Fermi Gamma-ray Space Telescope

LAT Pointed Observations

Fermi previously included pointed observations as part of its regular observing program. However, in early 2018 the Fermi spacecraft experienced an anomaly with its -Y Solar Array Drive Assembly (SADA), which left the -Y solar panel unable to rotate and necessitated the creation of a new observing strategy. Please see this page for more information regarding the event and Fermi's current survey observation strategy. Due to this anomaly pointed observations are no longer performed. Below we give information on how pointed observations were previously undertaken, and this should therefore be considered for historical purposes/reference only.

Fermi previously included pointed observations as part of its regular observing program. During the initial, somewhat more than 5 years, of operations Fermi operated primarily in a sky-survey mode with only occasional pointed observations. In December 2013 Fermi transitioned to a new observing strategy where sky survey was interspersed with pointed observations to increase exposure around the Galactic center. For normally scheduled LAT observations, Fermi's attitude is controlled by a weekly timeline that is generated by the Fermi Science Support Center (FSSC) and then included in uploads to the Fermi spacecraft by the Mission Operations Center (MOC).

Pointed observations were performed for several reasons that included:

  • Regular monitoring covering the Galactic center.
  • Accepted proposals from the annual proposal cycle that included requests for pointed observations.
  • Observations in response to accepted Target of Opportunity (ToO) requests. These included both observations that were motivated by already accepted proposals, and completely new observations.
  • Autonomous observations of gamma-ray burst locations that had durations of a few hours.

Pointed observations using the LAT were affected by two factors that are particular to this instrument. These are that (i) the LAT has a very wide field-of-view (FoV) and the Earth is a bright source of gamma rays (ii) the optimum LAT response is obtained by off-setting the pointing direction by a few degrees from the center of the FoV. However, there were no solar constraints on LAT observations, and the LAT could previously be pointed directly at the Sun. Because of the wide FoV, observations of a target were generally not undertaken when the source was too close to the Earth limb. During times when a target was not visible because of Earth angle constraints, during regular observations Fermi undertook other pre-planned observations. Typically these were sky-survey observations, although pointed observations of another target were possible.

Pointed observations performed for ToOs could be undertaken in two different ways. For the fastest changes to Fermi observations a "ToO order" was sent by the FSSC to the MOC who uploaded this to the spacecraft. In this mode Fermi pointed directly at the commanded location with the spacecraft automatically executing "limb-tracing" when the target was too close to the Earth, with observations of the target resuming when the Earth angle constraint was no longer violated. When sufficient time was available, ToO observations were preferably obtained by uploading a modified timeline to the spacecraft. In this case, typically pointed observations of the target would be interspersed with, for example, sky survey observations.

Exceptional Types of Pointed Observations

In exceptional cases the constraints on making observations close to the Earth limb could be overridden for special purposes. For example, the Earth limb has been deliberately observed in order to obtain a high flux of gamma rays for calibrations purposes. At some times in the mission, short weekly observations of the orbit poles were made to include Earth limb data to also aid in instrument calibration. Also, Fermi has at times been pointed directly at the Earth to look for Terrestrial Gamma-ray Flashes.