## Chandra Cycle 15 Estimated Maximum Exposure Time Information

To aid in proposal planning, the following plots 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 2014 (perihelion passage), April 2014, July 2014 (aphelion passage), October 2014, and near the end of Cycle 15 in January 2015. We provide plots for two different thermal planning limits to illustrate how exposure times can be affected by thermal constraints.

Here is an example of how the plots may be used. First, using PRoVis, compute the solar pitch angle as a function of time for your 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 2014) due to scientific constraints. The plots shows that in April 2014, the maximum allowed exposure time at 115 degrees in solar pitch is about 70ks if the planning limit is 144F, and 110ks if the planning limit is 147F. 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 these values.

Typical maximum dwell time vs. solar pitch angle in January, April, July, October and the following January for the upcoming cycle.

## Further Explanation

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 in the plots are computed with the following assumptions:

• The EPHIN starting temperature for the observing portion of each orbit (i.e., after emergence from the radiation zone) is calculated under the assumption that the spacecraft enters the radiation zone near perigee with EPHIN at a temperature of 144F (the current maximum allowed) or 147F (the planned maximum allowed) and then remains at an attitude that provides maximum cooling for the duration of radiation zone passage.
• The EPHIN temperature at each orbit start depends not only on the pitch history, but also on the season (owing to the eccentricity of Earth's orbit) and on slow degradation of EPHIN surfaces in space. The combination of these effects produces a slowly-rising temperature trend with a superposed annual oscillation.
• The models assume that 4 ACIS chips are used. This assumption affects the ACIS PSMC region (46-56 deg in pitch) as well as observations aft of 130 degrees in pitch. Using 5 or 6 chips will reduce the maximum allowable exposure time in these pitch regions.

Last modified: 12/13/12

The Chandra X-Ray Center (CXC) is operated for NASA by the Smithsonian Astrophysical Observatory. 60 Garden Street, Cambridge, MA 02138 USA.   Email: cxcweb@head.cfa.harvard.edu Smithsonian Institution, Copyright © 1998-2012. All rights reserved.