-F. Macchetto
As described in our previous report of mid-October, we have now completed the two special tests designed to verify the performance of the f/48 detector following the unexpected high-voltage breakdown of 17 September.
The first test was conducted on 22 October and consisted of a high-voltage turn-on with real-time commands issued from the ground. To allow for possible failure to turn-on, the test allowed several opportunities to try the switch-on procedures. This proved to be unnecessary, since the intensifier tube high- voltage was successfully turned-on at the first attempt. The high-voltage stayed on for the duration of the test and was then switched-off normally using, again, real-time procedures.
The second test was conducted on 28 October. This time the commanding was done through the pre-loaded on-board procedures. Note that for both of these tests, we had modified the way in which the high-voltage was turned-on to its full value. The way we implemented it was to allow the voltage to ramp-up to its minimum pre-set value, which is already about 80% of the final value, and then step-up the voltage in five steps with about two minutes of hold in each step. The theory is that this would give the electric charges time to stabilize, thus reducing the risk of corona breakdown. Another advantage, in case the high-voltage had tripped-off, was to give us a more precise value at which the event occurred and, thus, better diagnostics.
The second test was fully successful. The high-voltage was stepped-up to its maximum value without problems. After the usual wait time for thermal stabilization, we took three ''dark'' (background) frames and four ''flat- fields,'' the latter using internal calibration lamps (green-LED).
Both the ''dark'' and ''flat-field'' frames appear normal, with the same generic characteristics measured before the 17 September event.
From the data available, it is not possible to decide whether the September breakdown will occur again, nor with what frequency. However, it is the sense of all those involved that we must treat the f/48 detector as a limited resource and minimize the chances of shortening its life prior to the Maintenance and Refurbishment Mission (M&R).
Therefore, we have adopted a policy of allowing limited use of the f/48 detector for those programs that uniquely require its capabilities. Most users have already indicated their willingness to use the f/96 detector, instead. We will decide on a case-by-case basis which programs will still be done in f/48.
Following the M&R mission, we will probably wish to use the f/48 preferably to carry-out long-slit spectroscopy, the truly unique feature of this FOC mode.
Prior to restarting the use of the f/48 mode, we will modify the switch-on procedures along the lines used for the tests described above. While we have no strong positive data to mandate that we do this, it seems to be the prudent course of action.
Finally, for both f/96 and f/48, we have mandated a maximum of three switch-ons per week to limit as much as possible the stress on the detectors. We will also extend the maximum time between observations during which we would switch-on and off from twelve to eighteen hours to regain some of the scheduling efficiency lost because of this decision.