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WHT time lost to technical problems
The plot below shows for each month the fraction of night between dusk and dawn astronomical twilights when the planned observations could not be carried out due to technical problems. Engineering/commissioning nights, service/discretionary nights and nights when observers were using their own instrument, are excluded from the data. The raw data (more up-to-date) from which the plot was constructed are here.
The fraction of time lost to technical problems declined rapidly after 1992, as newly-commissioned instruments and software systems settled in. The spike in 1998-9 was due to faults in an ageing data-acquisition system.
The fraction of time lost was steady at around 2 - 3% from 2000 - 2010, but has risen slightly over the last 10 years, probably in part due to ageing of individual sub-systems, and in part due to pressure on ING resources from the upcoming deployment of a major new instrument, WEAVE.
The downtime is due to faults occurring in many different sub-systems, and is not easily reduced.
Technical downtime at other medium-sized optical telescopes is probably similar: as of the late 1990s, 3 - 4% was quoted for all of AAT, CTIO, KPNO, MMT and UKIRT.
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