arXiv subjects

H. U. Keller

Publications and source records attributed to H. U. Keller.

38 records · Page 3Linked to original sources

arXiv2006

Determination of the light curve of the Rosetta target Asteroid (2867) Steins by the OSIRIS cameras onboard Rosetta

Context: In 2004 asteroid (2867) Steins has been selected as a flyby target for the Rosetta mission. Determination of its spin period and the orientation of its rotation axis are essential for optimization of the flyby planning. Aim: Measurement of the rotation period and light curve of asteroid (2867) Steins at a phase angle larger than achievable from ground based observations, providing a high quality data set to contribute to the determination of the orientation of the spin axis and of the pole direction. Methods: On March 11, 2006, asteroid (2867) Steins was observed continuously for 24 hours with the scientific camera system OSIRIS onboard Rosetta. The phase angle was 41.7 degrees, larger than the maximum phase angle of 30 degrees when Steins is observed from Earth. A total of 238 images, covering four rotation periods without interruption, were acquired. Results: The light curve of (2867) Steins is double peaked with an amplitude of $\approx$ 0.23 mag. The rotation period is 6.052 $\pm$ 0.007 hours. The continuous observations over four rotation periods exclude the possibility of period ambiguities. There is no indication of deviation from a principal axis rotation state. Assuming a slope parameter of G = 0.15, the absolute visual magnitude of Steins is 13.05 $\pm$ 0.03.

astro-ph↗

arXiv1998

Isophot Observations of Comet Hale-Bopp: Initial Data Reduction

Comet Hale-Bopp was observed five times with ISOPHOT, the photometer on board the Infrared Space Observatory (ISO). Each time broadband photometry was performed using 4 different detectors, 5 apertures and, 10 filters covering the range between 3.6 to 175 $μ$m. Calibration measurements using the internal Fine Calibration Source were done together with each individual measurement. Background observations were performed with identical instrument settings at the same positions on the sky several days after the comet observations. The observation strategy and the initial data reduction steps are described in some detail and the resulting in-band power values of the Hale-Bopp observations and their uncertainties are derived. Initial reduction of these measurements was performed in 3 steps: (1) processing of raw data by removing instrumental and energetic particle effects, (2) averaging of the individual signals, and (3) determination of the detector responsivities and their uncertainties. The detector signal is determined by two different methods and the best value is chosen. At the present level of processing uncertainties range from 10% to a factor of 3 for the low power levels at short wavelengths. The in-band power levels at different wavelengths varied over 3 orders of magnitude.

astro-ph↗