The baseline sample size for the observation of stars in the Astro 1 and
Astro 2 BBPs is of and pixels, respectively.
These must be considered as starting points for the choice of the sample size
for galaxy observation, since it would be desirable to use the same size
to observe both stars and galaxies.
These two sample sizes, however, are very elongated across-scan, and this
could result in problems when trying to reconstruct the two-dimensional
morphology of galaxies.
Besides, the sample size can be reduced in the across-scan, but not in the
along-scan, direction without upsetting the TDI process.
Accordingly, four sample sizes of , , , and pixels
were considered. Note that the same sample sizes will be used in
Chapters 6 and 7 to numerically simulate
galaxy observations as they could be obtained by GAIA.
The expected all-mission accuracy in surface photometry in the band for these sample sizes and for different levels of surface brightness is given in Table 5.6, where a total number of 75 scans was assumed. Note that the band is very similar to the band (see Table 2.1) and that an average number of 75 scans is expected for each Astro from a 5-year mission (see Figure 2.4). From the tabulated values it appears that the surface brightness limit for surface photometry accurate at a given level increases with the sample size by about one magnitude per different sample size. For instance, the limit for surface photometry accurate to within 0.2 mag/arcsec is about 21.0 mag/arcsec for pixels/sample, 20.0 mag/arcsec for pixels/sample and so on.
The surface brightness at the effective radius of a galaxy of is typically about mag/arcsec for E galaxies and mag/arcsec for D galaxies, according to Table 4.5. Comparing these values with those in Table 5.6, it appears that a sample size of or pixels/sample is preferable in order to obtain multi-color surface photometry in the innermost regions, i.e. down to the effective radius, of most galaxies brighter than . This in turn suggests to carry out galaxy observations in the Astro 2, where both a sample size of and pixels could be adopted without upsetting the TDI process for star observations. Note, however, that while an increase in the sample size obviously increases the photometric accuracy, on the other hand this leads to a decrease in the achievable angular resolution. The previous considerations will therefore be combined with those developed in Chapters 6 and 7 to establish the best trade-off between photometric accuracy and angular resolution.