Specifying Optional CCDs for Cycle 8 and the Future

Because of changes in the Chandra thermal environment, the ACIS Power Supply and Mechanism Controller (PSMC) has been steadily warming over the course of the mission. Currently, observations at pitch angles less than 60 degrees which are longer than 50 ks and which consume maximum power within ACIS (6 CCDs clocking) are likely to approach or exceed the thermal limit for the PSMC.

The following figure shows the PSMC temperature as a function of spacecraft pitch angle using data from a series of observations from between 2004 and 2006, and for which the exposure times are at least 25 ks. The figure clearly illustrates the increase in PSMC temperature for observations at low pitch angles. Observations using 6 chips are plotted as red squares, and those using 5 chips are plotted as blue triangles with 2.5 degrees subtracted to show the predicted values with the current operating procedures; 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. Current scheduling practices provide a safe temperature "buffer" below the maximum. 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.

To mitigate excessive PSMC temperatures, all observers are now being asked to review their CCD selections and determine which, if any, of their CCDs can be turned off. Optional CCDs (marked as O1-O5) should be selected by the observers. When the observation is planned, one or more of the optional CCDs STARTING WITH O1 may be turned off to protect the PSMC. For Cycle 9, GOs are requested to specify up to two optional CCDs if 6 CCDs are being clocked. The GO may request up to 5 optional CCDs if they so desire.

In addition, the GO may specify that 6 CCDs are required for the science objectives of the proposal, but the GO should be advised that observations are typically scheduled at times when the pitch angle is favorable, so a requirement to operate 6 CCDs will, in part, define the windows into which it is likely that the observation will be scheduled. Alternatively, it may result in the observation being split into smaller pieces. While these restrictions apply to all observations, those most likely to be affected are those for which the approved observing time is larger than 25 ks and at least 5 required CCDs.

Observations that do not explicitly ask for un-interrupted scheduling can be split by Chandra Mission Planning team without further check with the PI. Observations explicitly asking for un-interrupted exposure are considered time-constrained and hence limited by the maximum amount of constrained time that can be approved at peer review.

Please refer to the Proposer's Guide for Cycle 9 for greater detail on how to specify optional CCDs and on constrained observations.

SOME RECOMMENDED CHIP SETS

The following illustrate some choices which may be useful to many Observers. However, note that any permutation of selected and optional chips may be chosen, subject to the rules above.

Last modified: 01/19/07

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