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 bands are given in Table 6.1 for a source of 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 bands (see Subsection 2.4.1)
from a G2V star of , whose spectrum was found to satisfactorily
approximate the spectrum of a typical galaxy in Section 5.4,
are listed in Table 6.1.
F439W | F450W | F555W | F606W | F675W | F702W | F814W | |
236 | 851 | 2396 | 4712 | 2713 | 4085 | 3249 | |
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 , and 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 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.