Pre-Phase A Report - Lisa - Nasa

130 Chapter 5 Payload Design
Once a particular diameter of the primary mirror is given, the intensity at the beam axis
in the far field is highest when the Gaussian diameter of the outgoing beam equals the
diameter of the primary mirror. The beam is then truncated at the 1/e2 contour line of its
intensity. In this case the power picked up at the far spacecraft is given by Eq. (3.1). For
reasons of cost and weight a diameter of only 30 cm for the primary mirrors was chosen;
it is adequate for the envisaged sensitivity level.
Coupling of changes in beam orientation to the interferometer signal is minimal when the
center of curvature of the wavefront at the receiving spacecraft sits inside the emitting
spacecraft. For the wavefront of a laser beam in the far field the center of curvature
coincides with the focus of the beam. The outgoing wavefront has therefore to be flat.
The final quality of the plane wavefront leaving the telescope is specified as λ/10 .
5.8.2 Telescope concept
The telescope widens the diameter of the beam from a few mm to 30 cm. Since the
space available is very limited, spherical optics would cause huge aberrations. Therefore
conical sections are used for the mirror surfaces instead of spherical ones. In this case
the imaging properties for point sources can in principle be perfect, if the focus of the
beam is positioned exactly at the focus of the particular mirror. This is the idea behind
the original Cassegrain telescope, where the primary is chosen to be a paraboloid and
the secondary a hyperboloid. Unfortunately, the tolerances for misalignments and the
usable field of view are very small in this case. There are several improvements over the
original Cassegrain, e.g. the Ritchey-Chretien telescope, minimising the first three Seidel
aberrations.
The transmitting and receiving telescope in LISA is therefore an improved Cassegrain
system, including an integral matching lens. It is mounted from the payload support
cylinder and protected by a thermal shield. The primary mirror is a double-arch lightweight
ultra-low expansion ULE design and has a diameter of 30 cm and also a focal length
of 30 cm. The secondary mirror, supported by a three-leg carbon-epoxy spider, is mounted
D Primary mirror
Secondary mirror
Figure 5.6 Geometrical arrangement of the telescope components
27.62 cm from the primary and has a diameter of 3.2 cm and a focal length of 2.6 cm. The
beam from the instrument package to the secondary mirror is expanded to a diameter
of approximately 3 cm by a suitable lens in the plane of the primary mirror. As just
mentioned, the optical elements are aspherics to reduce aberration in the f/1 telescope,
3-3-1999 9:33 Corrected version 2.08