Pre-Phase A Report - Lisa - Nasa

5.8 Telescope assembly 129
surface degradation effects will result in long term variations limited to 3 or 4 ◦C(with seasonal changes alone accounting for about 0.5 ◦C). Service module temperature changes could dominate the payload long term temperature
variations, with a sensitivity of nearly 0.5 K/K. The electronics boxes on the electronics
plate are a little on the warm side (in excess of 30 ◦C). Large temperature differences
(nearly 80 ◦C) exist along the front sections of the Y-shaped tube due to the aperture
seeing deep space. The payload support cylinder itself maintains a temperature difference
along its length of about 18 ◦C. To model the transient performance of the payload, numerical convergence criteria were
set to sufficiently small values so as to allow the detection of the very small temperature
changes important for LISA. Frequency response simulations were made at 10 −4 ,10 −3 and
10 −2 Hz and sets of ‘transfer functions’ relating the rms temperature of various payload
components to rms fluctuations in boundary conditions were calculated. Assuming the
power spectral density for observed insolation variations δL⊙ is given as
δL⊙ =1.3×10 −4 f −1/3 L⊙ Wm −2 / √ Hz , (5.1)
then for the optical bench we get temperature fluctuations of 2.0×10−4 K/ √ Hz at 10−4 Hz,
4.3×10−7 K/ √ Hz at 10−3 Hz, and < 8.0×10−1 K/ √ Hz at 10−2 Hz due to these solar
fluctuations.
The requirement of 1.0×10−6 K/ √ Hz at 10−3 Hz is met, but only by a factor 2. In
this case the fluctuations at 10−3 Hz in power dissipation for the payload electronics
on the electronics plate would have to be less than 6.8×10−4 W/ √ Hz and variations in
optical bench power dissipation would have to be less than 5.2×10−6 W/ √ Hz . Spacecraft
temperature variations would have to be less than 1.6×10−3 K/ √ Hz and laser electronics
dissipation variations less than 1.7×10−1 W/ √ Hz .
5.8 Telescope assembly
5.8.1 General remarks
The telescope has to fulfil two demands:
• The light power transmitted via the telescopes from the near to the far spacecraft
has to be as high as possible in order to reduce the shot noise level in the optical
readout system (see Eq. (3.2)).
• The wavefront of the outgoing beam has to be as flat as possible in order to minimize
the coupling of beam motions to the interferometer signal (see e.g. Eq. (3.6)).
Increasing the diameter of the primary mirror allows to reduce the divergence of the
outgoing beam and thus increases the intensity of the laser light at the far spacecraft.
In addition, the power picked up by the far telescope is proportional to the area of the
primary mirror there. The received lightpower is therefore proportional to the fourth
power of the mirror diameter D (see Eq. (3.2)).
Corrected version 2.08 3-3-1999 9:33