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Last modified: 19 December 2016

URL: http://cxc-dmz-prev.cfa.harvard.edu/proposer/threads/constrained_count/

Examples in Classifying Constrained Observations

Proposer Threads (Cycle 19)


Contents


Classifying and Counting Constrained Observations

If you have already completed the proposal forms, the number of constrained observations can be estimated simply by using the "Constraints/Slewtax" button in RPS. However, additional constraints specified in the "Remarks" section cannot be evaluated by that tool, so any classifications should be taken as estimates.

[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.

An observation is time constrained if there are any user-imposed limits on when it can be scheduled. Constrained observations are discussed in more detail in the FAQ for Constrained and Coordinated Proposals .

Having too many constrained observations impacts Chandra's observing efficiency. Therefore, the number of constrained observations that can be approved at the Peer Review is limited to 15% of the total number of observations. In addition, constrained observations will be classified as "Easy", "Average" or "Difficult" and counted against a classification quota.

Final constraint difficulty classifications for each proposal will be determined by the CXC after the proposal deadline, taking into account all declared constraints, including those that must be specified in the remarks. The maximum quota for each classification is then provided to the Chandra peer review, where each is enforced. The intent is to limit the most restrictive/difficult observations, while allowing more "easy" constraints to be approved.

Details of the grading scheme are given in Chapter 4 of the Call for Proposals. In particular, Table 5 lists the "grading parameter" for all allowed constraints. These are:

  • Window - the length of the time window in days
  • Phase - the period in days
  • Uninterrupted - the length of the exposure in ks
  • Observations coordinated with another observatory - the length of the window (in days) within which both observatories must observe the source
  • Roll Angle The nominal roll angle of Chandra changes with time. The grading parameter is the length of the time window (in days) for which the roll constraint is satisfied.
  • Monitor - dimensionless parameter depending on the exposure time, spacing and tolerance of the monitor series.
  • Group - dimensionless parameter depending on the time interval for the group and total number of observations.

In this thread, we apply the classification scheme outlined in the Call for Proposals to approved proposals from earlier cycles to determine the number/type of constrained observations for each program. Proposers are advised to have CfP Table 5 on hand when reading through these examples.


Example 1. PHASE and UNINTERRUPTED

This example is taken from proposal 08400908, Measuring the Distance and Dust Distribution to Cen X-3 with X-Ray Halo Variability

The science goal of this proposal is to provide information on interstellar grain properties along the line of sight to Cen X-3. The program consists of a single observation starting 5 ks before eclipse egress and ending 35 ks after eclipse egress. This observation has two constraints: it is phase constrained and it must not be interrupted. We must evaluate both constraints separately. The final classification will be that of the most difficult constraint.

How hard is the Phase Constraint?

According to Table 5 in the Call for Proposals, a phase constrained observation of a source whose period is less than 20 days falls in the "easy" category. The period of Cen X-3 is 2.087065 days, and hence the phase constraint is "easy".


How hard is the Uninterrupted Constraint?

The total length of the observation is 40ks, which puts it in the "Average" category for uninterrupted observations.


Final Classification

The most difficult classification for this observation is the Average uninterrupted length. It counts as ONE Average observation against the cycle quota.


Example 2. MONITOR

This example involves observations of a source that is slowly fading. The first observation is 10ks, the second 50ks 1-2 months after the first, and the final observation is 100ks 3-6 months after the second observation.

RPS parameters for the monitor

Required RPS parameters for a monitor observation are exposure time and minimum and maximum time intervals. The monitor table should be as follows:

Monitor Exposure Time Minimum Time Interval (days) Maximum Time Interval (days)
1 10
2 50 30 60
3 100 90 180

Evaluate the Constraint

The formula to calculate the constraint classification is given in Table 5 in the Call for Proposals.

  • Identify max(T), the largest exposure time of any observation. In this case it is 100ks, or 1.15 days.
  • For each exposure, Imin and Imax are the minimum and maximum intervals (e.g. for the 3rd observation Imin=90 and Imax=180 days).
  • Identify min(Imax). This is the smallest Imax value for all observations. In this case it is 60 days, corresponding to the interval between the first and second observations.

Compute the fractional tolerance for the interval with smallest Imax


	                 Imax-Imin   60-30
	       fractol = --------- = -----  = 0.333
		         Imax+Imin   30+60

Finally, compute the monitor parameter:

                            fractol    60 x 0.333
                min(Imax) x ------- =  ---------- = 17.3
                             max(T)       1.15

The monitor parameter is 17.3, so that each of the three observations falls in the Easy category. This program would count as THREE Easy observations against the Cycle quota.


Example 3. WINDOW

This example is taken from proposal 08100480, X-Ray Observations of Jupiter in Support of the New Horizons Flyby

The goal of this proposal is to obtain X-ray observations of Jupiter during the New Horizons Flyby. There are three components to this investigation: a time-variability study during approach, a multi-spectral morphology study near closest approach and a magnetotail dynamics study as NH heads to Pluto. Each of these components requires two 18ks (5 hour) observations (a total of 6x18 ks). The time windows for these observations are:

Observation Start Stop Window Duration
1 Feb 24 2007 9:30PM Feb 25 2007 2:30AM 5 hours
2 Mar 3 2007 5:00AM Mar 3 2007 1:00PM 8 hours
3 Mar 7 2007 5:30AM Mar 9 2007 2:30AM 45 hours
4 Mar 7 2007 5:30AM Mar 9 2007 2:30AM 45 hours
5 Feb 8 2007 8:30AM Feb 11 2007 1:00AM 64.5 hours
6 Feb 8 2007 8:30AM Feb 11 2007 1:00AM 64.5 hours

Each of these 6 observations must be done in a time window which is less than 3 days, which falls into the Difficult category. This program would therefore count as SIX Difficult observations against the Cycle quota.


Example 4. ROLL, UNINTERRUPTED and PHASE

This example is taken from proposal 08400850, A Chandra HETGS Study of LMC X-4 : Binary Disk and Wind Properties and Studies of Grain Distribution at Small Angles

Note that this example uses PRoVis plots and data from 2017 (Cycle 18). If you are proposing for Cycle 19, you should enter the appropriate dates into PRoVis.

The goal of this proposal is to obtain a high resolution spectrum of LMC X-4 to study the disk, wind and ISM of this source. It is multiply constrained. There is a roll constraint to minimize flux contamination from LMC X-1, a phase constraint to ensure the source is in a medium to high flux state and an uninterrupted constraint.

The Roll Constraint

The roll constraint is for angles 0-40 degrees or 90-180 degrees. The nominal roll angle of Chandra changes with time. Therefore a roll constraint translates to a window constraint, and the ease with which the observation can be scheduled is dependent on the length of the corresponding time windows.

[Using PRoVis for LMC X-1.]
[Print media version: Using PRoVis for LMC X-1.]

Figure 2: Using PRoVis

The PRoVis Results for LMC X-I.

The time windows can be calculated using the PRoVis tool. Enter the coordinates of LMC X-1 (RA=05:32:49.80, Dec=-66:22:13.80) in the box labeled "Target Coordinates" and hit the "Generate Plot" button. The resulting page is an interactive plot of roll, pitch and visibility values as a function of time. This plot can be used to determine when the roll constraint is satisfied during the nominal Cycle boundaries. Inspection of the plot data reveals that the roll constraints are satisfied as follows:

  • Roll angles 0-40 between 11/5/2017 - 12/15/2017
  • Roll angles 90-180 between 6/15/2017 - 9/16/2017

The duration of the corresponding windows are 41 and 94 days, for a total availability of 135 days. The roll constraint is classified as ONE Easy.


The Phase Constraint

The period of the super-orbital phase of LMC X-1 is 30 days. The phase constraint is therefore ONE Average.


The Uninterrupted Constraint

The total exposure time is 180 ks. The maximum exposure time depends on the pitch angle of a source (see the Proposers Guide for more details). For most locations in the sky, this will be at most 90ks (this estimate is used for Peer Review purposes). Therefore it will be necessary to split this observation into (at least) 2 segments. This counts as TWO Difficult uninterrupted constraints.


Final Count

The final category is that of the most restrictive (difficult) constraint. This observation counts as TWO Difficult constraints.


Example 5. TOO, MONITOR, COORDINATED

This example is taken proposal from 08501014, Tiny Hiccups To Titanic Explosions: Tackling Transients in Anomalous X-ray Pulsars

This is a series of observations to study the spectral and timing properties of an Anomalous X-ray Pulsar following a transient event (e.g. an outburst or flare). It consists of 4 observations: a fast Target of Opportunity Observation (TOO) followed by 3 observations spaced by 1-3 days. Ideally, these observations were to be simultaneous with RXTE.

The TOO Observation

The first observation (the TOO trigger) occurs within 4 days of the PI notifying the CXC that an event has occurred. This observation does not count as a constrained observation. Instead, it counts against the quota of Very Fast TOOs.


The TOO Follow-ups

The three TOO follow-ups are part of a monitor series and count against the quota of constrained observations. Details of how to evaluate a monitor constraint are given in the MONITOR example above. The RPS TOO table is as follows:

Followup Exposure Time Minimum Time Interval (days) Maximum Time Interval (days) Target Number
Initial 20
1 20 1 3
2 20 1 3
3 20 1 3

The formula to calculate the constraint classification is given in Table 5 in the Call for Proposals. For the three followups:

  • Identify max(T), the maximum exposure time. In this case the exposures are of equal duration (20ks=0.23 days)
  • For each exposure, Imin and Imax are the minimum and maximum intervals. In this case, Imin=1 and Imax=3 for all followups.
  • Identify min(Imax). This is the smallest Imax (maximum interval) value for all observations. In this case it is 3 days.

Compute the fractional tolerance for the interval with smallest Imax


	                 Imax-Imin   3-1
	       fractol = --------- = ---  = 0.5
		         Imax+Imin   3+1

Finally, compute the monitor parameter:

                            fractol    3 x 0.5
                min(Imax) x ------- =  ---------- = 6.52
                             max(T)      0.23

The monitor parameter is 6.52, so that each of the three observations falls in the Easy category.


Coordination with RXTE

All of the observations in this Cycle 8 proposal should have been simultaneous (exact overlap if possible) with RXTE. Thus the coordination window is less than 3 days. This requirement falls in the Difficult category.

For complete information about joint programs in the current cycle and information about requesting simultaneous or otherwise time-constrained observations, please refer to Chapter 5 of the Call for Proposals.


Final Count

This program consists of ONE Very Fast TOO and 3 Difficult constrained observations.



Last modified: 19 December 2016
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