Pre-Phase A Report - Lisa - Nasa

74 Chapter 3 Experiment Description
the science measurement band. Alternatively it might be better to ‘apply’ the UV in
a gated fashion only when it was necessary to discharge the proof-mass or in a timed
sequence aimed at matching the charging rate. For these modes we need some means of
controlling the UV intensity. This can be done using the lamp drive power over a limited
range by programming its supply current. A wider dynamic range can either be achieved
using a load switching technique or by using an electro-optic switch (Pockells cell). A
system to control the UV intensity involving a Pockells cell has been demonstrated in the
laboratory [94].
The UV discharge system incorporates 2 low-pressure mercury discharge lamps, associated
optical components, power supplies and control electronics contained within a single
enclosure measuring 10 cm×15 cm×7 cm. The system is internally redundant and only
1 unit is required per spacecraft. The unit is located on the radiator plate. Two electrical
connectors provide power and control function interfaces respectively. A digital
control interface, including internal h/k conversion, is assumed. Four optical fibre couplings
provide interfaces to fibre optic cables delivering independent UV photon fluxes to
the proof-masses within the telescope structures.
3.3 Drag-free/attitude control system
3.3.1 Description
LISA requires that the acceleration noise imposed on each proof mass in the sensitive
direction is smaller than
δa < 10 −15
f
1+
3×10−3
2
ms
Hz
−2 / √ Hz (3.13)
within the measurement bandwidth (MBW) from10 −4 to 10 −1 Hz. In terms of the requirements
on spacecraft control, taking into account the optical referencing from both
opposing faces of each proof masses, the LISA drag-free & attitude control system (DFACS)
must ensure that each spacecraft is controlled in translation and orientation such that the
relative displacement between the spacecraft and the proof mass in the sensitive direction
of each telescope arm is less than
δz < 2.5×10 −9 m/ √ Hz (3.14)
within the MBW. In the other axes, the requirement is relaxed to
δx = δy < 2.5×10 −6 m/ √ Hz (3.15)
within the MBW. The relative attitude requirement between each proof mass and the
spacecraft is
δθ < 1.5×10 −7 rad/ √ Hz (3.16)
within the MBW. The attitude of the spacecraft relative to each incoming wavefront must
be controlled to within the dc value of
φdc < 3×10 −8 rad , (3.17)
3-3-1999 9:33 Corrected version 2.08