2.4 The Payload Module

Although fixed or slowly variable biases are self-calibrated in the data reduction, variations at a frequency higher than or equal to the spin frequency cannot be corrected. It is therefore mandatory that the design ensures a basic angle stability (or at least a knowledge of the basic angle variations) at a level significantly below the nominal accuracy over the spin period of 3 hours. This stringent requirement is met by the present design, (Figure 2.6), which considers a monolithic toroidal structure entirely made of Silicon Carbide (SiC), a low-expansion, high-conductivity and homogeneous material. These qualities minimize the mechanical flexure and thermal expansion experienced by the truss structure, the mirrors and the detectors during the mission.

Figure 2.6: GAIA pay-load module. The two astrometric instruments and the spectrometric instrument are indicated.

The payload module is radiatively and conductively decoupled from the sun-shield by means of a thermal cover screening the whole module. Baffles and further covers are applied on the instruments' apertures, the latter being removed once in orbit. The payload temperature is thus passively stabilized at about 200 K. The payload module is also mechanically decoupled from the service module by releasing in orbit two of the three bipods connecting the two modules. A device was also designed for continuosly monitoring the basic angle variations with an accuracy better than $1~\mu\textrm{m}~\textrm{rms}$ and therefore guaranteeing the payload performance. It basically consists of a laser source illuminating simultaneously the two astrometric telescopes.

The three instruments are mounted on this axisymmetric structure with their lines of sight perpendicular to the satellite symmetry axis. The basic angle between the lines of sight of the two astrometric instruments is of 106 deg, while the line of sight of the spectrometric instrument lies on their axis of symmetry. The three instruments are essentially identical all-reflective telescopes. The mounting plates and mirrors are also made of SiC, and the focal planes are covered with CCDs.

Since the mission design has not been ``frozen'' yet, there still is some discussion about which would be the best choice for some critical instrumental parameters. This is the case e.g. for the photometric system, which will consist of a set of broad bands in the astrometric instruments and a set of medium bands in the spectrometric instrument. The choice of the photometric system to be implemented is of particular importance because the GAIA database will include such a large amount of high-accuracy information that it will be the reference for many decades. Several options are presently being discussed, differing both in the number of bands and in their response curves, and these uncertainties are reflected in the following, where only the broad-bands of the photometric system are of interest. In the description of the focal planes of the astrometric instruments, we will follow [Mérat et al. 1999], who generically consider a non-specified, four-color broad-band photometric system. In the simulation of galaxy observations, instead, we will consider the five-color system originally proposed by [Høg, Knude and Straizys 1999] and later variously modified, which we will hereafter refer to as the $fgriz$ system, owing to the fact that at the time of carrying out the simulations the expected photoelectron count rates were made available in this system. Finally, the broad bands of the Asiago Photometric System, one of the present candidate systems, are used to illustrate how the final GAIA broad-band photometric system could look like.