Examples in Classifying Constrained Observations
Proposer Threads (Cycle 15)
- Classifying and Counting Constrained Observations
- Example 1. PHASE and UNINTERRUPTED
- Example 2. MONITOR
- Example 3. WINDOW
- Example 4. ROLL, UNINTERRUPTED and PHASE
- Example 5. TOO, MONITOR, COORDINATED
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 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 5 of the Call for Proposals. In particular, Table 5.1 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 5.1 on hand when reading through these examples.
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.
According to Table 5.1 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".
The total length of the observation is 40ks, which puts it in the "Average" category for uninterrupted observations.
The most difficult classification for this observation is the Average uninterrupted length. It counts as ONE Average observation against the cycle quota.
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.
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)|
The formula to calculate the constraint classification is given in Table 5.1 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.
This example taken 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:
|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.
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
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 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.
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 11/6/2014 - 12/13/2014
- Roll angles 90-180 between 6/14/2014 - 9/14/2014
The duration of the corresponding windows are 37 and 92 days, for a total availability of 129 days. The roll constraint is classified as ONE Easy. Please note that the pitch angle of this source is always "bad" and hence the maximum exposure time is limited. This is discussed further below, in the section "The Uninterrupted Constraint".
The period of the super-orbital phase of LMC X-1 is 30 days. The phase constraint is therefore ONE Average.
The total exposure time is 150ks. 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.
Please note that the pitch angle of LMC X-1 is such that it will definitely be necessary to split the exposures into even smaller chunks. Inspection of the PRoVis plot for this source shows that the pitch angle is always in the restricted EPHIN range. The MaxExpo web page shows that for pitch angles around ~100 degrees (obtained from PRoVis) during the dates that meet the roll constraint between 90-180 degrees, the maximum exposure will be limited to ~60ks if the planning limit for EPHIN is 144F and ~100ks if the EPHIN planning limit is 147F. Observations made at the 0-40 degree roll contraint will have a much larger maximum exposure. For this reason, observations at the larger roll constraint will need to be broken up into multiple smaller observations. However, for the purposes of counting constrained observations at the Peer Review, we do not explicitly take pitch into account and so the final count stays at TWO Difficult constraints.
The final category is that of the most restrictive (difficult) constraint. This observation counts as TWO Difficult constraints.
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 would be simultaneous with RXTE.
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 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|
The formula to calculate the constraint classification is given in Table 5.1 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.
All of the observations should be simultaneous (exact overlap if possible) with RXTE. Thus the coordination window is less than 3 days. This requirement falls in the Difficult category.
This program consists of ONE Very Fast TOO and 3 Difficult constrained observations.