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The Scheduling of Observations

Now that Chandra operations are almost ``normal", observers may be interested in the steps that go into producing the schedule. The CXC generates a long term schedule and a detailed short term schedule, and there is interaction with users during the process. There may be revisions to the scheduling method for the second cycle and the user should keep an eye on the webpages for updates.

In the following Caveats section, we draw the attention of users to features of the process which are particularly important for them.


Two aspects related to scheduling are of particular interest to observers.

1) Observers cannot expect to make a ``quick look" observation and then turn around information from that observation in order to plan another observation within a short time scale. Minimum time between such observations should be 3 months; a month to get valid data, a month for analysis and reconfiguration decisions, then the observation must be finalized a month before it is carried out.

2) If simultaneous observations with other instruments are a vital part of your program and if they were part of your peer-reviewed proposal, a time-constraint should be already applied to your target. Barring malfunctions, it will be honored. After a proposal has been reviewed and accepted, the addition of constraints, or changes in object, position, or exposure time will usually not be allowed. If you have added coordinated observations later or if they are only a secondary part of the science, we cannot perturb the normal scheduling to give you a definite time in advance of the short-term schedule.

Long Term Schedule

Every year, new Chandra targets are generated by the calibration program, guaranteed time observations, successful proposals submitted in response to a NASA NRA, and proposals for Director's Discretionary Time (DDT).

A Long-term Schedule is developed for these targets with the aid of the Spike scheduling software. Spike places all available targets except TOO (Targets of Opportunity) and DDT (Director's Discretionary Time) into one week segments during which sky-viewing constraints (e.g., sun avoidance) and any user-imposed constraints such as time or roll angle can be satisfied. In this Long-term Schedule, targets are placed in one week blocks to fill about 70% of Chandra's available observing time. The remaining observations are unconstrained and kept in a pool of targets (listed in the last 3 weeks).

For each week an observation request (OR) list is generated from the long term target list for that week, plus some unconstrained targets from the pool to over-subscribe the available time. Oversubscription of the OR list allows the flexibility needed to construct the detailed schedule. Priorities are set so that all of the constrained observations (which must be accomplished) and a subset of the unconstrained targets are used in creating an efficient schedule. When the short-term schedule is generated, it consists of those targets from the OR list which will actually be observed in the absence of spacecraft anomalies. Targets not scheduled are returned to the pool.

The current Long-term and Short-term schedules are available to observers on the CXC www site:

It is our intent to follow the Long-term schedule. However, targets which are not constrained may at times be moved forward or back in time to fill requirements of the short-term schedules. Targets of opportunity and any instrument or spacecraft anomalies may also cause observing times of targets to shift. The long-term schedule is our present estimate of future order of observations. Please be aware of the uncertainties.

User Interaction

The WWW long-term schedule has links to our database which lead to a list of the requested observing parameters and to images of the proposed target field of view. Observers are encouraged to review these entries as early as possible and notify the CXC of any perceived problems. Note that these images assume a roll angle appropriate for the middle of the week to which the target has been assigned in the long-term schedule. This approximation may not always respect accepted roll constraints and will be corrected in the final scheduling process. Also, many targets allocated to the pool in the final three weeks of the long-term schedule will be moved ahead in time; the roll angles displayed now for these are thus unlikely to correspond to the final orientation.

Observers will be contacted by the CXC prior to the detailed scheduling of their targets for final confirmation of the observing parameters. Observers in the long-term schedule can expect to be contacted no later than two months before the start of the week in which their observation is scheduled. Pool observers will be contacted as soon as their targets are approved and cataloged. Observers are expected to reply within two weeks after CXC contact.

Short Term Schedule

After the long term schedule has been completed, the Flight Operations Team at the Operations Control Center in consultation with CXC staff go through each week and create a detailed time line of when each observation will be performed, as well as the detailed observing information to be sent to the satellite. The short term schedule is generated in this process. Ultimately, we hope that the short term schedule will be available a week before the start of the week's observations, and will be posted at that time. At present, the plans are often not finalized until a few days before the observations start.

- Fred Seward

next up previous contents
Next: Finding Chandra Targets and
Up: Chandra News
Previous: The Chandra Multi-wavelength Project