Generally speaking, we would like to observe galaxies in all sky areas that trigger the detection and from a suitable surrounding area, and to send the corresponding data to the ground. Such a surrounding area could be defined as composed by all the square areas of a given side that are adjacent to the areas where an excess surface brightness has been detected, as it is shown in Figure 5.3, where a side of 2 arcsec was chosen for illustrative purposes.
The outlined observation strategy appears to satisfactorily cover the galaxy regions whose surface brightness is just below the detection limit, thus allowing to study the galaxy morphology in greater detail. In the following, however, in order to estimate the telemetry rate required for galaxy observations, a few simplifying assumptions will be made. It is assumed that data are transmitted from circular areas centered on the galaxy center, and that the radius of this areas can be written as
Under these assumptions, the required telemetry rate after compression for galaxy observations can be estimated as
As mentioned in Section 2.6, a total telemetry rate of about 1 Mbit/s after compression is presently foreseen for GAIA. From Table 5.8, it appears that the observation of galaxies, carried out with or pixels/sample, as suggested in Section 5.7, would require a significant, but probably not unreasonable, part of the total telemetry. On the other hand, observing the assumed fraction of the sky with a smaller sample size would require a prohibitively high telemetry rate.