ACS was installed in HST as part of Servicing Mission 3B, on March 7, 2002. ACS operations have been smooth throughout the servicing mission orbital verification period after installation and throughout the first two cycles (11 and 12) of science observations. A complete Data Handbook is now available and calibrations and support are nearing a steady state.

2.1 ACS is a Maturing Instrument

ACS is still a relatively young instrument in comparison to others on HST, but its understanding has advanced well. At the time of Cycle 14 Handbook writing we have over two full years of combined science operations and calibration program execution and analysis from Cycles 11 and 12. Fortunately, operations, calibrations and science observations have continued to go well. Residual uncertainties from ground-based characterizations have been removed through a series of extensive on-orbit calibrations.

Instrument characteristics have for the most part remained stable with only minor or expected exceptions. An example of a minor exception is greater motion of the coronagraphic alignment than expected (see Section 5.2 discussion). An example of an expected change with time is the growth of Charge Transfer Efficiency losses for the CCDs (see Section 7.2 discussion).

The calibration state of the ACS will continue to evolve in the period prior to Cycle 14, but we are well beyond the period of rapid evolution in instrument calibration and characterization that applied two years ago. Most issues are now at a level still relevant for optimal science returns, but largely not useful at the Phase I proposing stage (e.g. sensitivity updates relevant for some filters may still occur at the ~1% level). As always, we will endeavor to keep users informed on new developments through the ACS WWW site and the ACS STANs (Space Telescope Analysis Newsletters), issued on an occasional basis.

2.2 SBC Scheduling Policies

The STIS MAMA control electronics were found in orbit to be subject to resets due to cosmic-ray upsets, therefore STIS MAMAs are operated only during the contiguous orbits of each day which are free of the South Atlantic Anomaly (SAA). Even though the design of the ACS MAMA control electronics in the SBC was modified so that it would not be susceptible to cosmic-ray hits, the background count rate still exceeds the bright object limits for the SBC during SAA passage. Consequently, the SBC will in general only be scheduled for use during SAA-free orbits. As we expect the SBC usage to be relatively low compared to the CCD cameras, we do not expect this to pose a problem to users.

2.3 Prime and Parallel Observing with the SBC

As explained in greater detail in Section 7.5, the MAMA detector that ACS uses in the ultraviolet is subject to damage at high illumination rates. To protect the instrument, we have established limits on the maximum count rate at which the detector may be illuminated. These count-rate limits translate into a set of configuration-dependent bright-object screening magnitudes. These are summarized in Table 7.8.

STScI will perform screening of all SBC exposures prior to scheduling. Targets not established as safe for the configuration in which they are being observed will not be scheduled. Observations that pass screening but are lost in orbit due to a bright-object violation will not be rescheduled. Observers are responsible for assuring that their observations do not violate the SBC count-rate limits. A detailed description of the SBC bright-object limits and the observers' responsibility is presented in Section 7.5.

To assure that STScI can adequately screen observations, special constraints are imposed on parallel observing with the SBC. In particular:

Table 2.1 below summarizes the policy with respect to SBC observing in Cycle 13.

***Table 2.1: Bright-Object Protection Policy for SBC Observations***
Type of Observing	Policy
Prime	Allowed if target passes screening
Snapshots	Not allowed
Coordinated parallel	Allowed only if ORIENT is exactly specified and field passes screening
Pure parallel	Not allowed

Targets that are one magnitude or more fainter than the magnitude limits in the screening tables generally automatically pass screening. For a target that is within one magnitude of the screening limits, observers must provide a calibrated spectrum of the source at the intended observing wavelength. If such a spectrum is not available, the prospective GO must request an orbit in Phase I for a pre-qualification exposure, during which the target spectrum must be determined by observation in an allowed configuration (see Section 7.5 for more details).

Please also note that if you are proposing SBC target-of-opportunity observations, we ask you to provide an explanation in your Phase I proposal of how you will ensure that your target can be safely observed.

2.4 Policy for Auto-Parallel Observations

As described in Section 8.7, ACS is able to make simultaneous observations using the Wide-Field Channel and the High Resolution Channel. Such observations are added automatically by the scheduling system if doing so does not impact the primary exposures. However, since the WFC and HRC share the same filter wheel, the filter used in the "parallel" channel is determined by that selected for the "prime" detector; the observer does not have the capability to select the parallel filter independently. This means that the possibility and character of these "Auto-Parallel" observations are purely a result of the choices made by the proposer of the prime program. For this reason, the following policies will be in effect for Auto-Parallel observations:

2.5 Use of Available-but-Unsupported Capabilities

We have established a set of core scientific capabilities for ACS which will be supported for Cycle 14 and are described fully in this Handbook. In addition there are a few capabilities with ACS, some of which are mentioned in this Handbook, for which limited access is available. These capabilities are "available-but-unsupported," and in consultation with an ACS Instrument Scientist can be requested. These include a few apertures, limited interest optional parameters, some GAIN options, and filterless (CLEAR) operation. If you find that your science cannot be obtained using fully supported modes, or that it would be much better with use of these special cases, then you may wish to consider use of an unsupported mode.

Use of unsupported modes comes at a price, and they should be used only if the technical requirement and scientific justification are particularly compelling. The following caveats apply:

Cycle 14 Phase I proposals that include use of unsupported ACS capabilities must include the following:

During the Phase II proposal submission process, use of available-but-unsupported modes requires formal approval from the ACS Branch at STScI. To request permission for use of an available-but-unsupported mode, please send a brief email to your Program Coordinator (PC) that addresses the above four points. The PC will relay the request to the contact scientist or relevant ACS instrument scientist, who will decide whether the use will be allowed. This procedure ensures that any potential technical problems have been taken into account. Note also that Archival research may be hindered by use of these modes. As a result, requests for use of unsupported modes which do not adequately address the above four points, or which will result in only marginal improvements in the quality of the data obtained, may be denied, even if the request was included in your approved Phase I proposal.


	*RAMP filters are fully supported with aperture WFC resulting in full field readouts. The WFC1-IRAMP, WFC1-MRAMP, WFC2-MRAMP and WFC2-ORAMP apertures (see Table 8.1) are available-but-unsupported.*

2.6 Data Volume Constraints

If ACS data are taken at the highest possible rate for more than a few orbits or in the CVZ, it is possible to accumulate data faster than it can be transmitted to the ground. High data volume proposals will be reviewed and on some occasions, users may be requested to break the proposal into different visits, consider using sub-arrays, or taking other steps to reduce data volume.

2.7 Charge Transfer Efficiency

Both the STIS and WFPC2 CCDs have shown a significant degradation in charge transfer efficiency (CTE) performance since their installation. The degradation is due to radiation damage of the silicon inducing the creation of traps that impede the clocking of the charge on the CCD. Since reading out the ACS WFC requires 2048 parallel transfers and 2048 serial transfers, it is not surprising that CTE effects have begun to manifest themselves in even the first years of ACS operation. For this reason, it is likely that some types of science, particularly those in which the source flux in each image is expected to be low (<0.1 electrons/second) and compact, will be most effectively performed during the first few years of ACS operation.

Our initial expectations for growth of CTE for the Wide Field Camera have proven to be conservative. After multiple iterations of special programs designed to track the growth of CTE in time we have found that after 1 year of operation there was a loss of approximately 1-2% in the counts from a star with between 50 and 150 total electrons, a nominal sky background of 30 electrons, and with placement at row 1024 (center) in one of the WFC chips. After 2 years the growth rate appears to be linear, thus such a target would experience a loss of approximately 4-8% in the middle (early 2006) of Cycle 14. A target placed at the WFC aperture reference point, near the maximum number of parallel shifts during readout, would have approximately twice the loss. Since the measurements to date extend only to 2 years post-launch of ACS, and the extrapolation to Cycle 14 is by a factor of two in baseline, these estimates must still be taken with an appropriate level of caution, however, the view now is that CTE is not developing as rapidly as previously expected, and is not yet a major driver affecting ACS science quality. Expected absolute errors after calibration of science data, at these low-loss levels, is expected to be of order 25% the relative loss.

As the CTE effects worsen in more remote future Cycles, users may want to consider using the post-flash capability (currently an "available-but-unsupported" mode) to add a background level to their images. This causes the Poisson noise from the background level to increase, and imposes a non-uniform background, but to-date only marginally improves the CTE performance of the detector. We do not recommend the use of the post-flash capability for any applications during Cycle 14, but we will continue to track this carefully and stand ready to revise this recommendation in a liberal way for future cycles. We are confident this is not yet the time to begin considering post-flash use. Please refer to Section 7.5 for more information on this topic.

2.8 Two-gyro Guiding

HST currently operates with three rate sensing gyros in the guiding control loop. At some point in the future additional gyroscopes on HST may fail, thus making it necessary to observe with only two gyroscopes instead of the usual contingent of three. This would have significant impacts on the scheduling of observations -- both the number of minutes available for observation per orbit, and the times of year when a given target can schedule, would be significantly reduced. It is also possible that the guiding would be less accurate -- there is a potential for a high-frequency (many Hz) pointing jitter to be superposed on the normal HST tracking, which would degrade the effective PSF. There is also some possibility that target positioning on the detectors would become less accurate. Tests of two-gyro guiding are planned for early 2005, where many of these impacts should become better defined. Additional tests would also be conducted in the event that gyro failures make two-gyro guiding necessary.


	Further discussion of how these impacts will affect the observatory and the instruments can be found in a separate Handbook, the HST Two-Gyro Handbook for Cycle 14. See the Two-Gyro Handbook for detailed information. All text in this ACS Handbook assumes three-gyro control.

HST is expected to transfer into two-gyro mode at some point in the future; since gyro failures are unpredictable this event may occur as early as Cycle 14. Therefore proposers are requested to provide information on two-gyro observing on their programs; see the Call for Proposals for details.

2.9 Status of STIS

As this Handbook is being finished for press it seems likely that the Space Telescope Imaging Spectrograph suffered a failure that will prevent its use in Cycle 14. Wording in this Handbook still refers to STIS in the present tense; see the Call for Proposals and web links for updates.

Advanced Camera for Surveys Instrument Handbook for Cycle 14

Chapter 2:
Special Considerations
for Cycle 14

2.1 ACS is a Maturing Instrument

2.2 SBC Scheduling Policies

2.3 Prime and Parallel Observing with the SBC

2.4 Policy for Auto-Parallel Observations

2.5 Use of Available-but-Unsupported Capabilities

2.6 Data Volume Constraints

2.7 Charge Transfer Efficiency

2.8 Two-gyro Guiding

2.9 Status of STIS

Advanced Camera for Surveys Instrument Handbook for Cycle 14

Chapter 2: Special Considerationsfor Cycle 14

2.1 ACS is a Maturing Instrument

2.2 SBC Scheduling Policies

2.3 Prime and Parallel Observing with the SBC

2.4 Policy for Auto-Parallel Observations

2.5 Use of Available-but-Unsupported Capabilities

2.6 Data Volume Constraints

2.7 Charge Transfer Efficiency

2.8 Two-gyro Guiding

2.9 Status of STIS

Chapter 2:
Special Considerations
for Cycle 14