How to Estimate Maximum Chandra Dwell Time

To aid in proposal planning, the following tables and plot may be used to estimate Chandra's maximum dwell time 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 determine 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 2010 (perihelion passage), April 2010, July 2010 (aphelion passage), and October 2010.

Here is an example of how the dwell time matrices may be used. We first compute the solar pitch angle as a function of time for our target (using, for example, PRoVis). Let us suppose that the pitch angle turns out to be 115 deg at a time of interest (April 2010) to which the observation may need to be constrained for scientific reasons. The table shows that the maximum uninterrupted duration of an observation under these circumstances is estimated at 22ks. It may be safer to plan with the more conservative estimate of 20ks derived from an orbit-start temperature higher by 2 deg F, and it is important to note that starting temperatures may be significantly higher (and maximum dwell times may be shorter) for mid-orbit observations.

Maximum dwell time vs. solar pitch angle in January, April, July, and October for the upcoming cycle. T_E is the assumed starting EPHIN temperature in Fahrenheit, and is based upon the current model for the thermal evolution of the spacecraft. The dashed lines at 20 and 30 ks are to aid the proposer in estimating maximum uninterrupted exposure times in the "bad pitch" region at various times of the year. To avoid overlapping histograms, lines connect values derived in pitch bins of 5deg.

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 matrices and associated plot 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 114 F (the current maximum allowed TEPHIN) 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. These effects are modeled and determine the initial temperature values in the left-hand column of the matrix. At each indicated date, the matrix shows maximum dwell times as a function of solar pitch angle for two initial temperatures: that determined by the model, and another 2 deg F higher to provide a more conservative estimate.
• The models assume that 4 ACIS chips are used. This assumption only affects the ACIS PSMC region (46-56 deg in pitch).

Last modified: 03/10/09

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