Humidity Restrictions

Why: Domes should be closed whenever there appears to be a significant risk of condensation at the level of the dome slit or the telescope. This is to avoid condensation dripping onto the mirror or other equipment.

Restrictions: When the humidity exceeds 90% all telescopes should close immediately. The telescopes can be reopened only after the humidity has dropped below 85% for more than 30 consecutive minutes. This should be done by opening the dome, and then checking that the mirror cover is dry before opening it. In case of a dispute, the telescope should be closed, or remain closed, and then the astronomer should refer to the La Silla Coordinator who has the final word.

Wind Restrictions

Why: Strong winds will affect the telescope mechanical structure, and also leave significant amounts of desert dust on the mirror.

Restrictions: The wind speed is displayed on the meteomonitor display, and also the limits are indicated by lights in the dome. The following wind restrictions apply:

In case of a dispute, the telescope should be closed and then the astronomer should refer to the La Silla Coordinator who has the final word.

Atmospheric Pressure

The pressure is measured at the Vaisala meteo station (the 30m tower near the 1m telescope) using a Rosemount sensor. The scale on the meteomonitor graph is given in hecto-Pascals (which are equivalent to the more familiar millibar). The pressure has been proven to be more reliable than any other source of information for forecasting local weather conditions, though occaisonally it produces false alarms! The lines which appear on the meteomonitor plot indicate the weather associated with the measured pressure.

Temperature

The temperature is measured at the Vaisala meteo station (the 30m tower near the 1m telescope) using senosrs at 2 meters (sensor 1), underground (sensor 2) and at 30 meters (sensor 3). The reading which appears in the meteomonitor is the average reading from the two above-ground sensors.

For astronomical observations, it is desirable to have a small diurnal change in temperature. Large changes in temperature lead to convection currents and consequently poor mirror and dome seeing.

Wind Speed

The wind speed is measured at the Vaisala meteo station (the 30m tower near the 1m telescope) at 3 levels (10, 20, and 30 meters for sensors 1, 2 and 3). The wind sensors (both speed and direction) use ball bearing mountings and optical encoders. Wind data from levels 2 and 3 should agree quite well and produce the best data for the mountain (the value given by the meteomonitor is an average of these two values).

The sensor at the 3.6-m usually gives higher readings than the meteomonitor. However, this sensor does not give a true reading as it is mounted on the top of the 3.6 dome, where a strong venturi forms.

The highest wind speed this winter (1997) was about 39 m/s average over one minute, and gusts in the 44 m/s range. The highest gusts ever recorded at La Silla slightly exceeded the measurement range of the sensors (50 m/s).

The most common wind direction is from the north-northeast. If the local wind direction is rotated 90 degrees anti-clockwise, the wind direction at high altitudes is obtained. This allows a prediction to be made as to whether a cloud system visible on the satellite pictures will pass over La Silla.

Popular opinion has it that southern wind brings good weather, though there is no strong evidence to support this idea. Usually a wind direction shift to the south occurs after some days of bad weather, which means that the system is now on its way to the Atlantic Ocean, and that we are in the backside of the air circulation.

Seeing

The seeing is measured using a differential motion monitor (DIMM) which is located close to the Schmidt telescope. The DIMM calculates the seeing using an analysis of the differential tilt of a wavefront at two locations 20cm apart. The standard deviation of the differential slope of 200 exposures is converted into the FWHM of a long stellar exposure.

Humidity and Dew Point

The humidity is measured at the Vaisala meteo station (the 30m tower near the 1m telescope) using a thin-film polymer humicap at an altitude of 2 meters. The dew point is computed from the humidity and the temperature measured at the same altitide as the humidity.

If a telescope is fitted with temperature sensors, the best protection against condensation can be obtained if the lowest temperature at or near the telescope is compared to the dew point. When the two meet, water will form, and it's too late! So the telescopes should be closed before the dew point gets close to the lowest telescope temperature.

If there are no local thermometers, the relative humidity can be used to judge whether condensation will form, but this is always something of a gamble. When there is some wind to equalize temperatures, the 90% rule may be appropriate, but in calm conditions it is very easy (you just need a surface cooled 1.5 degrees below ambient) to get condensation at 90% humidity! In calm conditions, a surface with even only moderate ability to radiate, can easily cool down 3 or 4 degrees below ambient, when it looks into the cold sky. At 4 degrees difference, you will get condensation at 70% relative humidity! So, it is not possible to set strict operational limits of relative humidity without knowing the radiation cooling of each area to be protected.

A incoming warm air mass can also send the dew point up and over the temperature of big structures having a long thermal inertia. So, increasing air temperature can be a sign that condensation may form.

Finally, if your telescope is air-conditioned to 8 degrees during the day, and then you open the dome into 12 degree warm air, 70% relative humidity will soak your telescope without any need for radiative cooling!

The average relative humidity at La Silla is about 25%, though this varies a lot between one year and the next.

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