Slew Times and Overheads
The Astronomer's Proposal Tool (APT) charges time to JWST observations for telescope slews of various kinds as a function of slew distance. Some slew times are calculated deterministically and others are charged statistically. Keeping dithers and offsets within the visit splitting distance for your target can reduce these overheads.
Slew and overhead times in this page are based on model expectations and are subject to change. The values listed here are as used in APT 2020.1 (pre-release).
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 Figure 1) based on current assumptions for JWST attitude control subsystem operations. Slews include:
- Initial slew to the target
- Examples of subsequent slews:
- dithers/nods (for improving data quality or obtaining background)
- mosaics (for 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 the average expected slew distance), and subsequent slews are charged deterministically (given each known slew distance). The charged times are shown in APT for the entire proposal (on the Proposal Information* page) and at the individual observation and visit levels. The times report exported by APT (via File → Export) also provides an ASCII listing of charged times that may be useful. Small slews are referred to as small angle maneuvers (SAMs).
Note that telescope rolls are charged identically as slews. So, for example, a 10° roll is charged the time of a 10° slew; even though the boresight (optical V1 axis) does not move, the observatory is being rotated by 10°. If 2 observations are offset by a range of position angles (e.g., 10°–14°), then the midpoint (12°) will be used to calculate the time charged for the roll.
Each slew (or roll) requires time for 3 operations:
- Slew to target
- Wait for observatory to settle
- Acquire (or reacquire) guide star
* Bold italics style indicates words that are also parameters or buttons in software tools (like the APT and ETC). Similarly, a bold style represents menu items and panels.
See also: APT Smart Accounting
The distance slewed from a previous target to start your observation of each of your targets cannot be known prior to actual scheduling, so a fixed average time is charged to all users, based on statistical expectations. Practice scheduling exercises indicate a value of 1,800 s (corresponding to a slew distance of 53°) will be typical, and so this initial slew charge is applied to each new observation (or first visit of a multiple visit observation). In APT, this initial slew time is accounted as Slew in each Visit Duration. The article on Smart Accounting has further details.
Guide star acquisitions are charged separately for the initial visit and all subsequent visits: 284 s (4.7 m). Guide star reacquisitions within a visit are charged as described 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 actual distance moved, as plotted in Figure 1. These are included among the Instrument Overheads in each Visit Duration. 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.
The times below are charged in APT using models that may differ from actual slew and overhead times.
Table 1. Slew time vs. distance
Slew time (s)
|>0.06" to 15"||20.48|
|>25" to 3°||109.312–825.6|
|>3° to 180°||521.216–3840.512|
Slew time expectations do not have sub-second precision, but values with such precision will be charged by APT.
Table 2. Wait (settling) + guide star acquisition time vs. distance
|Slew distance||Wait |
|Guide star |
|FGS operation required |
(plus subsequent steps; see below)
|0"–0.06"||5 s||5 s||#4. Fine Guide|
|>0.06" to 25"||10 s||37 s||#3. Track|
|>25" to Vist Splitting Distance||10 s||82.5 s||#2. Acquisition|
|Vist Splitting Distance –||30 s||284 s||#1. Identification|
Visit splitting overhead
See also: APT Visit Splitting
Some slews are large enough (individually or in combination within an observation) to require acquisition of a new guide star, which requires a new visit and incurs a larger overhead. The visit splitting distance used by APT is between 30"–80" depending on the Galactic latitude of the target. (Larger areas are serviceable by a single guide star at lower Galactic latitudes where more stars are available.) Any observation with a pair of pointings separated by a greater distance will require visit splitting and incur a new guide star acquisition overhead.
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.
Effect of Multiple slews
In some cases, a sequence of slews may incur overheads larger than expected for individual slews if the sum of the motions causes the overall motion to cross a threshold. 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|
As of APT version 27.1 and later, APT contains a graphical timeline that provides a visualization of the overheads for a selected visit or observation. The timeline does not break out the overheads in as much detail as given above, but it can still provide insight for understanding the sequence of activities and overheads that are being charged to your observations.