BRITE Constellation: data processing and photometry^⋆

¹ Instytut Automatyki, Wydział Automatyki Elektroniki i Informatyki, Politechnika Śląska, Akademicka 16, 44-100 Gliwice, Poland
e-mail: apopowicz@polsl.pl
² Instytut Astronomiczny, Uniwersytet Wrocławski, Kopernika 11, 51-622 Wrocław, Poland
³ Instytut Elektroniki, Wydział Automatyki Elektroniki i Informatyki, Politechnika Śląska, Akademicka 16, 44-100 Gliwice, Poland
⁴ Institute of Communication Networks and Satellite Communications, Graz University of Technology, Inffeldgasse 12, 8010 Graz, Austria
⁵ Institut für Astrophysik, Universität Wien, Türkenschanzstrasse 17, 1180 Wien, Austria
⁶ Département de physique, Université de Montréal, C.P. 6128, Succursale Centre-Ville, Montréal, Québec, H3C 3J7, Canada
⁷ Centre de Recherche en Astrophysique du Québec (CRAQ), Canada
⁸ Centrum Astronomiczne im. M. Kopernika, Polska Akademia Nauk, Bartycka 18, 00-716 Warszawa, Poland
⁹ European Organisation for Astronomical Research in the Southern Hemisphere (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
¹⁰ Department of Physics, Royal Military College of Canada, PO Box 17000, Station Forces, Kingston, Ontario, K7K 7B4, Canada
¹¹ LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 6, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
¹² Department of Astronomy & Astrophysics, University of Toronto, 50 St. George Street, Toronto, Ontario, M5S 3H4, Canada
¹³ Centrum Badań Kosmicznych, Polska Akademia Nauk, Bartycka 18A, 00-716 Warszawa, Poland
¹⁴ Institut für Astro- und Teilchenphysik, Universität Innsbruck, Technikerstrasse 25/8, 6020 Innsbruck, Austria

Received: 17 March 2017
Accepted: 22 May 2017

Abstract

Context. The BRIght Target Explorer (BRITE) mission is a pioneering space project aimed at the long-term photometric monitoring of the brightest stars in the sky by means of a constellation of nanosatellites. Its main advantage is high photometric accuracy and time coverage which are inaccessible from the ground. Its main drawback is the lack of cooling of the CCD detectors and the absence of good shielding that would protect them from energetic particles.

Aims. The main aim of this paper is the presentation of procedures used to obtain high-precision photometry from a series of images acquired by the BRITE satellites in two modes of observing, stare and chopping. The other aim is a comparison of the photometry obtained with two different pipelines and a comparison of the real scatter with expectations.

Methods. We developed two pipelines corresponding to the two modes of observing. They are based on aperture photometry with a constant aperture, circular for stare mode of observing and thresholded for chopping mode. Impulsive noise is a serious problem for observations made in the stare mode of observing and therefore in the pipeline developed for observations made in this mode, hot pixels are replaced using the information from shifted images in a series obtained during a single orbit of a satellite. In the other pipeline, the hot pixel replacement is not required because the photometry is made in difference images.

Results. The assessment of the performance of both pipelines is presented. It is based on two comparisons, which use data from six runs of the UniBRITE satellite: (i) comparison of photometry obtained by both pipelines on the same data, which were partly affected by charge transfer inefficiency (CTI), (ii) comparison of real scatter with theoretical expectations. It is shown that for CTI-affected observations, the chopping pipeline provides much better photometry than the other pipeline. For other observations, the results are comparable only for data obtained shortly after switching to chopping mode. Starting from about 2.5 years in orbit, the chopping mode of observing provides significantly better photometry for UniBRITE data than the stare mode.

Conclusions. This paper shows that high-precision space photometry with low-cost nanosatellites is achievable. The proposed methods, used to obtain photometry from images affected by high impulsive noise, can be applied to data from other space missions or even to data acquired from ground-based observations.