Next: 18.104.22.168 ACIS Period 2 Activities Up: 4.6.2 Science Instrument OV Previous: 4.6.2 Science Instrument OV
The period 1 ACIS activities include the initialization of the instrument, the verification of expected operating conditions, and the verification of the basic functions of the instrument. (Note that immediately after the IUS (Inertia Upper Stage: the final stage rocket motor attached to AXAF for orbital insertion) burn is completed, that valves will be opened to allow the outgassing process to start.)
The first set of activities will perform the hardware (HW) and software (SW) initializations. The very first command which must be sent to ACIS will turn on the instrument's power supplies. The Back End Processor (BEP) must then be turned on after a stable operating condition of the power supplies has been achieved. Many of the activities to be performed with ACIS will be executed by the flight SW running on the BEPs and FEPs; therefore the BEP and FEPs must be turned on before preceding with the initialization. After turn on, the BEP will be commanded to dump the flight SW to telemetry. We will verify from the outset that the flight SW has not been corrupted. The BEP will then run through a series of checks of its internal memory and its interfaces. The FEPs should be turned on during this stage. We will then establish the capability to send commands to the BEP and verify their execution.
The next stage of activities will verify the expected operating conditions of the instrument. ACIS has several temperature monitors located on the electronics boxes, the radiators, the camera housing, and the focal plane, and one pressure monitor inside the camera housing. The sequence of events will be to simply turn on the monitors and immediately start to read them out. We will then use the temperature information to confirm our preflight thermal models.
The HW and SW initialization and the verification of the physical conditions must occur as soon as possible. There may be a delay before the next stage of activities which is the CCD initialization. The first step in this stage is to apply power to the CCD analog electronics and immediately start to clock out the CCDs. The CCDs will start to produce data which must be processed by the FEPs and BEP and sent to telemetry. The first data will be produced in ``diagnostic mode'' which produces the most useful information for identifying problems. If everything is working, we will configure the CCDs in the timed exposure mode and start collecting data. An important test at the beginning of this data collection will be the bias computation. The events produced by the internal calibration source will provide useful diagnostic data on the state of the CCDs (specifically, has any contamination built up) but the event rate is low enough so as to not adversely effect the bias calculation. After executing in timed-exposure mode, we will load another one of the ACIS onboard parameter blocks and operate in that configuration. We will then load a parameter block from the ground and operate in that configuration. It is crucial to verify both methods of loading parameter blocks because both will be heavily used during normal operations.
The next stage is the detailed checkout of the CCD operations. The ACIS flight software will monitor the signal from the EPHIN detector and turn off power to the CCDs (by powering down the DEAs and FEPs, but not the BEPs) when the signal indicates a high radiation environment. The flight software continues to monitor the radiation signal and after a programmable number of low signals, it will turn the CCD power back on. We will verify this performance by manually setting the radiation signal high and then low and observing the flight software's response. The detailed checkout will also include bias measurements made with different exposure times; these data are particularly useful in characterizing the CCDs' performance. Finally, the detailed checkout will include data collected in timed exposure mode and continuous clocking mode with several configurations to verify the capability to collect useful data in these modes; these data will also be useful in characterizing the CCDs' performance. These measurements will be conducted as normal operations commanded through parameter blocks.
The last activity of the period 1 ACIS activities is the opening of the vent valve. This is the only activity which we believe could potentially cause irreparable damage to the instrument if not executed properly. See the Functional Test Procedure for proper precautions and sequences which need to be observed to safely open the door. Key elements to this procedure include opening the low conductance valve and waiting for at least three hours to allow accumulated gas to vent prior to opening the large (hot-wax) valve.