Chapter 3
Offset Pointing, Visibility, and other Constraints

3.1  Introduction

3.2  Offset Pointing

3.3  Visibility

3.3.1  Radiation Belt Passages

3.3.2  Avoidances

1. Sun avoidance - cannot be overridden - viewing is restricted to angles larger than 46 degrees from the limb of the Sun. This restriction makes about 15% of the sky inaccessible on any given date, but no part of the sky is ever inaccessible for more than 3 months.
2. Moon avoidance - viewing is restricted to angles larger than 6 degrees from the limb of the Moon. This restriction makes less than 1% of the sky inaccessible at any time. This avoidance can be waived, but at the price of a reduced-accuracy aspect solution (see Chapter 5).
3. Bright Earth avoidance - viewing is restricted to angles larger than 10 degrees from the limb of the bright Earth. This restriction makes less than 5% of the sky inaccessible at any time, but there are certain regions which can only be viewed, continuously, for up to about 30 ks. The avoidance can be waived, but at the price of a reduced-accuracy aspect solution (see Chapter 5). Figure 3.4 illustrates the point that the Earth avoidance region is nearly stationary. This is a consequence of the combination of high elliptical orbit and radiation belt passages. This partially blocked region moves several degrees per year, reflecting the evolution of the orbital elements.
   The greatest amount of observing time is available in the vicinity of apogee, when the satellite moves most slowly and the earth and its avoidance zone occupy an approximately stationary location on the sky, visible in Figure 3.4 as the extension to the south of the sun avoidance band.
4. Roll angles - the spacecraft and instruments were designed to take advantage of the Observatory having a hot and a cold side. Thus, the spacecraft is preferentially oriented with the Sun on the −Z side of the X−Y plane, where +X is in the viewing direction, the Y-axis is parallel to the solar panel axes, and +Z is in the direction of the ACIS radiator (see Figure 1.1). In this orientation there is only one "roll angle" (rotation about the viewing- or X-axis - positive West of North) for which the solar panels can be rotated so that they are directly viewing the sun - the nominal roll angle. Small deviations ( ∼ degrees) from the nominal roll angle may be allowed depending on the viewing geometry. The roll-angle constraint imposes further visibility restrictions. These can also be evaluated with the ObsVis tool.

3.3.3  Pitch Angle Constraints

Figure 3.3: Diagram to illustrate ranges of solar pitch angle (the angle between the pointing direction and the satellite-Sun line) within which different observing limitations apply. Table 3.1 provides quantitative estimates for the observing limitations in the solar pitch angle ranges shown in the drawing.

1. The EPHIN detector is subject to possibly degraded performance at elevated temperatures that may affect its use in safing the science instruments from high levels of particle radiation. During long observations at pitch angles of between approximately 60 and 135 degrees the EPHIN may reach temperatures that may result in anomalous performance. If this isn't complicated enough, the exact performance depends on the near term thermal history. The CXC has developed a model to predict EPHIN temperature as a function of time and pitch angle to aid in Mission Planning.
2. Solar pitch angles greater than 170 degrees are not accessible. This is necessary to prevent excessive cooling of the propellant lines, which might then rupture. For cycle 10 we urge that you carefully consider how to configure your observation such that it does not require a pitch angle greater than 170 degrees. This may be done, for example, by imposing no constraints on the observation, or by using the ObsVis tool at http://obsvis.harvard.edu to see if your time or roll constraint is acceptable within the allowable pitch angles, i.e. between 46 and 170 degrees. Even this must be tempered by considering the EPHIN-imposed pitch-angle constraints discussed above. It is possible that a peer-review accepted proposal may, in fact, not be accomplished because of these safety constraints. To avoid this unpleasant possibility, observers are urged to carefully plan their observations using all the proposal preparation tools and contacting the CXC  HelpDesk if necessary.
3. There are observing restrictions in the pitch angle range 152-170 degrees imposed by the need to prevent propellant lines from dipping to low temperatures before line heaters switch on.
4. Owing to the changing thermal environment, the ACIS Power Supply and Mechanism Controller (PSMC) Detector Electronics Assembly (DEA) has been warming and is expected to continue so doing. The temperature is affected by both the solar pitch angle and by the number of ACIS chips in use. As a result, allowable durations for 6-chip ACIS observations at solar pitch angles of less than about 56 degrees will be limited. ACIS observers are being asked to specify optional chips and the priority order in which these may be turned off; this information will be used as needed during the mission planning process to control ACIS temperatures (see 6.20.1). Observers are encouraged to avoid specifying ACIS observations that require 5 or 6 chips and that must also be executed at solar pich angle below about 56 degrees.

• Observations in the 60-135 degree pitch zone can be performed, but if too long will be broken into shorter durations, which may be separated by a day or more. The maximum continuous duration is highly dependent on the specific pitch as well as on the preceding pitch history of the Observatory. If such observations have roll constraints, the observations must either be brief or the roll constraints must be generous enough to allow multiple segments at their different, time-dependent, roll angles. Constraining roll angles to be constant for multiple segments is discouraged, as achieving off-nominal roll angles creates additional thermal problems/concerns and can only be done for a very limited range of off-nominal roll angles around the nominal value.
• Simultaneous longer-duration observations with telescopes (such as XMM-Newton) with a preferred pitch angle in the 60-135 degree range may be very difficult, or even impossible, to schedule.
• Targets near the ecliptic poles (such as the Magellanic Clouds) are especially affected by these considerations since their pitch angles are always close to 90 degrees.

Figure 3.2: The Chandra visibility showing contours of fractional visibility averaged over the 12-month interval of Cycle 10. The darker the shade of gray, the lower the visibility. The three contour levels correspond to 70%, 80%, and 90% average visibility

3.4  Other Constraints and Considerations

3.4.1  Instrument Constraints and Considerations

• The HRC has a brightness limit which limits the flux per microchannel plate pore.
• The HRC has a telemetry limit. Exceeding this limit, amongst other consequences, reduces observing efficiency.
• The HRC has linearity limits. Exceeding these limits voids the effective area calibrations.
• The ACIS has a telemetry limit. Exceeding this limit, amongst other consequences, reduces observing efficiency.
• The ACIS is subject to the effects of pulse pileup. Dealing with this effect requires careful planning of the observation.
• The ACIS has a limit for the total amount of allowed flux in a pixel during an observation. The limit only impacts a small number of potential observations, primarily those of very bright sources that request the dither to be turned off. Please see Section 6.18

3.4.2  User-Imposed Constraints

Time Constraints:

Time Windows - specific time intervals in which observation must be scheduled. Such constraints are primarily for use in coordinated observing campaigns or for arranging an observation to coincide with some time-critical aspect of the target.

Monitoring Intervals - for observing a target at semi-regular intervals for a specified duration.

Phase Interval - specific phase intervals for observing sources with long, regular periods.

Coordinated Observations - targets specified to be observed by Chandra and another observatory in a give time period are time constrained.

Continuity of observation - specifying that an observation may not be interrupted (up to 160 ks).

Group Observation - a target which needs to be observed within a particular time range with other targets in the program.

Roll Constraints:

- specifying a particular roll angle and tolerance.

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