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Last modified: 19 December 2016


FAQ for Constrained and Coordinated Proposals

Proposer Threads (Cycle 19)


What is a time constrained observation?

An observation is time constrained if the proposer restricts the time during which that observation can be performed. This may be because the user has specified that the target can only be observed at certain times, or when an observation cannot be scheduled until triggered (a TOO). Constrained observations are reviewed by the CXC for feasability before proposals go to the Peer Review.

Many targets can only be observed at certain times of the year because of pitch angle or sun block restrictions. These observatory-imposed restrictions do not count as "constraints". However, proposers should check that any user-imposed constraints do not push the target into the "cannot do" pitch range (see the Visibility and Constraints Chapter of the Proposers Guide for details). Proposers can also check the visibility of a target object using PRoVis.

What makes an observation time constrained?

The following will result in a constrained observation:

  • 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.
  • TOO Followups - for repeated observations of a TOO target at specified intervals.
  • Group Observation - a target which needs to be observed within a particular time range with other targets in the program.
  • 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 180 ks).
  • Roll Constraints - specifying a particular roll angle and tolerance.
  • Any other constraints not listed above that are specified in the "Remarks" box in the "Constraints" section of RPS.

Preferences vs. Constraints

It is possible for a proposer to designate a constraint as a "preference". In this scenario, the major goals of the project can be achieved without a hard constraint but superior results might be expected if the constraint is met.

A "preference" is met on a best-effort basis, unlike "constraints", which have to be fixed into the appropriate slot in the Long Term Schedule to meet the constraints.

I want 4 observations of a target, but don't care about the spacing -- days or even months apart is fine. Does this count as time constrained?


If you have any scientific requirements for the spacing of observations that are not compelled by target visibility alone, you must mark the observation as constrained. The most efficient way to schedule multiple observations of the same target may be to do them back-to-back. Any requirement that they NOT be scheduled this way would indeed qualify as a constraint.

The best way to specify such a constraint on the RPS form is a monitor with large tolerances.

How are time constrained observations counted?

Time constrained observations are designated as such in the CXC observation database if any of the RPS fields described above (window, phase, roll etc) are filled out, or if constraints are specified in Remarks. Each time constrained observation in a proposal will be counted as "Easy", "Average" or "Difficult", with annual quotas for each. See Chapter 6 in the Call for Proposals for details on how constraints are classified and the Examples in Classifying Constrained Observations thread for real-life examples. Note that the count is done on an observation, not target, basis. Thus a series of 5 monitoring observations of a single source may count as a single target in RPS (only one target form need be filled out) but will count as 5 time constrained observations.

The number and classification of constraints can be calculated using the "Constraints/Slewtax" tool, accessible by button from the top of the RPS page.

[An image of the top of the RPS window with the Constraints/Slewtax button highlighted.]
[Print media version: An image of the top of the RPS window with the Constraints/Slewtax button highlighted.]

Figure 1: Top of RPS Showing Constraints/Slewtax Button

An image of the top of the RPS window with the Constraints/Slewtax button highlighted.

Note that additional constraints specified in the Remarks section cannot be evaluated by this tool, so any classifications should be taken as estimates. The calculator will also assess the additional slew time required for each pointing, which contributes to the total time for the proposal.

What is the difference between a Joint Proposal and Coordinated Observations?

A proposal is classified as "joint" if time is requested on one or more observatories (e.g. HST or the VLA) in addition to Chandra. Please see Chapter 5 in the Call for Proposals for more details.

An observation is "coordinated" if the Chandra observations are to be scheduled in conjunction with another space observatory or NRAO. For example, if the observations are required to be simultaneous with, or offset from, those of another observatory.

Unless indicated as coordinated in the proposal, the awarding of joint Chandra time with another observatory does not mean that the observations will be coordinated.

Please note that with the exception of NRAO, coordination with ground-based observatories is only available as a preference.

If I apply for joint time, do I also have to check the coordinated box on the RPS forms?

Yes, if the Chandra observation is to be coordinated with another observatory

Many joint projects do not require the observations to occur within the same timeframe. The Chandra mission planners will have no way of knowing that an observation is coordinated if it is not designated as such.

I have observing time on another facility, and they are willing to slave to Chandra's schedule. I will tell them when my Chandra observation is scheduled. Do I need to check the coordinated box?


If the "coordinated" flag is not set, the observation is unconstrained. Observations that are completely unconstrained are often designated as "pool" targets. Pool targets may be scheduled at any time during the cycle to fill gaps between constrained observations. Therefore they are often scheduled with only a few days notice, making it almost impossible to give the other observatory time to change their schedule.

Last modified: 19 December 2016
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