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Last modified: 17 Jan 2017


Slew Tax and Constrained Observations for Grids

Proposer Threads (Cycle 19)


Grid (or Raster) Basics

A grid (or raster) consists of several contiguous, or nearly contiguous, observations. The term "grid" or "raster" is slightly misleading in that:

  • The region can be irregular
  • The pointings will not "line up" in a grid unless a roll or group constraint is applied (see the Grids thread for further discussion)

A "slew tax" of 1.5 ks is added to each observation at the Chandra peer review. This represents the average slew, settle and set-up time required for an observation. Because some or all of the pointings in a grid can often be done consecutively, the average slew time is less than the nominal 1.5 ks and is calculated differently. To qualify for the reduced slew tax, the observations in a grid must satisfy the following conditions:

  • Maneuvers from one observation to the next must be less than or equal to one degree
  • There can be no change in instrument configuration.
  • Individual exposure times must be 44 ks or less (which corresponds to a minimum to 2 grid observations per 90 ksec group - including slew time)

Programs which qualify for reduced slew tax also qualify for a reduced "time constrained count." Time constrained observations impact Chandra's observing efficiency (see the FAQ for Constrained and Coordinated Proposals). Short, closely spaced observations which can be scheduled as a group have less of an impact on Chandra's efficiency than the same observations spread over the sky. Please note that grid programs are unconstrained unless the proposer specifically requests a constraint.

The "Constraints/Slewtax" tool (Figure 1), accessible from the top of the RPS form, can properly estimate the number and category of constraints that a grid observation will entail. Note that additional constraints or difficulties imposed in the Remarks section cannot be evaluated by this method. The tool will also assess the slew time associated with each pointing in the grid, and give an accurate count of the total time required for your observation.

[Click the Constraints/Slewtax Button to calculate the slewtax for your observation]
[Print media version: Click the Constraints/Slewtax Button to calculate the slewtax for your observation]

Figure 1: Top of RPS Showing Constraints/Slewtax Button

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

In this thread, we show how the slew tax and time constrained count for observations in a grid are calculated.

Slew Tax and Constrained Observations for Grids

The rules for calculating the slew tax and constrained observations for a grid are as follows:

  • Pointings will be assembled into one or more groups with a maximum of 90 ks per group, including slew tax.
  • The slew tax for the first observation in a group will be 1.5 ks. Subsequent observations in the same group are charged at a rate of 0.5 ks.
  • Each group of 90 ks will be assesed as ONE time constrained observation.


Consider a survey with 20 pointings each of 5 ks. The first 16 pointings can be assembled into a group with a total time (exposure plus slew) of 89 ks. The slew tax is 1.5 ks for the first observation plus 15x0.5 ks for subsequent observations in that group. The remaining 3 exposures form a group of 14.5 ks (slew plus exposure), with a total slew of 1.5+2x0.5 ks. The total slew tax is therefore 12 ks.

There are TWO groups, so the total constrained count=2.0 Note that this calculation assumes all the pointings are constrained. The default assumption is that grid observations are NOT constrained. The proposer must explicitly impose a constraint in the RPS forms for these observations to count as constrained.

The breakdown is as follows:

Group No Exp Total Time   Group Tax  Total Exp
  1     1        6.5        1.5        5.0
  1     2	12.0	    2.0	      10.0
  1	3	17.5	    2.5	      15.0
  1	4	23.0	    3.0	      20.0
  1	5	28.5	    3.5	      25.0
  1	6	34.0	    4.0	      30.0
  1	7	39.5	    4.5	      35.0
  1	8	45.0	    5.0	      40.0
  1	9	50.5	    5.5	      45.0
  1	10	56.0	    6.0	      50.0
  1	11	61.5	    6.5	      55.0
  1	12	67.0	    7.0	      60.0
  1	13	72.5	    7.5	      65.0
  1	14	78.0	    8.0	      70.0
  1	15	83.5	    8.5	      75.5
  1	16	89.0	    9.0	      80.0
  2	17	 6.5	    1.5	      85.0
  2	18	12.0	    2.0	      90.0
  2	19	17.5	    2.5	      95.0
  2	20	23.0	    3.0	     100.0

 Number of pointings: 20 
 Expo per pointing: 5    
 Number of groups: 2    
 Total exposure: 100  
 Total slew tax: 12  
 Total Constrained Count: 2  
 (assumes all targets constrained)
 Total exposure + tax = 112  
When filling out the RPS form use:
  • Is this observations (part of) a grid? -- Y
  • Unique Grid Name -- Grid1 (use any unique name for your grid)
  • Number of Grid pointings -- 20
  • Total Observing Time (ks) -- 100
With this information entered, clicking on the "Contraints/Slew Tax" button in the active RPS site will return this output:
Chandra Constraints/Slew Tax for

The Constraint/SlewTax button provides an ESTIMATE of the constraint grade
that may be assigned to each target for accounting against maximum quotas
of each grade allowable at peer review (see the section on Constraints in
the CfP).  Constraints in Remarks are not quantified in RPS data, and so
cannot be accounted for by this tool.

|        | Grade *   |       |       |       |        |       |         |       | TOO      |
| Target | Pointings | Unint | Coord | Roll  | Window | Phase | Monitor | Group | Followup |
| 001    |           |       |       |       |        |       |         |       |          |
Total constraints for proposal: 

Total Time for Proposal =  100.00  
Total Slew Time         =   12.00


Target   = Target Number
Grade    = Easy, Average(Avg), Difficult(Diff)
Pointings= Number of pointings charged for grade
Unint    = Uninterrupted
Coord    = Coordinated with other observatories
Roll     = Roll Constraints
Window   = Time Window Constraints 
Phase    = Phase Constraints
Monitor  = Monitoring Observations
Group    = Group Interval
Followup = TOO Followups

Last modified: 17 Jan 2017
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