Moving Target Overheads
Observation overheads for moving targets are the same or marginally longer than for fixed targets in most cases. The primary exception being additional time needed to acquire the guide star and set up tracking.
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 Stars, Moving Target Acquisition and Tracking
The initial visit overhead for acquiring the guide star for a moving target observation is about 3× longer than for a fixed target, based on average performance in the first several years of operations. A significant part of that is due to the fact that the observatory may slew to the initial attitude for the moving target observation, but then have to wait until the earliest start time of the track for the selected guide star (i.e., it is an artifact of the scheduling system). Once an observation has begun, overheads for dithers are about 48 s longer than for a fixed target. Because dithers are normally an integral part of observations, these overheads can add up. For observations with 4 dither positions exposures of about 500 s per optical configuration and 2 optical configurations, moving targets are charged about 30% more (Total Duration in APT) than fixed targets.
Time critical observations
Words in bold are GUI menus/
panels or data software packages;
bold italics are buttons in GUI
tools or package parameters.
- 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
- 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 observation. By comparison, 10 1-hour observations each constrained to start within a 30-minute window would be charged considerably more.
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.
Making use of smart accounting
See also: APT Smart Accounting
Smart Accounting is available for 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, and reduces the total amount of charged time for slewing. 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 slew time because the observatory is already pointed at the target. 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. Note that while a program will benefit from the savings of Total Charged time when multiple observations of a given target are included, that does not mean the observations will be scheduled consecutively. To specify the order and possible timing of the individual observations, Special Requirements or Solar System windows can be specifed. If such requirements prevent observations from being taken consecutively, they will not benefit from the Smart Accounting time savings described above.
To run smart accounting in APT, follow the instructions on the APT Smart Accounting page.