Geoscience Reference
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between the photocenter of a recorded signal and the projection of the
object's barycenter on the sky plane. In turn, this shift will play a major
role in determining the final accuracy of asteroid astrometric measurements,
with obvious implications for the derivation of the orbital parameters of the
objects. From these simple considerations it is easy to understand how the
reduction of the dynamical and physical properties of the asteroids observed
by Gaia will be as a whole complex and iterative procedure.
The big advantage of Gaia, with respect to ground-based telescopes
of larger aperture, will be obviously the fact of being diffraction-limited.
The image of an object on the Gaia focal plane will be the result of the
convolution of the incoming wave front with the optical system of Gaia. In
addition, several other subtle effects will play a role in the generation of the
final signal recorded by the detectors: these effects include the motion of the
asteroid across the focal plane, the discrete (not ideally uniform) implemen-
tation of the time-delayed integration (TDI) read-out technique, the fact
that the detector consists of arrays of discrete pixels, so that the exact
location of the incoming light on the grid of pixels has some influence on
the recorded signal. Other important elements to be taken into account are
the quantum eciency of the detectors at different wavelengths, and, more
important, the fact that the signals will consist of the numbers of photoelec-
trons collected along a limited number of pixel columns. The physical size
and rectangular shape of the pixels has also, obviously, a major influence
and must be taken into account. In addition to the above properties of the
interaction of the incoming wave front with the Gaia optics and detectors,
it is also necessary to model the intrinsic properties of the signals. Asteroid
emission at visible wavelengths consists of sunlight scattered by the surface.
The actual flux of photons incident on the Gaia focal plane will then be
the final result of a complex interaction of solar photons with the aster-
oid surface. The object's size, albedo, shape, macroscopic and microscopic
roughness, and light-scattering properties will all play a role in the final
properties of the incoming signal. We should stress that this complicated
physical process is currently not fully tractable by means of purely analytic
means, and no definitive theory of light scattering is presently available.
The formation of an asteroid signal on the Gaia astrometric focal plane
is modeled by means of a numerical algorithm, in which all the effects men-
tioned above are taken appropriately into account. The general algorithm
is based on a ray-tracing approach, and a Monte-Carlo implementation. An
example showing how an incoming signal is finally converted into a recorded
Gaia signal is shown in Fig. 1.
