Slew Tax and Constrained Observations for Grids

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

• 1. The region can be irregular
• 2. 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 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:

• 1. Maneuver from one observation to the next must be less than or equal to one degree
• 2. There can be no change in instrument configuration.
• 3. Individual exposure times must be less than 40 ks.

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 observations). 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.

Slew Tax and Constrained Observations for Grids

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

Example

Consider a survey with 20 pointings each of 4 ks. The first 17 pointings can be assembled into a group with a total time (exposure plus slew) of 77 ks. The slew tax is 1.5 ks for the first observation plus 16x0.5 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        5.5        1.5	       4.0 
  1     2       10.0        2.0        8.0 
  1     3       14.5        2.5       12.0 
  1     4       19.0        3.0       16.0 
  1     5       23.5        3.5       20.0 
  1     6       28.0        4.0       24.0 
  1     7       32.5        4.5       28.0 
  1     8       37.0        5.0       32.0 
  1     9       41.5        5.5       36.0 
  1     10      46.0        6.0       40.0 
  1     11      50.5        6.5       44.0 
  1     12      55.0        7.0       48.0 
  1     13      59.5        7.5       52.0 
  1	14	64.0	    8.0	      56.0 
  1	15	68.5	    8.5	      60.0 
  1	16	73.0	    9.0	      64.0 
  1	17	77.5	    9.5	      68.0 
  2	18	5.5	    1.5	      72.0 
  2	19	10.0	    2.0	      76.0 
  2	20	14.5	    2.5	      80.0 
 
 Number of pointings 20 
 Expo per pointing 4    
 Number of groups 2    
 Total exposure 80  
 Total slew tax 12  
 Total Constrained Count 2;  
 (assumes all targets constrained)
 Total exposure + tax 92  

Look-Up Table for Slew Tax and Constrained Observations for Grids

Below is a look-up table giving slew tax and number of constrained observations for different values of the number of pointings and the exposure time per pointing. The number in parentheses is the number of constrained observations, assuming all observations in the grid are constrained. Proposers can use the look-up table, calculate the tax using the rules described above or submit a query to the CXC Helpdesk.