Chapter 3
Offset Pointing, Visibility, and other Constraints

3.1 Introduction

This chapter gathers together several topics pertaining to observation planning, irrespective of focal-plane instrument and grating configuration, to serve as additional guidelines for preparing proposals. Most of these topics are automatically addressed by the target visibility interface webtool (ProVis) or the observation visualizer software (ObsVis) available as part of CIAO. The intention here is to familiarize the user with the considerations.

3.2 Offset Pointing

The offset pointing convention for Chandra is that a negative offset of a coordinate moves the image to more positive values of the coordinate and vice-versa. Examples of offset pointings of the ACIS instrument are shown in Figure 3.1. Examples using the HRC are shown in Figure 3.2. ./images/newacisoffsetsA1crop.jpg

Figure 3.1: Image created with ObsVis shows the ACIS field of view overlaid on the optical image field of the galaxy NGC 891. North is up and East is to the left. Roll is measured positive, West of North. The roll angle shown is 10^°. Five chips are turned on (solid outlines, with dashed node boundaries shown) and five off (dashed chip outlines). Top Panel: Target is centered at the nominal ACIS-S aimpoint. Offset (Y,Z) coordinate system is also shown, refer to Figure 6.1 Middle Panel: The target has been offset 90 arcsec in the negative Y direction: (Y,Z) offset of (-1.5, 0) in arcmin. In the Bottom Panel the offset is (-1.5, -3) in arcmin ./images/newhrcoffsetsA1crop.jpg

Figure 3.2: Example of offset pointing with HRC, overlaid on a DSS-I R-band optical image. North is up and East is to the left. Roll is measured positive, West of North. The roll angle shown is 10^°. Top Panel: The target, the edge-on galaxy NGC 891, is at the nominal HRC-I aimpoint. Offset (Y,Z) coordinate system is also shown, refer to Figure 6.1. Center Panel: The target is offset with (Y,Z) offset of (-5,0) arcmin. Bottom Panel: The offset is (-5,-5). Note the small dot at the location of the HRC-I aimpoint.

3.3 Visibility

There are a number of factors that limit when observations can be performed. These are discussed in the following subsections.

3.3.1 Radiation Belt Passages

High particle-radiation levels are encountered as the Observatory approaches perigee. Data acquisition ceases whenever certain particle-radiation thresholds are exceeded. A working number for the altitude at which this takes place is about 60,000 km. Cessation of observations and protection of the instruments in regions of high radiation results in approximately 20% of the 63.5 hour Chandra orbit being unusable.

3.3.2 Avoidances

The following constraints are necessary to ensure the health and safety of the spacecraft and science instruments. Proposals which violate these constraints may be rejected.

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.
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).
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 3.3 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 3.3 as the extension to the south of the sun avoidance band.
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 ProVis tool.

3.3.3 Pitch Angle Constraints

Figure 3.3: The Chandra visibility showing contours of fractional visibility averaged over the 12-month interval of Cycle 15. The darker the shade of gray, the lower the visibility. The five contour levels correspond to 50%, 60%, 70%, 80%, and 90% average visibility. Gradual changes in the thermal properties of the spacecraft with time require us to impose restrictions in the durations of observations at various solar pitch angles (i.e., angles between viewing directions and the direction to the Sun). These restrictions are evolving with time; observers are urged to consult Announcements on the CXC Proposers webpages for updates (http://cxc.harvard.edu/proposer/). The principal pitch restrictions are of several kinds:

The Electron Proton Helium Instrument (EPHIN, Section 2.5) detector, used in safing the science instruments from high levels of particle radiation, suffers degraded performance at elevated temperatures. During extended observations at pitch angles of between approximately 65 and 135 degrees the EPHIN, which also acts as a proxy for the temperatures of other spacecraft components affected by -Z side solar heating, can reach temperatures that may result in anomalous performance. 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.
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. We urge that you carefully consider how to configure your observation so 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 Chandra Pitch Roll and Visibility tool (ProVis) at http://cxc.harvard.edu/soft/provis/ to see if your time or roll constraint can be achieved within the allowable pitch angles, i.e. between 46 and 170 degrees. (Even this must be tempered by consideration of other pitch-angle constraints discussed in this section.) It is possible that a peer-review accepted proposal may, in fact, not be accomplished because of these safety constraints. To avoid this possibility, observers are urged to plan their observations carefully using all the proposal preparation tools and contacting the CXC HelpDesk if necessary.
There are observing restrictions in the pitch angle range 156-170 degrees imposed by the need to prevent propellant lines from dipping to low temperatures before line heaters switch on.
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 doing so. 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 60 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 Section 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 pitch angle below about 60 degrees.
The Integral Propulsion System (see Section 2.2) tank pressure and temperature approach potentially unsafe levels as a result of accumulated dwell in bi-modal pitch regions (45-70 degrees and to a lesser extent 110-130 degrees). The time constants are long, and the associated restrictions affect principally the development of the long-term schedule (see Section 2.7.1).
The ACIS electronics can heat to unsafe temperatures during long dwells at high solar pitch angles, resulting in limitations on observations at 130-170 degrees. These limitations depend to some extent on the number of ACIS chips in use.

These spacecraft constraints have several implications for proposers:

Observations can be performed over the full range of accessible solar pitch angles (46 to 170 deg), but if they are too long for the particular observing conditions they will be broken into shorter durations, which may be separated by a day or more. The maximum continuous duration depends on the target pitch and thermal history of the spacecraft. It can be estimated from the figure given on the MAXEXPO web page (http://cxc.harvard.edu/proposer/maxexpo.html). Observations with roll constraints must either be of appropriately limited duration 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 may cause planning difficulties, as achieving off-nominal roll angles can only be done for a limited range of off-nominal roll angles around the nominal value (accessible from ProVis).
Simultaneous longer-duration observations with telescopes that have a limited range of accessible pitch angles (such as XMM: approximately 70-110 degrees) 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.

3.4 Other Constraints and Considerations

The instrument constraints are discussed in the chapters devoted specifically to the instruments. User-imposed constraints are discussed in the instructions for completing the Chandra Remote Proposal Submission (RPS) form. We summarize these here.

3.4.1 Instrument Constraints and Considerations

For details on the following limitations, please refer to Chapters 6 (for ACIS) and 7 (for HRC).

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

Chandra users may need to specify a number of observing constraints particular to their observations. In general, the specification of a user-imposed constraint decreases the efficiency of the observatory and therefore should be well justified in the proposal. Note that only a limited number of constrained observations can be accommodated (see the CfP for details). User imposed constraints are summarized here.

Time Constraints:
: Time Windows - specific time intervals in which an 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 repeatedly, with intervals and durations specified.
: Phase Interval - specific phase intervals for observing sources with long, regular periods.
: Coordinated Observations - targets specified to be observed by Chandra and another observatory within a given time period.
: Continuity of observation - specifying that an observation be performed in a single (or the fewest possible) segment(s).
: 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.

INDEX

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.

Chapter 3 Offset Pointing, Visibility, and other Constraints