5.5 Sky Background and Readnoise

The sky background as it is observed from the ground and from space was discussed in Subsection 3.2.2, and it was concluded that for our purposes a conservative value of $\mu_{bg,I}=21.0$ mag/arcsec$^2$ can be assumed for the sky background. In the following it will be furtherly assumed that the sky background is unifom within the aperture and has the same G2V spectrum assumed for galaxies in the previous Section. As already noted, however, the sky background will be negligible in the global error budget of GAIA surface photometry.

The total readnoise levels of GAIA CCDs in the ASM1 and in the BBPs of the Astros can be estimated following the recipe of [Vannier 1999]. The values obtained for the presently planned sampling of the ASM1 are given together with the values obtained for four different sample sizes in the BBPs in Table 5.2. Even though the sample size in the ASM1 appears fairly well-established, the values obtained for different sample sizes in the ASM1 are also included to illustrate how a decrease in the required accuracy of two-dimensional position measurements in the ASM would result in a substantial reduction of the readnoise.

Table 5.2: Readnoise levels of GAIA Astro CCDs in the ASM1 and BBPs. Full readout of the CCDs was assumed both for the ASM1 and the BBPs. Values corresponding to three and four sample sizes are given for the ASM1 and the BBPs. Note that $1\times8$ and $6\times8$ pixels/sample are presently planned for the observation of stars in the Astro 1 and Astro 2 BBPs, respectively.

Parameter	Unit	ASM1	ASM1	ASM1	BBPs	BBPs	BBPs	BBPs
Sample Size	pixels	$2 \times 2$	$3\times3$	$4\times4$	$6\times8$	$6\times4$	$6\times2$	$1\times8$
Readnoise	e$^-$ rms	10.63	7.92	6.72	5.44	6.74	8.79	10.44