5.4 Sensitivity of the Astros

In the estimation of an instrument's sensitivity, the transmittance of the telescope, the Quantum Efficiency of the detector and the spectrum of the observed object must be taken into account. On the basis of the data for the baseline mission design provided by Matra Marconi Space, expected photoelectron count rates for stars of different spectral types in different photometric bands were provided by L. Lindegren in a private communication. To determine the stellar spectrum most closely approximating the spectrum of a typical galaxy, a comparison between Kinney template galaxy spectra and Kurucz theoretical stellar spectra was carried out. It was thus found that a good approximation to the Kinney spectra is given by the Kurucz spectrum for a G2V star, as is shown in Figure 5.2, where the Kinney spectrum of a Sb galaxy is used for illustrative purposes.

Figure 5.2: Comparison of Kinney Template Galaxy Spectra and Kurucz Theoretical Stellar Spectra. The solid line shows the Kinney spectrum for a Sb galaxy, whereas the dotted line shows the Kurucz spectrum for a G2V star. Galaxy spectrum from [Kinney et al. 1996], stellar spectrum from [Munari and Castelli 2000].

The expected photoelectron count rates for a G2V star in the $G$ band and in the bands of the $fgriz$ photometric system (see Subsection 2.4.1) are listed in Table 5.1 for stars of magnitude $I=15.0$ and interstellar extinction $A_I=0.0$.

Table 5.1: GAIA expected photoelectron count rates. Rates of detected photoelectrons in the CCDs of GAIA Astros for a G2V star in the $G$ and $fgriz$ photometric bands. $I=15.0$ and $A_I=0.0$ were assumed. Count rates expressed in electrons/s. The corresponding count rates for some spectral types earlier and later than G2V are included for illustrative purposes. Courtesy of Lennart Lindegren, Lund Observatory.

ST	$G$	$f$	$g$	$r$	$i$	$z$
A5V	32552	9705	6780	4894	3065	1463
F6V	23783	5574	4872	4236	2936	1507
G2V	20079	3984	4096	3892	2866	1541
K3V	16104	2416	3208	3558	2766	1564
M0V	11015	844	1640	2470	2545	1632