More on high cadence LAT

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December 11, 2020 - Reijo Keskitalo, Julian Borrill and Sara Simon


This post continues the development of the high cadence scanning strategy. Earlier posts on the topic:

Here, we explore the area of the sky that is continuously observed within some time window, driven by the requirements of the transient science goals. In particular we need to be able to follow the evolution of GRB light curves in order to be confident that we can reject false positives, which entails observing the same sky on several consecutive days.


We use the azimuth-modulated high cadence strategy with the following parameters:

  • 140 degree throw
  • 30 degree avoidance radius around the Sun and the Moon
  • 40 degree observing elevation
  • observe with the full CHLAT : FOV = 8.4 degrees


Top of the plot shows daily hit maps for first four weeks of January. We took all 365 hit maps and evaluated the common sky area in various time-windows. The common sky fraction in windows of various lengths is shown in the bottom of the plot.

Fsky vs wlen.png

We find that the common sky area depends strongly on the length of the window and day of the year. The dips in the common sky area occur when the Sun and the Moon obstruct disjoint sets of pixels. Longer windows allow for more lunar motion leading to reduced common sky area.

The following table gives the minimum and median common sky areas, together with the minimum when excluding the worst 10% of the time since we do not need (or expect) 100% observing efficiency for the transient science.

Common sky area
Window [days] Minimum fsky Min. fsky (best 90%) Median fsky
1 0.380 0.423 0.468
2 0.327 0.389 0.439
5 0.244 0.303 0.360
7 0.200 0.254 0.315
10 0.117 0.193 0.257
14 0.003 0.095 0.199