Stellar Calibration of the Smart Panoramic Optical Sensor Head (SPOSH)

Abstract

The SPOSH camera is designed for observations of transient phenomena on the night hemisphere of Earth (or other planets) from an orbiting spacecraft. It is currently being tested during observation campaigns for meteors from ground. The camera features a highly sensitive (1024 x 1024) CCD chip, a wide (>120°) field of view, is typically operated at a high rate (2 frames / sec) and has sophisticated built-in software for event detections and reporting. To calculate celestial positions and trajectories for recorded meteors it is necessary to determine the parameters for the inner and exterior orientation of the camera. Our camera model includes three orientation angles w, f, k, the focal length, the coordinates of the principal point and a second order polynomial describing the radial distortion. The model parameters are determined by stellar calibration using images of star fields recorded during recent meteor campaigns. Star positions in the images are determined from the maxima of Gauss functions fitted to the observed pixel patterns. The use of stars with their precisely known positions taken from a star catalog warrants high calibration accuracy. In recent tests we identified approximately 5.000 stars in a single image, more than 1700 of which could be related to catalog entries. After the analysis, the residual error of star positions is on average 0.25 pixels. The calibration software is developed under IDL (Interactive Data Language) and is intended to be user-friendly for less experienced users