Calibration Plans

The Cycle 16 calibration plan will be developed after selection of GO observations, and submission of Phase II programs. Cycle 16 calibrations will continue to be performed to support the science program. The timing of the anticipated Servicing Mission 4 may influence these calibration plans. 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 Table 3.2 in Chapter 3 of the ACS Cycle 16 Instrument Handbook.

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 (Sections 3.2.3, 3.4.2 and 3.5.3 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.

The Cycle 15 calibration plan has been developed and all Phase II programs submitted by November 2006. This plan includes calibrations intended to cover up to the next servicing mission, and therefore has been submitted with six extra months to June 2008. Routine calibrations continue to be performed to support the Cycle 15 science program. Results from Cycles 11, 12 and 13 have been almost fully obtained, and most of the Cycle 14 calibration observations are in hand with analyses underway.

New for Cycle 15 is the recognition that instrument characteristics may have changed slightly as a result of the Side 2 electronics switch and lowering the WFC operating temperature by 4 degrees C in July 2006. The Cycle 15 plan will provide full calibration of the ACS in this configuration.

ACS Cycle 15 Calibration Plan

The goal of the Cycle 14 calibration program is 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 are being carried forward at the proper cadence. Also, resources are being 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.

The Cycle 14 program included special observations to improve ramp, grism and prism calibrations. The fundamental geometric distortion calibration for the SBC is being derived, and the sensitivityof QE and CTE to detector operating temperature was assessed.

At the time of writing (July 2006) programs 10736, 10737, and 10738 were slightly modified to obtain calibration data after the switch to Side 2 electronics and lowering of the WFC temperature setpoint. The following programs were added for lower level testing:

• 11005 Functional Test — MEB2 Switch
• 11006 SBC Filter Wheel Checkout
• 11007 ACS Side 2 Dump Test and Verification of ACS Memory Load
• 11008 ACS CCD Side-2 Temp Setpoint
• 11009 ACS Science Data Buffer Checks

ACS Cycle 14 Calibration Plan

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 started on October 2004 

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.

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.

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.

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. The proposal IDs and titles for the interim and standard Cycle 11 calibrations may be found below.

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:

1. 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.
2. 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.
3. 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.
4. Characterization of detectors. This includes charge transfer effects, long wavelength fringing, hot pixel growth, etc.

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.

Interim Calibrations

Standard Calibrations

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.

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.