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Next: Grating Activities Up: 4.6.2 Science Instrument OV Previous: ACIS Period 1 Activities ACIS Period 2 Activities

The period 2 ACIS activities will include some of the most dramatic in the OV phase, such as opening the ACIS door, opening the aft and forward contamination covers on the HRMA, opening the HRMA sunshade door and conducting the first astronomical observation with AXAF.

The first activity is the opening of the ACIS door. Unfortunately, a great deal of uncertainty currently exists concerning when it is safe to open the ACIS door. The problem is that no one on the project can accurately predict the out-gassing rates of the OBA. Since ACIS is the coldest surface on the satellite (filter $\approx-60$ C and CCDs $\approx-120$ C), most of the contaminants will eventually freeze out on ACIS. Therefore, it is crucial that the ACIS door not be opened until the OBA has out-gassed to an acceptable level. Currently, 25 days has been proposed as the length of time required; however, this is not even an estimate, merely a number chosen on past experience and it may have little bearing on reality. Before opening the ACIS door, the ACIS internal pressure monitor and the SIM pressure monitor will be checked. If the values are significantly different, it will be necessary to wait longer. If they are the same, it may be safe to open the door but it is not guaranteed. The launch-lock position is on the ACIS-S array; therefore ACIS will be configured to collect data with the central 2 chips of the ACIS-S array and the 4 chips of the ACIS-I array. It is not clear at this time if the ACIS door should be opened first or if the HRMA aft contamination cover (ACC) should be opened first. It is possible that there will be some residual out-gassing after the ACC is opened; in this case the ACC will be opened first and some time will be allowed for this material to vent overboard. If not, the ACIS door will be opened and data will be collected on the internal background of the satellite. The ACIS door should be opened during a real-time contact since it uses a paraffin actuator which may be monitored during the process. After the ACIS door is opened, data will be collected from the Forward Contamination Monitor (FCM) sources. These data will be used to determine if a contamination layer had built up on the HRMA. The data will be collected in the ACIS timed-exposure mode which will be commanded by an on-board parameter block.

At this point, it is time to open the HRMA sunshade and collect first light with AXAF. AXAF should be positioned on the first target and the sunshade opened. ACIS will be collecting data as the door is opened and we should verify in real-time that operation has executed normally. We will use the ACIS data to verify that the door has opened completely and is not obstructing the optical path. We will also collect bias and diagnostic data to measure any increases in the background rate. Multiple observations will be conducted to find the boresight of the ACIS-S array and the ACA, this will entail changing the pointing direction many times. Multiple observations will be conducted to find the best focus of the ACIS-S array, this will entail moving the focal plane along the optical axis many times. Finally, the plate scale of the ACIS-S array will be measured in two ways. First, a point source will be moved across the field of view by changing the pointing direction and second, an observation with several objects separated by known (but small) distances will be conducted. These observations should all be conducted with normal operating modes commanded through ACIS parameter blocks.

The SIM will be moved in the Z direction for the first time in the next stage. We will first move the SIM in small steps to keep the HRMA aimpoint on the ACIS-S array. If the operations have been successfully executed and verified, we will then move the SIM so that the HRMA aimpoint falls on the ACIS-I array. We can them move the aimpoint around on the ACIS-I array and further verify the SIM performance. We will then find the boresight, focus and plate scale of the ACIS-I array using the same procedures above. All of these operations will be conducted with ACIS in a normal operating configuration.

The last activity involves inserting the High Energy Transmission Gratings (HETG) and collecting data. The SIM must be translated to place the HRMA aimpoint on the ACIS-S array. The HETG will then be inserted as ACIS is collecting data. We will collect data from several targets to verify the ability to center a target for the gratings and to verify the focus of the ACIS-S array at its outer extremes. After these operations are completed the HETG will be moved out of the way.

Finally, ACIS will be moved to the stow position. The time in the stow position is extremely useful for ACIS because there will be three X-ray calibration sources which will be used for the most detailed performance check of the CCDs. These sources will always be on and ACIS will always see them when in the stow position. ACIS will require the full 24 kbps for some of this time. ACIS will run in a normal operating mode when collecting these data.

next up previous contents
Next: Grating Activities Up: 4.6.2 Science Instrument OV Previous: ACIS Period 1 Activities

John Nousek