6.2.1 Photoelectron Count Rate

The sensitivity of an instrument employing a CCD as a detector can be conveniently defined as the number of detected electrons per second, or electron count rate, generated by a given reference source.

As far as WFPC2 is concerned, the photometric calibration obtained by [Holtzman et al. 1995b] and the A/D gains given by [Biretta et al. 1996] can be combined to estimate the photoelectron count rates in different WFPC2 bands generated by a source of given standard magnitude and color indices. Values thus calculated for WFPC2 bands approximately corresponding to the $BVRI$ bands are given in Table 6.1 for a source of $I=15$ and typical galaxy color indices given by [Prugniel and Héraudeau 1998].

As for the BBP, expected photoelectron count rates for stars of different spectral types and in different photometric bands were provided by Lennart Lindegren in a private communication. The rates expected in the $fgriz$ bands (see Subsection 2.4.1) from a G2V star of $I=15$, whose spectrum was found to satisfactorily approximate the spectrum of a typical galaxy in Section 5.4, are listed in Table 6.1.

Table 6.1: HST WFPC2 and GAIA BBP Electron Count Rates. Rates generated by an $I=15$ star are given for WFPC2 and BBP bands approximately corresponding to the $BVRI$ bands, and are grouped accordingly. Rates expressed in e$^-$/s. .

$B$		$V$		$R$		$I$
F439W	F450W	F555W	F606W	F675W	F702W	F814W
236	851	2396	4712	2713	4085	3249
$f$		$g$		$r$		$i$	$z$
3984		4096		3892		2866	1541

From Table 6.1 it appears that the sensitivity of GAIA BBP is generally slightly higher that that of HST WFPC2, notwithstanding the much bigger aperture of HST, but this can be explained by the seven reflections taking place in HST OTA (2) and WFPC2 (5), and to a minor extent by the higher quantum efficiency of GAIA CCDs. Specifically, the two instruments are of nearly equal sensitivity in the $V$, $R$ and $I$ bands, even if in the case of GAIA BBP the latter band is divided into two bands of smaller bandwidth, whereas GAIA BBP is substantially more sensitive in the $B$ band. The correspondences between HST WFPC2 and GAIA BBP bands outlined in Table 6.1 could in principle be used in the simulation of GAIA observations to convert WFPC2 electron counts into the correspondent electron counts for the BBP, but in practice a different approach is followed, assuming equal sensitivity for the two instruments. Such a very conservative assumption is believed to reinforce the validity of the obtained results in case the presently assumed specifications for the GAIA payload should change resulting in lower electron count rates than it is now foreseen.