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2. INSTRUMENT DESCRIPTION 
The main task of the GAIA astrometric instrument is the 
measurement of angles between stars belonging to two 
widely separated sky regions. Basically, the instrument is 
composed by an optical part and a detection system 
(CCD sensor mosaic) placed on the focal plane. 
The feasibility study has suggested two different 
configurations: the interferometric (with two apertures) 
and the monolithic option (single aperture). In the 
following we will concentrate on the former option. 
Figure 1: Optical interferometer configuration and star image on the focal plane 
2.1 Interferometer Optical configuration 
The Optical Instrument is constituted by two elements 
(Cesare, 1998): 
■ Korsch-type interferometer which produces star 
images on the focal plane with an intrinsic high 
spatial resolution in the along-scan direction. 
■ A beam combiner (BC) which endows the optical 
interferometer with two lines of sight. 
The optical interferometer (figure 1) consists of two 
elliptic primary mirrors (M1+, M1.) with 0.65 m aperture 
and separated by a 2.45 m baseline, an hyperbolic 
secondary mirror M2, another elliptic mirror (M4), and 
two flat mirrors (M3, M5). The mirrors M2 to M5 are 
monolithic and lie on the optical axis of the 
interferometer. 
The geometric features and the parameters of the 
telescope optical configuration are summarized in table 
2. 
Table 2: geometric features of GAIA telescope 
Parameter 
Value 
Baseline 
2.45 m 
Aperture Diameter (= entrance pupil 
diameter) 
0.65 m 
Effective Focal Length 
40 m 
Overall field of view 
1.4°x1.4° 
One of the goals of GAIA is to determine “absolute 
parallaxes” of the stars, i.e. parallaxes which are 
independent of distance and motion of background 
stars. To achieve this goal, GAIA must be able to 
perform measurements of the separation angle 
between stars belonging to widely separated sky 
regions (i.e. of stars with very different parallax factors). 
In the present design this capability is achieved by 
endowing the telescope with two different lines of sight 
(LOS1 and LOS2) separated by a wide angle (the basic 
angle, BA) by means of an optical system called “beam 
combiner”. 
The beam combiner (figure 3) consists of four flat 
mirrors placed in front of the primary mirrors. They 
intercept the light coming from two directions separated 
by a basic angle of 54° (which is defined by the angle 
between the normals to the four mirror surfaces) and 
reflect it towards the interferometer apertures. 
This solution is similar to that adopted in the 
HIPPARCOS mission and the main benefits are: 
■ The BC constitutes a physical realisation of the 
basic angle, i.e. one of the fundamental quantities 
which affect the accuracy of the star position, 
parallax and proper motion determination. At the 
same time, the BC allows to identify a conceptually 
simple way of monitoring and controlling its 
stability, i.e. through the control of the relative 
orientation of the BC mirrors.