reduced, with a consequent decrease of the read-out
noise and of the CCD read-out time. Of course, the
reduced read-out noise is advantageous for the
detection of fainter stars.
In the Astrometry area, the nominal TDI shifting period
(with angular velocity of 120 as/sec and pixel size of 4
mm) is Tshitt * 172 ps.
The Photometry area and the Star Mapper are covered
by mosaics of 40 and 28 CCDs respectively. In these
areas, the sampling requirements are more relaxed. A
typical sampling of a star image of 3 x 3 pixels is
sufficient to determine photocenter coordinates. The
pixel size is 60pm x 120pm, corresponding to a TDI
shifting period of T S hift * 2580 ps.
The major characteristics of the CCD for all the areas
are summarized in table 6 and 7, the integration time
reported is referred to an elementary exposure, that is
the exposure on a single CCD during the star transit on
the detector.
Table 6: CCD characteristics
Astrometry
Area
Photometry
Area
Star Mapper
CCD linear
size
23.3x70 mm
35 x 70mm
35 x 70mm
Pixel size
4 pm x 50
pm (linear)
60pmx120
pm (linear)
60pmx120
pm (linear)
Number of
pixel in CCD
-5800x1400
580 x 580
580 x 580
Integration
time
1 sec
1.5 sec
1.5 sec
Table 7: CCD sensor requirements for the astrometry area
Image
sampling
> 3 pixel/fringe period
Quantum
efficiency
> 60%
in the X = 750 ±100 nm observation
band
Read-out-
noise
< 3 e
Charge
transfer
inefficiency
<10' 6
3. METROLOGY
The metrology for the GAIA interferometric instrument
is one of the most crucial issues to the success of the
mission.
The optical part of the instrument must be kept stable
essentially for two reasons: formation and persistence
of the interference fringes, and stabilization of the basic
angle (Cesare, 1998).
Concerning the former, the fringe visibility must be
always close to its nominal value (visibility loss < 5%)
across the astrometric field. The second reason is
nevertheless crucial, in fact while the basic angle
between two lines of sight needs to be controlled within
5 pas, the stability of the relative position of star images
in two different FOVs is relaxed to 10 pas to take in to
account geometric field distorsion variability, in the time
scale from 0.75 sec (minimum integration time of a star
image on the astrometric field) to 3 hours (great circle
scan period).
The most critical elements to be stabilised are the
primary mirrors of the optical interferometer which can
be subjected to rigid body motions like translations
(tolerances: 5 nm) and rotations (tolerances: 1.8 nrad)
and the beam combiner mirrors (tolerances: 17 prad in
rotation, corresponding to a linear translation of 6 pm).
The mirror shape variations are less critical, in fact they
have smaller effects on the fringe visibility loss, and
hence on the image position shift, and can be passively
controlled (i.e. by passive thermal stabilisation).
The rigid-body relative movements of the most critical
mirrors (beam combiner and primary mirrors of the
optical interferometer) are monitored and controlled by
means of an Optics Active Control System composed
by 15 laser interferometers (each measuring the
distance between two reference markers) and 5 tip-tilt
mechanisms, each controlling the mirror in three
different directions: one translation along the mirror
longitudinal axis, and two rotations - cx-tilt, P-tilt - of the
plane normal to this axis.
Three markers are placed on each of the primary
mirrors (M1*), 6 on the mirror M2, and 18 are distributed
on the beam combiner mirrors (figure 8). The markers
are attached to the mirrors through optical contact.
The laser interferometers monitor the distance variation
between reference markers (caused by rigid-body
movements of the mirrors) and the tip-tilt mechanism
moves the “active mirrors” to compensate the distance
variations. Moreover, several tests and simulations
have demonstrated that, as a consequence of the
distance control between the selected reference points,
also the fringe contrast, the star relative position and
the basic angle are also kept under control.
Error analyses have shown that the measures of both
absolute and relative distances must be performed by
laser metrology with the following accuracy:5s/s = 4-1(r
1a (relative error for the absolute distance
measurements), and 5s/s = 8-10‘ 12 1a (relative error on
distance variation measurement). In order to attain
these results the laser frequency variation must be
stable at 8v/v < 8-10' 12 1a over 0.75 s + 3 h time scales.
Figure 8: Position of the reference markers (L, Mi, Rj, Si,)
on the optical interferometer mirrors
150