To aid in proposal planning, the following plot may be used to estimate the maximum allowable time that Chandra may stay at a particular solar pitch angle. The dwell time may include time for target acquisition, settling, and target exposure.
Chandra Mission Planning schedules all observations, and will split exposures as necessary if they are longer than a full orbit above the radiation zones (currently about 180ksec), or if temperature limits will be exceeded. The information here can be useful for proposers whose observations require constraints for scientific reasons, to estimate the maximum uninterrupted exposure time for a given target, once its pitch angle is known, e.g., using PRoVis.
The data are provided in 3 month intervals: January 2013 (perihelion passage), April 2013, July 2013 (aphelion passage), October 2013, and near the end of Cycle 14 in January 2014.
Here is an example of how the plot may be used. First, using PRoVis, compute the solar pitch angle as a function of time for our target. Suppose that the pitch angle turns out to be 115 degrees at the time when we wish to observe the target (for instance, April 2013) due to scientific constraints. The plot shows that in April 2013, the maximum allowed exposure time at 115 degrees in solar pitch is about 50ks. However, it is important to point out that due to the thermal history of the spacecraft during the orbit in which this observation is to be scheduled, the maximum allowed time may actually be less than 50ks.
Typical maximum dwell time vs. solar pitch angle in January, April, July, October and the following January for the upcoming cycle.
The need to keep various components of the spacecraft within allowed temperature ranges adds to the constraints that limit where Chandra can point. As the outer surfaces of Chandra age with ongoing exposure to the space environment, the thermal properties of these surfaces change and alter the operational constraints to which Chandra is subject.
A major source of constraints to Chandra's ability to dwell for an extended time at any of a broad range of solar pitch angles is heating of the EPHIN (the Electron Proton Helium INstrument), which is used to warn of high radiation environments and can initiate safing of the instruments, but there are several additional causes of thermal limitations. For a more detailed explanation of these, please see section 3.3.3 of the current Proposer's Observatory Guide.
The curves on the plot are computed with the following assumptions:
Last modified: 10/18/11
