149 
Figure: 4: Focal Plane Layout (X = 750 nm, AX = 200 nm) 
The preceding Star Mapper views all the stars 
during the scan motion of the satellite, and implements 
different and important functions, such as: 
• detection of the presence of each star entering the 
FP, up to a limiting magnitude V = 20 
■ estimation of the times at which the star will transit 
on the successive FP zones (i.e., the preceding 
Photometry area, the Astrometry area, the 
following Photometry area and the following Star 
Mapper); transmission of these data to the relevant 
detector control electronics 
■ determination of the position occupied by the star 
on the Star Mapper at a given time 
This area of the focal plane allows to perform a 
significant data reduction at the detector level. In fact 
using the information from the preceding Star Mapper, it 
is possible to read out just the detector zones in which 
the presence of a star is expected, discarding the 
empty pixels. 
All the CCDs are operated in Time Delay Integration 
(TDI) mode, which consists in shifting all the charges of 
a vertical line (row) to the next row at a velocity equal to 
the apparent star velocity (figure 5). This CCD 
operation mode is implemented to accumulate charges 
for a sufficient time (established by the along-scan pixel 
size), at the same time avoiding the image smearing 
otherwise caused by the continuous change of the 
telescope line of sight. 
The apparent star velocity is different from chip to chip 
because of the residual telescope field distorsion. 
Therefore, the TDI line shift must be clocked by 
independent oscillators. 
The Astrometry area is covered by a mosaic of 632 
CCDs of three different sizes. Each CCD is made of 
rectangular pixels with size 4 x 50 pm (optimized for X = 
750 nm). 
The interference fringes are present only in the along 
scan direction, due to the telescope optic design, while 
in the cross scan direction, the photons distribution 
follows the diffraction pattern. Given that each fringe 
must be sampled with at least 3 pixels to properly 
reconstruct the signal, the along scan direction is 
characterized by the smaller pixel dimension. 
In the cross-scan direction the star image can be 
sampled with a much lower resolution: a single sample 
of the Airy disk of the image is sufficient. In this 
direction the linear size of the Airy disk is 226 mm, 
which correspond to ~ 5 pixels. To obtain a single 
sample, a technique for the image acquisition named 
binning has been proposed, which consists in building a 
macropixel made of 5 pixels containing the full Airy 
disk. In such a way, the number of pixels processed is