The Photometric Data
The Hipparcos mission produced photometric data derived from transits over the main grid as well as for transits over the star mapper slits. For the main grid a wide pass band “Hp” was used, effectively determined by the transmission characteristics of the optics (the telescope mirrors and the lenses in the Image Dissector Tube assembly, IDT) and the S20 photocathode responses (Varma and Ghosh 1973). To be optimized for astrometric measurements, it was required to collect as much light as possible in the Hp band. For the star mapper slits the light was split into a “B T ” and a “V T ” pass band by means of a dichroic prism. The subscript T refers to the use of those transits to create the Tycho catalogue, a largely complete astrometric and photometric survey of about 2 million stars down to magnitude 11 (Hoeg et al. 2000b,a), which is based on the complete star mapper photon count records. All three pass bands evolved, and in particular the responses for the main grid photometry (Hp) deteriorated significantly over the mission (see below).
The purpose of the photometry for the main field was to act in the first instance as a detector of duplicity (see Chapter 4) through the relation between the mean signal and the amplitude of the first harmonic in the modulation, referred to as M1 (Eq.2.33 and Section 2.2.4, page 54). The Hp photometry also served very well as a detector of variability (van Leeuwen et al. 1997a,b) and as a global uniform reference system for photometric calibration (ESA 1997). The photometry obtained from the star mapper transits provided colour information which was used in a range of instrument parameter calibrations. Unfortunately, it was limited to stars brighter than 10th magnitude only. For fainter stars, measured or derived ground-based data was used in those calibrations.
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