South Pole LAT scan strategy

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January 11, 2021 - Reijo Keskitalo, Julian Borrill and Sara Simon


One of the goals of LAT observations from the South Pole is to delens the SAT BB spectrum. To that end, it is important to focus the LAT observing time to coincide with the SAT observations. This is not trivial, due to the two telescopes having very different focalplane footprints. The SAT FOV is about 28 degrees and the LAT FOV is about 8 degrees. For the two telescopes to observe the same fields, LAT boresight must target a much larger field. Furthermore, to achieve the same shape of the observed fields, LAT boresight cannot be confined in a rectangular area like the SAT boresight.

Proposed solution

We propose splitting the LAT observations into three concentric fields to approximate the SAT hit distribution. In the following worked example, SAT boresight observed a rectangular field defined by RA = [15, 65] degrees, Dec = [-55, -52.5] deg. Each constant elevation scan was azimuth-locked for 84 minutes at a time, extending the observed field due to Earth's rotation by 21 degrees in RA. To approximate the SAT hit pattern, LAT observed three fields:

  • RA = [-21, 100], Dec = [-60, -47.5]
  • RA = [-16, 93], Dec = [-62.5, -45]
  • RA = [-9, 85], Dec = [-66, -42]

Each LAT scan was azimuth-locked for 60 minutes at a time (15 degrees RA drift). Elevation step was 1.0 degrees, up to 22 elevation steps to cover a field. Shorter scans would allow for smaller elevation steps but would compromise ground pickup removal.

Here we show the resulting hit maps and their differences. Since we anticipate little statistical weight to be associated with the poorly sampled boundary of the SAT map, we define a 90% SAT mask as the deepest-integrated 90% of the SAT-observed pixels.

LAT vs SAT hits.png

Statistics indicate that we achieved nearly complete LAT observations of the SAT field without much loss of LAT observing efficiency:

Comparing hit maps
Map fraction
Raw SAT fsky 5.5%
Raw LAT fsky 5.4%
SAT pixels not seen by LAT 3.0%
best 90% of SAT fsky 4.9%
best 90% of SAT not seen by LAT 0.7%