Tracking a source

Everything in Definitions works in the ground frame: an array pointed at alt 70°, az 180° stays pointed there whatever the hour. Real sources rise and set, so Observation ties the ground frame to a site and a time and converts a sky position into the alt/az pair divergent_pointing() takes. A configuration chosen once therefore does not stay the configuration in force all night:

CTAO-North tracking the Crab Nebula at a fixed div = 0.04. Over eleven hours the stereoscopic footprint moves between 100 and 132 deg², and the mean multiplicity crosses the stereoscopic floor of two twice, once on either side of transit — because the same altitude is reached rising and setting. A configuration picked at the start of the night is not the configuration a fixed div gives at the end of it; matching one to the other means re-pointing during the night, not choosing div once.

Try it yourself

The plot above is one fixed div, read off at a handful of hours. The notebook behind it lets you change any of that: pick a different source, site or night, drag div, and step through the night hour by hour to see the sky map update. It opens on the same array and source as the plot above — CTAO-North, the Crab Nebula, div = 0.04 — on a night chosen so the source stays up for the whole window, so the first thing it shows is that same trade-off, before you touch anything.

Open the demo in its own tab for the full-screen version.

This runs on a Python interpreter compiled to WebAssembly: nothing is installed, no code leaves your machine. It is built from examples/marimo/observing_a_source.py; run or edit it locally with marimo edit examples/marimo/observing_a_source.py.