From geometry to efficiency: updated CFHT astrometric camera pointing model and estimated on-sky time savings

Abstract

Efficient target acquisition is important for the Canada-France-Hawaii Telescope (CFHT). The ESPaDOnS and SPIRou instruments use guide fields only about 10 × 10 arcsec across, so a pointing error can require extra telescope moves and use observing time. This paper updates the calibration of CFHT's AstroCam astrometric camera. The new model has three stages. First, a global geometric model represents broad effects such as zero-point offsets, axis alignment, collimation, polar misalignment, and gravity-related flexure. Second, a low-degree spherical-harmonic model corrects smooth patterns across the sky. Third, a two-dimensional polynomial corrects repeatable local patterns in the guider or focal plane. Nighttime science images were matched with telescope telemetry, corrected for timing, stellar motion, observing location, and atmospheric refraction, and filtered to reduce outliers and oversampling. Tests on held-out observations from AstroCam, ESPaDOnS, SPIRou, WIRCam, and MegaPrime reduced the raw twoaxis root-mean-square (RMS) error from 73.5–170.0 arcsec to 13.4–17.2 arcsec after the global stage and to 8.29–11.29 arcsec after all three stages. These held-out rows were not used to fit the model, although they may come from the same observing nights as the training data. An observing scenario based on about 24 acquisition visits per night estimates that saving 30 s per acquisition would recover about 12 min per night, or 33.4hr per year. This is a planning estimate, not a measured operational saving, and should be tested prospectively on the telescope.