The Astronomer's Proposal Tool (APT) charges time to JWST observers for telescope slews as a function of slew distance. The Visit Splitting Distance, defined here, contributes significantly to overheads.
Slew and overhead times given on this page are based on model expectations and are subject to change. The values listed here are as given in APT 25.4.
JWST observing programs are charged time and overheads for slews by the Astronomer's Proposal Tool, APT. The slew overheads model in APT includes discrete jumps at certain distances (see details below), based on current assumptions for JWST Attitude Control Subsystem operations. Slews include:
- The initial slew to target
- Subsequent slews, for example, to:
- dither / nod (improving data quality)
- mosaic (observing a wider field)
- move from target acquisition to the science target (if the pointings are offset or an offset target is used)
- move a science target behind a coronagraphic occulting mask or within a spectroscopic slit or shutter
- roll the telescope from one Position Angle to another
Initial and subsequent slews are charged differently as quantified below. The initial slew distance from the target observed previously cannot be known prior to scheduling. Therefore initial slew times are charged statistically (based on expectations), and subsequent slews are charged deterministically (given each known slew distance). The charged times are shown in APT at the proposal level, the visit level, and in the exported times report. Small slews are referred to as small angle maneuvers (SAMs).
Note telescope rolls are charged identically as slews. So, for example, a 10-degree roll is charged the time of a 10-degree slew, even though the boresight (optical V1 axis) does not move. If two observations are offset by a range of Position Angles (e.g., 10 – 14 degrees), then the midpoint (12 degrees) will be used to calculate the time charged for the roll.
Each slew (or roll) requires time for three operations:
- Slew to target
- Wait for observatory to settle
- Acquire (or reacquire) guide star
Initial visit slew
The initial slew to target is charged 1800 seconds (30 minutes). This corresponds to a slew distance of 53 degrees. The distance slewed from the previous target cannot be known prior to scheduling, so an average estimated time is charged to all users, based on statistical expectations. In APT, this initial slew time is charged as "Slew Time". (Some subsequent visit slews are charged Slew Time as well; see Smart Accounting.)
Guide star acquisitions are charged separately for the initial visit and all subsequent visits: 282 seconds (4.7 minutes). Guide star reacquisitions within a visit are charged as described below in Table 2.
After the initial slew to the target, subsequent slews (e.g., for dithers, mosaics, or target acquisitions) are charged times as a function of distance slewed, as plotted in Figure 1. The slew itself is charged as in Table 1. Guide star acquisition and settling times are given in Table 2 and described in more detail below. Note that "slews" < 0.06" do not actually involve slewing the telescope; instead they are executed by moving the Fine Steering Mirror.
Note the times below are charged to observers by APT using models that may differ from actual slew and overhead times.
Table 1. Slew time vs. distance
|Slew distance||Slew time|
|0" – 0.06"||0 s|
|> 0.06" – 15"||20.48 s|
|15" – 25"||20.48 – 26.112 s|
|> 25" – 3°||109.312 – 825.6 s|
|> 3° – 180°||521.216 – 3840.512 s|
Full table: slew_times_APT_25.4.txt
Slew time expectations do not have sub-second precision, but these exact values will be charged by APT. Linear interpolation is used for distances between those listed in the table.
Table 2. Wait (settling) + guide star acquisition time vs. distance
|Slew distance||Wait (settling) time||Guide star acquisition time||FGS operation required (plus subsequent steps)|
|0" – 0.06"||5 s||5 s||#4. Fine Guide|
|> 0.06" – 25"||10 s||17.5 s||#3. Track|
|> 25" – Vist Splitting Distance||10 s||65 s||#2. Acquisition|
|Vist Splitting Distance –||30 s||282 s||#1. Identification|
Table 2 applies only to fixed (stationary) targets. Moving target overheads will be higher; they have yet to be fully modeled and implemented in APT.
Visit splitting distance
Some slews are large enough (individually or combined) to require a new guide star, incurring larger overheads. A new visit is defined for each new guide star acquisition. The Visit Splitting Distance is between 30" – 80" depending on the Galactic latitude of the target. Any pair of observations separated by a greater distance will require visit splitting. A statistical analysis was used to define the Visit Splitting Distance vs. Galactic latitude rules plotted in Figure 2 and used by APT. Larger areas are serviceable by a single guide star at lower Galactic latitudes where more stars are available.
Guide star overheads
Guide star acquisition and settling times are given in Table 2. A new guide star is required for slews greater than the Visit Splitting Distance. In this case, the guide star acquisition time is reported separately by APT. For smaller slews, the guide star acquisition time is included in the time charged for SAMs.
- Fine Guide
Once in Fine Guide mode, very small slews may be performed with the fine steering mirror (see Table 2). Larger slews require returning to earlier steps in the sequence, which takes more time. The largest slews require identification of a new guide star and repeating all the steps.
Moving targets are observed using FGS Track mode. Only the first 3 steps are performed.
In some cases, a sequence of slews may incur larger overheads than expected for individual slews. For example, consider a series of 6 slews, 15" each, all in the same direction along a line 90" long. Though all slews are < 25", a larger overhead will be charged for exceeding the Visit Splitting Distance; a new guide star will be required. For this threshold, APT considers the maximum distance between all pairs of pointings.
However, consider a series of 7 slews, 4" each, along a line 28" long. The total distance is not considered in this case. Each 4" slew is charged the smaller overhead for being < 25". In this case, the threshold depends only on the pointing accuracy of each individual slew.
For smaller pointing shifts < 0.06", multiple shifts must again be considered. In this case, in order to remain in Fine Guiding mode, all pointings must remain within +/- 0.06" of the initial pointing in both axes of FGS ideal coordinates. Otherwise, the guide star will stray too far within the FGS subarray, and the FGS will have to perform Track mode again before resuming Fine Guiding.
These rules are summarized in the table below.
Table 3. Distance considerations for multiple slews
|0.06"||Must stay within +/- 0.06" of initial pointing in FGS ideal coordinate axes|
|25"||Consider only individual slew distances|
|Visit Splitting Distance||Consider maximum distance between all pairs of pointings|
JWST Observing Overheads and Time Accounting Overview
Visit Overheads Timing Model
JWST Attitude Control Subsystem
JWST Pointing Performance
Fine Guidance Sensor, FGS
JWST Guide Stars
JWST Astronomers Proposal Tool, APT