Data base

With the integration of the End-Of-Night & End-Of-Run reports in our data-base system it is now possible to provide better defined data on down-time, specifically due to weather. The weather down-time still somewhat depends on the observers as the definition of bad weather is not always the same (e.g., thin clouds does not in general affect differential photometry or spectroscopy much, but makes absolutely calibrated observation in principle impossible), but it should give a good general indication of the time lost to weather.

With the new system we can also define the amount of dark time more precisely compared to the rough estimate of 8 hours per night assumed in Table 1. To partly include the time spend during twilight on regular observations with high-resolution spectroscopy or IR observations, or on flat fielding or standard star observations which are an integral part of normal optical imaging observations, we have defined the dark time taking nautical twilight (the Sun 12$^{\circ}$ below the horizon) instead of astronomical twilight (the Sun 18$^{\circ}$ below the horizon). This makes the nights somewhat longer but also means that problems, e.g., during flat fielding can generate down-time in contrast to the old definition, and a whole night lost will mean more observing time lost. Give the normal practice of observing we believe that this is a more correct definition.

For period 33 the total amount of available dark time was 1658hr 37m (99517 min), a total of 196hr 58m (11818 min, or 11.9%) was lost due to bad weather and a total of 15hr 35min (935 min, or 0.94%) was lost due to technical problems. Note also that if we `correct' the total amount of dark time for the typical difference between nautical and astronomical twilight it turns out that assuming an average length for the night in the semester of 8 hr as assumed it Table 1 is actually fairly accurate and the corresponding percentage of down-time is 1.1% as given above.

The weather down-time statistics depend on input from the observers and is likely not extremely accurate, but the overall result is similar to the weather down-time reported for the WHT and INT telescopes over the same period. Also, a simple cross correlation with the weather reports shows that all nights with too high humidity or wind speed were reported as lost and likely there is little under-reporting of time lost.

In the future we intend to improve the data-base system further so we can provide the data in more details, e.g., we will have a monthly report created automatically.