Fermi Gamma-ray Space Telescope

Short- and Long-Term Changes to Fermi Observations

For the last year Fermi has been performing a modified observing strategy where inertially pointed observations, to increase exposure around the Galactic center, have been interleaved with sky survey observations. Starting on 2014 December 4, Fermi will return to routinely performing only sky survey observations, with alternate orbits having Fermi rocked north and south of the zenith. Short term changes to this mode may still occur in response to targets of opportunity.

In addition to this long term change, there will be a change in the rock angle used for sky survey observations between 2014 December 3 to December 10. During this interval the angle between Fermi's orbital plane and the direction to the sun (beta angle) will be high. Therefore, to keep the boresight of the LAT farther from the Sun, Fermi will use an increased rock angle of 55 degrees instead of the usual 50 degrees.

Note that data analysis which performs cuts based on rock angle (for example to reduce contamination from the Earth limb) may exclude data from this time depending on the cut used.