Moving Target Overheads

Except for an additional overhead for guide star acquisition and tracking set-up, there are no other overheads exclusive to moving targets. Examples of general overheads for specific moving target observations are provided.

On this page

Final overheads on moving target observations are calculated and reported by the Astronomers Proposal Tool (APT). Proposers do not need to compute the overhead times on their own; there are also many more overheads that apply to moving targets, but are not specific to them, that are not discussed in this article.

Guide star acquisition and tracking

See also: JWST Guide StarsMoving Target Acquisition and Tracking

Guide star acquisition and initiation of tracking for moving targets will incur a 90 s overhead. An additional 30 s will be applied for each dither. These overheads are unavoidable for all moving target observations.

Time critical observations

Words in bold are GUI menus/
panels or data software packages; 
bold italics are buttons in GUI
tools or package parameters.

Any observations specified to occur within a short window is considered time critical and will incur an additional overhead. Please see Time Constrained Observations for more details. Examples for solar system targets include, but are not limited to, observing:

  • an object at a specific date and time,
  • an object at a specific rotational phase,
  • an object at a particular phase angle,
  • a satellite at a particular elongation or position angle, and
  • a satellite during a transit or when in shadow.

See Solar System Special Requirements and Timing Special Requirements for complete lists of constraints available for moving target observations in APT.

It is recommended that efforts be taken to avoid making your observations time critical, but in some cases, the scientific program may require it. Careful consideration of science requirements can avoid this overhead.

If complete longitudinal coverage is desired for a quickly rotating body, remaining on the object during an entire rotational period is recommended. For example, full rotational coverage of Jupiter can be obtained in ~10 consecutive hours of observations and avoids the overhead because specific phases are not required.

Observations of faster moving objects (asteroids, NEAs, nearby comets, etc.) at specific phase angles are more likely to incur the overhead than observations of slower moving objects (Centaurs, KBOs, the giant planets, etc.). 

The Phase special requirement and Central Meridian Longitude Solar System window can be used interchangeably for any targets with defined coordinate systems. However, keep in mind that the entire observation must occur within the window specified by a Central Meridian Longitude constraint, whereas the Phase constraint only specifies when the observation should begin. In other words, the schedulability window for the start time of an observation with a Central Meridian Longitude constraint will be reduced by the length of the observation; if the observation is longer than the specified window, it will not be schedulable. The Phase constraint thus increases schedulability, but there is a possibility that the observations will not actually occur between the desired rotational phases.

Target of opportunity (ToO) observations

See also: JWST Target of Opportunity Observations, JWST Target of Opportunity Program Activation

There are 2 different categories for ToO observations: disruptive and non-disruptive. These are discussed in more detail on the Target of Opportunity Observations page.

Making use of smart accounting

See also: APT Smart Accounting

Smart Accounting is available for planning moving target observations. The Smart Accounting tool looks at the full set of proposed observations in a proposal and decides which sub-groupings can logically be scheduled together to reduce the total amount of charged time for slewing; however, this does not guarantee that the observations will actually be scheduled together during execution. Without smart accounting, the slew time for each visit is set at 35 minutes (2100 s) by default. In reality, consecutive visits to the same target do not require 35 minutes of time to slew to the target because the observatory is already pointed at the target. Using Smart Accounting, the slew time overhead charged to the program can be significantly reduced. For example, an observation of Titan with the NIRSpec IFU followed right afterwards by an observation of Titan with the MIRI MRS would be charged 35 minutes for the initial slew to Titan for the NIRSpec observations and a significantly shorter time (<5 minutes) for switching instruments to MIRI, target acquisition, and re-acquiring a guide star.

To run smart accounting in APT, follow the instructions on the APT Smart Accounting page.

Notable updates
    Updated the default slew time.

    Added a general caveat about computing overheads.

    Linked to CfP pages and removed specifics for overheads.

  • Updated wording in the Smart Accounting section.
Originally published