"Star coverage": a simple tool to schedule an observation when FOV rotation matters

TL;DR

This paper introduces a simple tool to simulate and schedule observations by calculating the star coverage affected by FoV rotation, aiding in calibration and optomechanical assessments of telescopes.

Contribution

The paper presents a novel software tool that simulates FoV rotation and star coverage, specifically designed for scheduling calibration observations of alt-azimuth telescopes.

Findings

The tool accurately predicts the angular extension of FoV rotation.

It helps optimize calibration observation scheduling.

Potential to produce sky images using the Variance method.

Abstract

During a tracking mode observation, every telescope with an alt-azimuthal mount shows a rotation in the field of view (FoV) due to the diurnal motion of the Earth. The angular extension of the rotation depends mainly on the time length of the observation, but also on the telescope's latitude and pointing, because it is determined by the evolution of the parallactic angle of the target, which is a function of those two parameters. In many cases, the rotation of the FoV can be exploited to assess some optomechanical properties of the telescope, e.g. the alignment of the optical elements or the motors' precision during the tracking. As a consequence, it could happen that a proper simulation of the FoV rotation is crucial to program an observation aiming at calibrating the whole system. We present a tool to simulate the apparent rotation of the FoV, calculating the actual "star coverage" exploitable for scientific goals. Given the FoV and the pointing direction, the software calculates the angular extension of the rotation, considering only the stars observable by the telescope below the magnitude limit. This tool will be adopted to schedule the pointing calibration runs of the innovative ASTRI-Horn Cherenkov telescope, developed by INAF for gamma-ray ground-based astronomy, but with the potentiality to produce sky images as an ancillary output, using the so-called Variance method. By exploiting the FoV rotation with the Variance method, the critical assessment of the camera axis can be successfully performed.