Specifying Optional CCDs

Last modified: 12/10/12

THERMAL LIMITATIONS ON THE NUMBER OF OPERATIONAL CCDs

Because of changes in the insulating layers on the exterior surfaces of the Chandra spacecraft, many of the spacecraft components have been reaching higher temperatures over the course of the mission. Starting in AO-8, limitations were placed on the number of operational CCDs and/or the duration of observations with solar pitch angles less than 60 degrees to keep the ACIS Power Supply and Mechanism Controller (PSMC) temperature within operational limits. Starting in AO-13, the CXC has been encouraging GOs to select 5 or fewer CCDs unless the science objectives require 6 CCDs. This is a departure from the previous recommendation of the CXC which asked GOs to request 6 CCDs to maximize the archival utility of the observations. Starting in Cycle 13, the Operations team may turn off one optional CCD if a total of 6 CCDs are requested (where total is the sum of required "Y" plus optional "OPT#") for observations with pitch angles greater than 130 degrees in order to keep the ACIS Detector Electronics Assembly (DEA), Digital Processing Assembly (DPA), and Focal Plane (FP) within operational limits. GOs may still select 6 CCDs if their science objectives require 6 CCDs, but this may add to the complexity of scheduling the observation. Starting in Cycle 14, GOs will not be allowed to specify "Y" for 6 CCDs in the RPS forms when they submit their proposal. If a GO requires 6 CCDs for their observation, they are to specify 5 CCDs as "Y" and one CCD as "OPT1" at the time of proposal submission. If the proposal is selected, the GO may work with their User Uplink Support Scientist and change the "OPT1" to a "Y" if the sixth CCD is required.

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.

SPECIFYING THE OPERATIONAL CCDs

The observer may specify that a given CCD must be on for an observation by entering "Y" for that CCD at the appropriate place in the RPS form. If a CCD must be off one enters an "N". Finally, one may specify 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 "OPT1-OPT5". The first to be turned off, if necessary, would be designated by "OPT1", the second to be turned off would be designated by "OPT2", etc. Observers are encouraged to use 5 or fewer CCDs if their science objectives are not significantly affected by turning one or more CCDs off. If the observer's science objectives require 6 CCDs, the observer should set 5 CCDs to "Y" and one CCD to "OPT1". The sixth CCD may be changed from "OPT1" 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 6 CCDs is beneficial in keeping the ACIS focal plane and electronics temperatures within the required operating ranges. 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. Each of these temperatures can be reduced by reducing the number of operating CCDs. If observers desire the most accurate gain calibration for their observations (provided by a cold and stable focal plane temperature), they may wish to clock 5 or fewer CCDs for those observations. This can be accomplished by setting 5 CCDs to "N" and 5 CCDs to "Y" and/or "OPT1-OPT5". The observer may even wish to set 6 CCDs to "N" and only 4 CCDs to "Y". For example, if the observer is using the ACIS-I array for imaging, they could select the four I array CCDs and one of S2 or S3 or they could select just the four I array CCDs. If the observer is using S3 for imaging, they could select S2, S3, S4, I2 and I3 and turn S1 off or they could select just S1, S2, S3, & S4.

If no optional CCDs are selected, and six CCDs are on 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 specify 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 (5 CCDs) 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 5 CCDs on.

FIGURE 2: ACIS-I (5 CCDs) 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.

FIGURE 3: ACIS-I (6 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 one of the two optional CCDs would be turned off.

FIGURE 4: ACIS-S (5 CCDs) 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 and resulting 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.

FIGURE 5: ACIS-S (5 CCDs) 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 I2 turned off.

FIGURE 6: ACIS-S (6 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 one of the two optional CCDs would be turned off.

FIGURE 7: ACIS-S (6 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 need to specify all 6 ACIS-S CCDs to be on. 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 one of the two optional CCDs would be turned off.

BACKGROUND ON THE THERMAL ENVIRONMENT

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. The ACIS PSMC is most affected in the range of pitch angles from 45-60 and the ACIS DEA, DPA, & FP are affected in the range from 130-170.

FIGURE 8: PITCH DEPENDENCIES

Figure 9 shows the PSMC temperature as a function of spacecraft pitch angle using data from a series of observations from between 2007 and 2012, 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

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.

FIGURE 10: ACIS DPA TEMPERATURE vs. PITCH

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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