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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 less equal 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 beta symbol 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 beta symbol angles.

Figure 1 shows five of these curves, covering the typical range 90° less equal beta symbol less equal 130°; curves for any other desired beta symbol angle can be readily extrapolated from the data collected.

Fig. 2 shows the derivatives of the temperature curves for beta symbol = 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°< beta symbol less equal 130°: 4.5 units (12.5hr)

Maximum observation duration for 95°< beta symbol less equal 110° : 9 units (25 hrs)

Basically unrestricted duration for beta symbol less equal 95°

Practical durations for 95°< beta symbol less equal 130° will depend upon the starting temperature, but in most cases a 3 unit observation should be feasible.

P. Ferri

Figure 1

CMA temperature versus time

Figure 2

CMA temperature derivative versus temperature

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Last modified: Thursday, 26-Jun-2003 13:48:32 EDT