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Selecting Required and Optional CCDs


Last modified: 12/09/13

THERMAL LIMITATIONS ON THE NUMBER OF REQUIRED AND OPTIONAL CCDs

Many of the spacecraft components have been reaching higher temperatures over the course of the mission because of changes in the insulating layers on the exterior surfaces of the Chandra spacecraft. The ACIS electronics and Focal Plane(FP) temperatures can reach operational limits depending on the orientation of the spacecraft and the number of operational CCDs. The number of operational CCDs and/or the duration of observations is limited for observations with solar pitch angles less than 60 degrees and solar pitch angles larger than 130 degrees. Starting in Cycle 15, the CXC has been encouraging GOs to select 4 or fewer required CCDs unless the science objectives require 5 or 6 CCDs. This is a departure from the previous recommendation of the CXC which asked GOs to select 6 CCDs to maximize the archival utility of the observations. Please note that the solar pitch angles specified on this page, while they correspond to the pitch range boundaries given in the Proposers' Observatory Guide and in other CXC documents, are practical representations of more complex physical behavior and should be understood as approximate.

SELECTING THE REQUIRED AND OPTIONAL CCDs in RPS

The observer may select that a given CCD is required for an observation by entering "Y" for that CCD at the appropriate place in the RPS form. The observer may select that a CCD must be off for the observation by entering an "N". Finally, the observer may select rank-ordered optional CCDs which will be turned on or off at the discretion of the mission schedulers depending on the specific thermal situation of the Observatory. This is done by entering "Off1-Off5". The first to be turned off, if necessary, would be designated by "Off1", the second to be turned off would be designated by "Off2", etc. Observers are encouraged to select 4 or fewer required CCDs if their science objectives are not significantly affected by turning one or more CCDs off. If the observer's science objectives require 5 CCDs, the observer may select 5 required CCDs by entering "Y" for the desired CCDs and "N" for the remaining CCDs. If the observer's science objectives require 6 CCDs, the observer should select 5 required CCDs and one CCD as an optional CCD by entering "Off1". RPS will not allow the observe to select six required CCDs. The sixth CCD may be changed from "Off1" to "Y" after the proposal has been accepted during the interaction with the User Uplink Support Scientist. The CXC will do its best to schedule the observation to meet the thermal requirements of the Observatory. But observers should be aware that 6 CCD observations will be increasingly more difficult to schedule as the mission progresses.

Using fewer than 5 CCDs is beneficial in keeping the ACIS focal plane and electronics temperatures within the required operating ranges. If observers desire the most accurate gain calibration for their observations (provided by a cold and stable FP temperature), they should select 4 or fewer required CCDs. This can be accomplished by selecting 6 CCDs to be off by entering "N" for those CCDs and selecting 4 required CCDs by entering "Y" for those CCDs. For example, if the observer is using the ACIS-I array for imaging, they could select just the four I array CCDs. If the observer is using S3 for imaging, they could select S2, S3, S4, and I3 or they could select S1, S2, S3, & S4.

If 5 required CCDs and one optional CCD are selected and the observation is not constrained in such a way as to prohibit it, the observation is likely to be scheduled at a time for which the pitch angle is greater than 60 degrees and/or less than 130 degrees. The user should be aware that such an observation may be listed in the long term schedule (LTS) at a date for which the pitch angle for this target is less than 60 degrees or larger than 130 degrees. However, when the observation finally appears in the short term schedule, it may be at a date for which the pitch angle will be greater than 60 degrees and/or less than 130 degrees, or may be segmented.

SOME RECOMMENDED CHIP SETS

Observers should select the chip set that is best for their primary science. The following suggestions have proven to be popular, and would facilitate a more useful and homogeneous archive.


FIGURE 1: ACIS-I (4 Required CCDs, 1 Optional CCD) imaging, nominal aimpoint

The rationale for the configuration above is that, in the unlikely event of major background flares, telemetry might saturate more rapidly if S3 were on and the FP temperature and electronics temperatures will be lower with only 4 or 5 CCDs on. Given the current thermal performance of the spacecraft, it is possible that the optional CCD would be turned off.

FIGURE 2: ACIS-I (4 Required CCDs, 1 Optional CCD) imaging, nominal aimpoint

For this configuration, the rationale is that S3 is generally more sensitive and closer to the ACIS-I aimpoint, so more sensitive to serendipitous source detection. Given the current thermal performance of the spacecraft, it is possible that the optional CCD would be turned off.

FIGURE 3: ACIS-I (4 Required CCDs, 2 Optional CCDs) imaging, nominal aimpoint

The rationale for the configuration in Figure 3 is that it is desired to have both S2 and S3 on, but it is not required. Given the current thermal performance of the spacecraft, it is probable that the first optional CCD would be turned off and possible that the second optional CCD would be turned off also.


FIGURE 4: ACIS-S (4 Required CCDs, 1 Optional CCD) imaging, nominal aimpoint

For this ACIS-S imaging chipset, chips farthest from the aimpoint (where the PSF is degraded) would be turned off first. On the other hand, S4 has significant noise streaks resulting in decreased sensitivity, so some users may prefer to turn it off earlier in the optional sequence.
The rationale for the selection in Figure 4 is that S1 will have a higher count rate than a FI CCD in the case of a background flare and thus it might be desirable to have S1 off. Given the current thermal performance of the spacecraft, it is possible that the optional CCD would be turned off.

FIGURE 5: ACIS-S (4 Required CCDs, 1 Optional CCD) imaging, nominal aimpoint

The rational for the selection in Figure 5 is that the observer may want to use S1 to help model the background on S3, therefore it is desirable to have S4 turned off. Given the current thermal performance of the spacecraft, it is possible that the optional CCD would be turned off.

FIGURE 6: ACIS-S (4 Required CCDs, 2 Optional CCDs) imaging, nominal aimpoint

The rational for the configuration in Figure 6 is that it is desirable to have 6 CCDs on, but not required. Given the current thermal performance of the spacecraft, it is probable that first optional CCD would be turned off and it is possible that the second optional CCD would be turned off also.


FIGURE 7: ACIS-S (4 Required CCDs, 2 Optional CCDs) spectroscopy, nominal aimpoint

The optimum ACIS-S spectroscopy chip set depends strongly on the expected spectrum of the target. Typically the maximum signal is desired, so the HETG and LETG observer is most likely to insist on all ACIS-S chips. If the science does depend strongly on the flux received on the S0 and S5 CCDs, the observer may desire that all 6 ACIS-S CCDs be on for the observation. The observer should select 5 CCDs as required and one CCD as optional on the RPS forms when they submit their proposal, and then work with their User Uplink Support Scientist if their proposal is accepted to change the one optional CCD to be required. However, the observer should be aware that the amount of useful flux on S0 and S5 with the LETG is typically quite low given they are both FI CCDs. Given the current thermal performance of the spacecraft, it is probable that the first optional CCD would be turned off and it is possible that the second optional CCD would be turned off also.

BACKGROUND ON THE THERMAL ENVIRONMENT AND ITS IMPACT ON ACIS

There are several components on the Chandra spacecraft which have reached elevated temperatures at a variety of pitch angles. Figure 8 displays approximate pitch ranges and the components sensitive in those ranges. Within ACIS, the Power Supply and Mechanism Controller (PSMC) heats at pitch angles less than 60 degrees and the Focal Plane (FP), Detector Electronics Assembly (DEA), and Digital Processing Assembly (DPA) heat at angles larger than 130 degrees. Under current conditions, and assuming an initial PSMC temperature of less than +30 C, observations at pitch angles less than about 60 degrees, longer than 50 ks, and with 6 CCDs operating are likely to approach or exceed the PSMC thermal limit. Likewise observations at pitch angles larger than about 130 degrees, longer than about 50 ks, and with 6 CCDs operating are likely to approach or exceed the DPA and DEA thermal limits. Finally, the ACIS FP temperature will increase above the desired operating temperature of -119.7 C for observations with pitch angles larger than 130 degrees. Each of these temperatures can be reduced by reducing the number of operating CCDs. The Operations team may turn off one or two optional CCDs if a total of 5 or 6 CCDs are selected (where total is the sum of required "Y" plus optional "Off#") if the thermal models indicate that operational limits might be reached for a given observation.

FIGURE 8: PITCH DEPENDENCIES

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Figure 9 shows the PSMC temperature as a function of spacecraft pitch angle using data from a series of observations from between 2007 and 2013, and for which the exposure times are at least 50 ks. The figure illustrates the increase in PSMC temperature for observations at low pitch angles. Observations using 6 chips are plotted as red triangles, and those using 5 chips are plotted as blue triangles; variations in the maximum temperature at a particular pitch angle within these two cases correspond primarily to variations in the starting temperatures for the observations. The maximum allowable PSMC temperature is indicated by the yellow horizontal line. The PSMC temperature has been controlled since 2008 through the use of a model which predicts the temperature for a given week. If the predicted temperatures exceed the planning limits, adjustments are made such as turning off an optional CCD, splitting an observation, or rescheduling an observation at a more favorable pitch angle. It is evident from the plot that using one less CCD can reduce the temperature by a few degrees, thus allowing somewhat longer observations to be carried out at low pitch angles.

FIGURE 9: ACIS PSMC TEMPERATURE vs. PITCH

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In tail-Sun orientations (pitch angles larger than 130 degrees), the ACIS FP temperature, the Detector Electronics Assembly (DEA) temperature, and the Digital Processing Assembly (DPA) temperature can warm outside of the desired range. Each of these three temperatures can be reduced by reducing the number of CCDs being clocked. Figure 10 displays the DPA temperature as a function of pitch angle for 5 and 6 CCD configurations. The DPA temperature has been controlled since 2012 through the use of a model which predicts the temperature for a given week. If the predicted temperatures exceed the planning limits, adjustments are made such as turning off an optional CCD, splitting an observation, or rescheduling an observation at a more favorable pitch angle.

FIGURE 10: ACIS DPA TEMPERATURE vs. PITCH

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Figure 11 displays the DEA temperature vs. pitch angle for 5 and 6 CCD observations. Both of these figure show the lower electronics temperatures for 5 CCD observations.

FIGURE 11: ACIS DEA TEMPERATURE vs. PITCH

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