mack et al. § The Photometric Stability of ACS: Revisiting the Hubble Deep Field, ACS ISR 2004-17, by A. Riess § Detector Quantum Efficiency and Photometric Zero Points of the ACS, ACS ISR 2004-08, by G. De Marchi et al. § Results of UV Contamination Monitoring of the ACS, ACS ISR 2004-05, by F. Boffi et al. 2.6. SBC MAMA Recovery

The SBC MAMA Recovery program (#10735) is used to turn on the ACS MAMA and return it to its normal op erational state after an anomalous shutdown. This program is invoked only when needed (less than once p er year). 3. Cycle 14 Sp ecial Calibration Programs

Several sp ecial calibration programs complement the Cycle 14 routine calibration programs describ ed ab ove. These sp ecial programs, which are listed in Table 3, provide basic calibrations for the SBC and ramp filters, information ab out the UV narrow-band red leak, and the dep endence of CTE and QE on CCD temp erature. Results from this latter test (program #10771) will b e used in assessing the need for installation of the Aft Shroud Cooling System during the next Hubble servicing mission. Table 3: Cycle 14 Sp ecial Calibration Programs
Program ID 10722 10731 10741 10742 10743 10771 Total (Cy PI Maiz Chiab erge Suchkov Fruchter Larsen Sirianni cle 14 sp ecial Title SBC Geometric Distortion UV Narrow-Band Red Leak Continuum L-Flats (Ramps) Ramp, Grism Wavelengths Improved Wavelengths (SBC Prism) CTE/QE Temp erature Dep endence programs) Orb Ext. 6 2 3 4 2 12 29 its Int. 4 0 0 0 2 12 18 Note Basic calibration Resp onds to early failed cal Basic calibration Resp onds to early failed cal QSO Ly lines (1400-1800ђ) A ASCS supp ort test

A few sp ecial programs from Cycle 13 were completed recently; they are listed in Table 4. Some of these were functional checks for capabilities to b e used only if needed (programs #10449, 10450), and others improved calibrations for the p olarizers and prisms


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Sembach

(programs #10378, 10391). The Short Annealing Test (program #10453) verified the effectiveness of using 6 hour anneals (see з2.2). Table 4: Cycle 13 Sp ecial Calibration Programs
Program ID 10378 10391 PI Biretta Larsen Title Orbits Ext. Int. 12 0 1 11 0 0 0 1 13 7 2 0 0 20 Note Last visit in July 2005 Last visit in August 2005 Use program only if needed Use program only if needed Adopted for routine use Recently submitted

Polarization Calibration Wave, Flux for Prisms (SBC and HRC) 10449 Cox SBC Filter Wheel Checkout 10450 Sirianni Functional Test of MEB2 Switch 10453 Sirianni Short Annealing Test 10720 Riess Monochromatic PSF in the Red Total (Cycle 13 sp ecial programs)

4.

SBC Calibrations

SBC observations currently account for only 3% of the ACS observing time, but observations in this channel have received more attention since STIS shut down. The Cycle 14 SBC Geometric Distortion program (#10722) is intended to improve the geometric distortion solution in the ACS/SBC imaging modes and the PR130L prism mode. The geometric distortion correction is needed to upgrade the SBC L-flats to a level approaching that of the CCD L-flats. Several sp ecial calibration programs designed to improve the SBC prism wavelengths (#10743, 10391) enhance previous calibrations. In Cycle 14, prism observations account for ab out one quarter of the SBC observing time. See the accompanying articles in this volume by Cox and Larsen for more information ab out SBC calibration activities. 5. Polarized Filters

Polarization calibration activities (such as Cycle 13 program #10378) have b een used to characterize the ACS p olarizers and their p ossible uses. A series of ISRs describing the p olarizers describ es recent progress in calibrating the p olarized filters. § ACS/HRC Polarimetry Calibration IV. Low-Frequency Flat-Fields for Polarized Filters, ACS ISR 2005-10, by V. Kozhurina-Platais & J. Biretta § ACS/HRC Polarimetry Calibration III. Astrometry of Polarized Filters, ACS ISR 2004-11, by V. Kozhurina-Platais & J. Biretta § ACS Polarization Calibration II. The POLV Filter Angles, ACS ISR 2004-10, by J. Biretta & V. Kozhurina-Platais § ACS Polarization Calibration I. Introduction and Status Report, ACS ISR 2004-09, by J. Biretta et al. 6. Grism/Prism Sp ectroscopy and Ramp Filters

In Cycle 14, prism/grism sp ectroscopy accounts for approximately one tenth of the WFC observing time and one-quarter of the HRC and SBC observing time. The ST-ECF is


ACS Calibration Status

9

resp onsible for the calibration of the ACS sp ectroscopic modes. Work continues on characterizing b oth the wavelength solutions and sensitivities of these modes. Recent ISRs include: § Updated Wavelength Calibration for the WFC/G800L Grism, ACS ISR 2005-08, by S. Larsen & J. Walsh § Flat-Field and Sensitivity Calibration for ACS G800L Slitless Spectroscopy Modes, ACS ISR 2005-02, by J. Walsh & N. Pirzkal The ST-ECF sp ectral extraction software, aXe, is the primary software used to manipulate ACS slitless sp ectroscopy images. The most current version of this software (v1.5) can b e linked to from the ACS web page. See the accompanying article in this volume by Walsh et al. for a description of the software and its uses. Two Cycle 14 sp ecial programs will provide up dated calibrations for the ACS ramp filters. Program #10742 will calibrate the throughputs of the ramp filters as a function of wavelength by obtaining observations of a flux standard with the filters crossed with the grism. This should also calibrate the zeroth order of the grism, which may allow some users to avoid having to obtain additional direct images of the fields they observe with the grism. Program #10741 will provide continuum L-flats for the ramp filters, which will improve the characterization of the total filter transmissions. 7. Coronagraphy

Coronagraphic observations account for roughly 3-4% of ACS observing time. They dep end strongly on the p ointing stability of the telescop e and the ability to flat field detector artifacts. These issues are discussed in the following two ISRs. Observers should note that there is no appreciable difference in the quality of coronagraphic observations b etween two-gyro mode and three-gyro mode. § ACS Coronagraph Performance in Two-Gyro Mode, ACS ISR 2005-05, by C. Cox & J. Biretta § ACS Coronagraphic Flat Fields, ACS ISR 2004-16, by J. Krist 8. Point Spread Function

The ACS p oint spread function (PSF) dep ends on wavelength. To date, there has b een only a limited amount of information ab out the PSF shap e. Encircled energies are given by Sirianni et al. (2005), and the shap e and stability of the PSF has b een documented in the following ISRs: § Characterization of the ACS/HRC Point Spread Function in Two-Gyro Mode, ACS ISR 2005-11, by M. Sirianni et al. § Two-Gyro Pointing Stability of HST Measured with ACS, ACS ISR 2005-07, by A. Koekemoer et al. § Multi-Filter PSFs and Distortion Corrections for the HRC, ACS ISR 2004-15, by J. Anderson & I. King Several talks at this workshop also addressed the issue of ACS PSFs. The ACS Calibration Team is using Cycle 14 sp ecial calibration program #10720 to characterize the monochromatic PSF of the WFC in the red using a combination of the F850LP filter and several ramp filters. The purp ose of this program is to improve the precision of photometric measurements obtained with the F850LP filter.


10 9.

Sembach Astrometry

With the advent of Guide Star Catalog 2, it should b e p ossible to improve the absolute astrometric solutions for ACS images by roughly an order of magnitude over current solutions. An accuracy in the range of 0.1-0.3 may b e achievable for fields containing sufficient numb ers of identifiable guide stars. The following ISR documents the technique for achieving an astrometric accuracy comparable to the resolution of the telescop e. § Demonstration of a Significant Improvement in the Astrometric Accuracy of HST Data, ACS ISR 2005-06, by A. Koekemoer et al. 10. WFC CCD Gain Change for Cycle 14

The new default gain setting for the WFC in Cycle 14 is GAIN=2. This change helps to alleviate image ghosts caused by electrical cross-talk, which is documented in the following I S Rs : § Cross-Talk in the ACS WFC Detectors II. Using GAIN=2 to Minimize the Effect, ACS ISR 2004-13, by M. Giavalisco § Cross-Talk in the ACS WFC Detectors I. Description of the Effect, ACS ISR 2004-12, by M. Giavalisco 11. Documentation and Web Site Up dates

Several up dates to ACS documentation should make it easier for observers to find information ab out ACS. First, the ACS Instrument Handbook (Gonzaga et al. 2005) has b een reduced in length by ab out 15% without loss of content by consolidating information and moving some of the ACS calibration plan material to the ACS web site. The index for the handb ook has also b een up dated, and a series of summary tables at the front of the handb ook have b een added to make it easier to find basic information ab out the instrument and its supp orted modes of op eration. The ACS Instrument Handbook was released with the Cycle 15 Cal l for Proposals. A short up date to the ACS Data Handbook is planned (Pavlovsky et al. 2006). The up date will include minor corrections to wording and syntax of some of the examples, as well as clarification of some of the text related to prism/grism reductions. The up dated handb ook is exp ected to b e released in early 2006. ACS long term usage statistics are now b eing charted on the STScI metrics web page at http://www.stsci.edu/hst/metrics/SiUsage/ACS LT. This page contains b oth graphical and tabular information ab out requested ACS modes, filters, and exp osure times. Acknowledgments. Calibration of the ACS is truly a team effort. I thank the memb ers of the ACS Calibration Team for their dedicated efforts to sp ecify and conduct the ACS calibration program describ ed in this article. I thank the PIs of the calibration programs listed in Tables 2-4 for their descriptions of the calibration programs. I am particularly grateful to Ron Gilliland, the ACS calibration lead, for his willingness to distill a large amount of calibration information and organize it into tabular form. References Gonzaga, S., et al. 2005, ACS Instrument Handbook, Version 6.0, (Baltimore: STScI) Pavlovsky, C., et al. 2006, ACS Data Handbook, Version 5.0, (Baltimore: STScI) Sirianni, M., et al. 2005, PASP, 117, 1049