CMA1 Temperature and Detector Stability
During the early phase of the mission, the CMA1 detector/HT supply combination commonly exhibited unstable behaviour (ref. EXPRESS No. 3 p.3 ) at elevated detector temperatures. Operational procedures were instituted to maintain a low average CMA1 temperature, namely a systematic switch off of the 28V power supply (A1 line, analogue electronics and HT convertors) for maneouvres, perigee passes and some non-LE prime observations, and a restriction of the maximum solar aspect angle to
130°.These procedures have reduced to practically zero the occurrence of instability and indeed long observations in the second half of 1984 have demonstrated that there is no immediately obvious degradation in performance if the temperature is maintained below 27°C.
In order to see how these constraints affect the total allowed duration of an
observation it is important to quantify the CMA1 detector temperature
behaviour as a function of the 
angle and the total A1 line on-time. Data
from observations carried out during the last three months has been analysed
to give a set of curves describing the average time profile from the A1 line
switch on of the CMA1 temperature at different
angles.
Figure 1 shows five of these curves, covering the typical range 90°
130°;
curves for any other desired
angle can be readily extrapolated from the data collected.
Fig. 2 shows the derivatives of the temperature curves for
= 90° and 130°
and indicates that the likely stable temperature for long observations would
be 27.5° and 29.5° respectively. Based on this analysis, the following
guidelines will be followed for planning and executing all future
observations:
Maximum observation duration for 110°<
130°: 4.5 units (12.5hr)
Maximum observation duration for 95°<
110° : 9 units (25 hrs)
Basically unrestricted duration for
95°
- NB.
- Practical durations for 95°<
130° will depend
upon the starting temperature, but in most cases a 3 unit observation
should be feasible.
Figure 1
Figure 2
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