3.2.1 Philosophy

Two basic roles facilitate the description of command scenarios: the ``observer'' determines the science operations of ACIS, controlling it using the various AXAF-I mission planning and control resources such as the AXAF Science Center, Operations Control Center, the Deep Space Network, and the Spacecraft. The ``maintainer'' determines the maintenance operations of ACIS, such as code patches and diagnostics. The ``maintainer'' also controls the instrument using the various AXAF-I mission planning and control resources.

For the purposes of this document, the term ``science run'' will be defined as a contiguous period of time during which ACIS is observing a source using a single set of parameters. Many science runs may be combined to form an ``observation,'' or a time during which the instrument is pointed at a specific target and data are being taken. However, starting a science run involves some overhead, as no X-ray data can be collected during the instrument electronic settling time and during the calculation of new bias maps. More than one observation is possible during a single science run, and it will be an operational issue for the AXAF schedulers and operations staff to decide how to break up observations of targets with ACIS science runs.

To perform a science run, an observer commands ACIS to accept a set of parameters to use for the run then instructs ACIS to execute the run. The parameters and commands define the Science Mode parameter block and options to use for the run. ACIS then sets up the hardware and software elements according to the specified mode and proceeds to acquire, process, and telemeter event data from the selected CCDs. The observer allows ACIS to continue its data acquisition for the desired period of time then commands ACIS to terminate the run. Figure 3.1 illustrates this process.

**Figure 3.1:** Science Run Data Flow Diagram
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Upon receipt of a ``Load Parameter Block'' command, the Command Interface dispatches the command to load the specified parameter block into the list of available parameters. When it receives an ``Execute Science Run'' command, the Command Interface dispatches the command to a Science Mode. The Science Mode then retrieves the parameter block specified in the command. It then configures an Event Processor to prepare for events, tells the DEA Interface to set up the DEA hardware and sequencer, and instructs the FEP Interface to load code and parameters into the FEPs and to start executing. Finally, the Science Mode instructs the DEA Interface to start clocking the CCDs.

As the DEA clocks the CCDs, pixel data are produced and forwarded to the FEPs (not shown in Figure 3.1 above). The FEPs select events from the generated pixel data and forward the event data to the BEP. The BEP's interface to the FEP then instructs the Event Processor to process the events. As the Event Processor selects, processes, and formats the events for telemetry, it instructs the Telemetry Interface to send the packaged event data.

When the observer sends the command to terminate the Science Run, the Command Interface instructs the running Science Mode to stop the run. The Science Mode, in turn, instructs the DEA Interface and FEP Interface to stop clocking the CCDs and producing events. When the FEP Interface receives an indication from the FEP that the data set is complete, it tells the Event Processor to stop. The Event Processor then completes its event processing and tells the Science Mode that it has finished. The Science Mode then forms an Observation Report and tells the Telemetry Interface to send the formed report. The detailed form of all these commands is outlined in the tables below.

Table 3.4 describes the contents of a `generic' load parameter block command. The content of each specific type of parameter block is Science Mode dependent and is described for each Science Mode later in this chapter. ACIS will be capable of storing at least four parameter blocks in its software ROM for use during on-orbit checkout and for commonly used observational modes. This helps the maintainer to reduce the number of commands required to configure the instrument.

**Table 3.4:** Generic Load Parameter Block Command Packet
Field Name	Description
Packet Word Length	Total number of 16-bit words in the command packet.
Packet Sequence #	Identifies the command packet in a series.
Command Opcode	``Load < type > Parameter Block'' command opcode. This instructs the ACIS Science Instrument Software to store the ``Parameter Block Data'' contained within the packet.
Block ID	Block within the ACIS.
Block CRC	Checks the load for errors in transmission.
Parameter Block Data	Parameter block data to store.

Tables 3.5 and 3.6 outline generic `Start Science Run' and `Stop Science Run' commands, respectively. There are several different telemetry formats, matched to the different ways data may be obtained with ACIS (see Section 3.3.2), but all the telemetry forms contain at least the start time of the Science Run. This allows the data to be uniquely tagged. After being commanded to stop a Science Run, the ACIS software produces a report of the complete run. Science Run Reports must contain at least the information described in Table 3.7.

**Table 3.5:** Generic Start Science Run Command
Field Name	Description
Packet Word Length	Total number of 16-bit words in the command packet.
Packet Sequence #	Identifies the command packet in a series.
Command Opcode	``Start < Science Mode > Run'' command opcode. Instructs the ACIS Science Instrument Software to retrieve the parameter block indicated by the ``Block ID'' below, and setup and run the requested Science Mode. ACIS will continue to run the mode until commanded to stop.
Block ID	< Science Mode > block parameter block to use for the run. This block may have been loaded just prior to the Start Run command, or may be part of either the pre-loaded library, or one of the blocks provided at launch.

**Table 3.6:** Generic Stop Science Run Command
Item	Description
Packet Word Length	Total number of 16-bit words in the command packet.
Packet Sequence #	Identifies the command packet in a series.
Command Opcode	`Stop < Science Mode > Run' command opcode. Instructs the ACIS Science Instrument Software to stop the current science run.
Block ID	< Science Mode > parameter block to be used for this run.

**Table 3.7:** Stop Science Report Information
Field Name	Description
Science Run Start Time	ACIS time-stamp sampled relative to the start of the Science Run. All data produced by the run is relatable to this time-stamp.
Data Summary	Indication of how much information was produced by the run. The intent is to provide the observer with a confirmation that all the run's data have been received.
Discrepancy Summary	Summary information of any discrepancies uncovered during or after the run.