Advanced Camera for Surveys Instrument Handbook for Cycle 14
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| Advanced Camera for Surveys Instrument Handbook for Cycle 14 | |||||
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13.1 Ground Testing and CalibrationChapter 13:
Calibration Plans
13.2 SMOV Testing and Calibration
13.3 Cycle 11 Calibration
13.3.1 Calibration Priorities
13.4 Cycle 12 Calibration
13.5 Cycle 13 Calibration
13.6 Cycle 14 Calibration
In this Chapter we describe the current status of ACS calibrations, as well as plans for the Cycle 14 period of primary interest in this Handbook.
At the time of writing, the analysis of SMOV (Servicing Mission Orbital Verification) and most Cycle 11 data has been completed. Most of the Cycle 12 calibration observations have been obtained, and calibrations of fundamental items such as geometrical distortions and absolute sensitivity are well in hand. Details and results for low use modes are being actively pursued.
For further information please access the STScI web page for ACS, and/or consult the
Helpdeskfor staff input to specific questions.13.1 Ground Testing and Calibration
Ground calibration and testing was a prime responsibility of the ACS Investigation Definition Team (Principal Investigator Holland Ford, JHU) and was carried out at Ball Aerospace in Colorado. Thermal vacuum (and supplemental dry-nitrogen environment) testing in which orbital conditions were simulated was conducted pre-launch at Goddard Space Flight Center. Filter transmission curves, and detector quantum efficiency curves, were derived at GSFC and JHU. These tests characterized the basic properties of the optics, the detectors, and the mechanisms. During ground calibration the highest priority was given to those measurements essential to establish that instrument design specs were being met, and to those measurements that could not be obtained on-orbit. Most of the non-unique ground test data were superseded by on-orbit measurements as part of the STScI Cycle 11 calibration plan. Successive cycles of the calibration program both maintain routine calibrations (such as providing darks and biases and tracking evolving CTE) and also brings maturity to determinations of the overall QE and low-frequency flat fields.
13.2 SMOV Testing and Calibration
The primary goal of the SMOV3B was a timely commissioning of the HST observatory for normal science operations. For ACS this has included basic testing of the instrument functionality as well as testing/setting of the focus (internal and external), measuring the sensitivity in all filters, establishing the geometric distortion and plate scale, quantifying the point spread function for each camera, and adjusting flat fields to properly capture low-frequency variations for which ground calibrations are always difficult. Data from SMOV proposals are non proprietary and fully accessible through the HST archive. We list below the program IDs and proposal titles. Details of the Phase II programs may be found via: http://presto.stsci.edu/public/propinfo.html. Some programs are of very limited technical interest (e.g., Science data buffer check--9003), while others (e.g., WFC flat field stability--9018) collected large amounts of data in standard filters on objects of potential archival science interest.
Table 13.1: ACS SMOV Proposals
13.3 Cycle 11 Calibration
The SMOV calibration and testing period extended for roughly two months taking routine monitoring of ACS performance through April 2002. With the nominal start of Cycle 11 at July 1, 2002 and with a desire not to define the final calibration program for Cycle 11 until some flight experience from SMOV could be obtained, a decision was made to break the Cycle 11 calibration into two parts: (1) an interim program continuing routine monitoring (darks, biases) and extending characterization in areas not well covered by SMOV, and (2) the standard full program expected to start in July 2002 and run for one year. Tables 13.2 and 13.3 list program IDs and titles for the interim and standard Cycle 11 calibrations respectively.
13.3.1 Calibration Priorities
As for any instrument the ACS calibration plan represents a compromise between the desire to calibrate the instrument as well as possible and the availability of finite resources both in terms of primary HST orbits and in terms of human resources at STScI. The list of priorities that guided the Cycle 11 calibration planning was:
- Monitor the Health and Safety of the Instrument. This includes obtaining all data necessary to verify that the instrument is performing as planned and to insure a useful lifetime as extended as possible.
- Update and Maintain Pipeline Reference Files. Dark, biases, flat fields and sensitivities used in the pipeline calibration need to be accurate and current. Information on newly released reference files is announced via the Space Telescope Analysis Newsletter and posted on the ACS web page. The updated list of recommended reference files to be used with each data set is available through the HST Archive, and will be automatically applied when calibrated data are requested from the Archive.
- Characterization of Optical Performance. The point spread function and its variation across the field of view needs to be carefully determined as a function of wavelength.
- Characterization of detectors. This includes charge transfer effects, long wavelength fringing, hot pixel growth, etc.
Table 13.3: Cycle 11 ACS Standard Calibrations
The Cycle 11 calibration program was intended to most effectively balance the needs of the community for obtaining excellent science results from the instrument with the limited resources available (e.g., a nominal limit of 10% time available for calibration). Common uses of the instrument were fully calibrated.
13.4 Cycle 12 Calibration
The goal of the Cycle 12 calibration plan was to complete the definition of the ACS calibration by refining the measure of a number of important parameters of the instrument. These included the geometric distortions, L-flats, sky flats, and the quantum efficiency of the two CCD detectors. Monitoring programs followed hot pixels, the stability of bias and dark reference frames, the photometric calibration stability, and tracking the degradation of photometric performance due to CTE losses induced by the continued exposure to cosmic radiation.
The Cycle 12 calibration plan started on October 2003 and Table 13.4 lists the programs.
Table 13.4: Cycle 12 ACS Standard Calibrations
The CCD daily monitoring program continued to provide dark and bias frames to build the corresponding bi-weekly reference files. Separate programs provided bias reference files for subarrays, and measured the SBC detector's dark current and stability. The CTE monitoring program characterized the CCDs' CTE losses as a function of time, field crowding and background levels, and defined recipes to calibrate them. Also, accurate measures and monitoring of available post-flash background levels were made to allow its use in future cycles in mitigating CTE losses. Annealing of CCD hot pixels continued as during Cycle 11, and possible contamination to the UV throughput was monitored. The photometric zero points of all cameras will be further refined through observations now obtained of four spectro-photometric standard stars. The photometric calibration for compact or point sources with very red spectral energy distribution was checked through observations of extreme red stars. The stability of the photometric calibration, geometrical distortions and flat fielding will also be measured and monitored through dedicated programs. Polarization calibrations were taken to fully characterize the internal polarization of the instrument. Data to support accurate wavelength measures and L-flats for the ramp filters were acquired. The plan also included accurate measures of the wavelength calibration of the grism and the prism.
13.5 Cycle 13 Calibration
The goal of the Cycle 13 calibration plan is to continue refining the definition of the ACS calibration by measuring a number of important parameters of the instrument, but with a level of resource commitment in orbits that will be lower for areas exhibiting excellent stability to date. These include the geometric distortions, L-flats, sky flats, and the quantum efficiency of the two CCD detectors. Monitoring programs followed hot pixels, the stability of bias and dark reference frames, the photometric calibration stability, and tracking the degradation of photometric performance due to CTE losses induced by the continued exposure to cosmic radiation. New observations will be aimed at testing the wavelength stability of primary filters and further refining the measure of their bandpasses, and continuing to build up polarimetric calibration.
The Cycle 13 calibration plan will start on October 2004 and Table 13.5 lists the currently planned programs.
Table 13.5: Cycle 13 ACS Standard Calibrations
The CCD daily monitoring program will continue to provide dark and bias frames to build the corresponding bi-weekly reference files. The CTE monitoring program will continue to characterize the CCDs' CTE losses as a function of time, field crowding and background levels, and define recipes to calibrate them. Also, rough measures and monitoring of available post-flash background levels were made to allow its use in future cycles in mitigating CTE losses (these observations do not -- and will not for Cycle 14 -- constitute calibrations sufficient for reducing science data). Annealing of CCD hot pixels will continue as during Cycles 11 and 12, and possible contamination to the UV throughput will be monitored, although less frequently. Observations will be obtained to further define zeropoints, and to check filter bandpasses. The photometric calibration for compact or point sources with very red spectral energy distribution will be provided through observations of extreme red stars. The stability of the photometric calibration, geometrical distortions and flat fielding will also be measured and monitored through dedicated programs. Polarization calibrations will fully characterize the internal polarization of the instrument.
13.6 Cycle 14 Calibration
The Cycle 14 calibration program will be developed in detail during mid-2005 at which time: (1) instrument performance will have been tracked for three years on-orbit; (2) the results from Cycles 11 and 12 and, at least in part, Cycle 13 calibration programs will be known with documented results; (3) feedback will be available from the community on the effectiveness of ACS calibrations to date; (4) the approved Cycle 14 science program and implied instrument use will be known.
The goal of the Cycle 14 calibration program will again be to optimally support science results from the community while balancing the program with available resources (HST orbits and staff analysis time). Routine calibrations such as darks will be carried forward at the proper cadence. Resources will also be allocated for further characterization of capabilities and calibration of science observations as required in response to evolution of either performance or needs reflected in the science program as a whole.
In special circumstances proposers may wish to request additional orbits for the purpose of calibration. These can be proposed in two ways and should be for calibrations that are not likely to be in the core calibration programs. An example of a non-core calibration would be one that needs to reach precision levels well in excess of those outlined in Tables 12.1, 12.2 or 12.3.
The first type of special calibration would simply request additional orbits within a GO program for the purpose of calibrating the science data to be obtained (see Section 4.3 of the Call for Proposals). In this case the extra calibration would only need to be justified on the basis of the expected science return of the GO's program.
The second type of special calibration would be performed as a general service to the community via Calibration Proposals (Section 3.6 of Call for Proposals). In this case the calibration observations should again be outside the core responsibilities of the ACS group to perform, and furthermore should be directed at supporting general enhancement of ACS capabilities with the expectation of separately negotiated deliverables if time and/or funding are granted.
Proposers interested in obtaining either type of special calibration should consult with Instrument Scientists from the ACS Group via questions to the Help Desk at least 14 days before the proposal deadline in order to ascertain if the proposed calibrations would be done at STScI in the default program.
Observations obtained for calibration programs will generally be flagged as non-proprietary.
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