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arXiv · 2609.15724

Automatic astrometric calibration of low-cost all-sky cameras

J. R. González Fernández·Laura Hermosa Muñoz·María Fernández Alonso·Lucía González Cuesta
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Abstract

All-sky cameras support weather monitoring, amateur astronomy, fireball and space-debris detection, and measurements of light pollution. These applications require reliable astrometric calibration despite imperfect camera alignment and variable observing conditions. We present an open-source procedure that combines a compact fisheye model, an exact rotation to account for camera tilt, and automatic star matching. It can initialise a calibration from a single frame given the observing site coordinates and observation time. We evaluate the procedure on nine cloud-free, moonless frames, obtained with a Raspberry Pi camera and a 1.55 mm lens. Fitting on eight frames and evaluating on the ninth in turn gives a median residual of 0.70 px (95\% interval 0.64-0.80 px) across 4172 matches, reduced to 0.60 px by two optional decentering parameters. All nine single-frame calibrations pass the quality criteria and predict positions on other frames with a median residual of 0.78 px, taking about six seconds per calibration. We also apply the clear-sky calibration to 73 frames with varying lunar illumination or cloud cover, obtaining comparable residuals for accepted matches on moonlit frames (0.62 px). These results support reusing a calibration while the camera geometry remains unchanged and enough stars remain visible for reliable matching. The procedure is available as the Python package ascal.

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J. R. González Fernández, Laura Hermosa Muñoz, María Fernández Alonso, Lucía González Cuesta. 2026-09-14. Automatic astrometric calibration of low-cost all-sky cameras. https://arxiv.org/abs/2609.15724

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