Temperature sensors
ETH-2 has 8 temperature inputs, each of them measuring temperature in -40°C to +60°C range with precision and accuracy better than ±0.1°C. In AS2 ETH-2 terminology, these eight channels are designated "raw" temperature channels t1 to t8.
Device firmware enables an arbitrary number of raw temperature input channels (out of 8 available) to be organized in so called “composite channels”, so that the device calculates average composite channel temperature and relative deviations of each of raw component channels that belong to it. This is a very useful feature if one wants to measure average temperature of large objects such as telescope primary mirrors and at the same time to monitor relative temperature changes over their surfaces as primary mirror temperature deviations can lead to mechanical stresses and cause optical distortions. Data from component channels cannot be displayed in a regular display cycle (but can be at the operator request), instead only the average temperature of the composite channel can be displayed regularly. For more details about ETH-2 display subsystem, read under "
display and power" tab.
For each channel, both raw and composite, operator can set four limiting thresholds:
-
Two absolute limits, one defining minimum, the other one defining maximum permitted channel temperature. Absolute temperature limits define the range of temperatures in which safe and proper operation of scientific equipment is expected.
-
Two relative limits, each one specifying the “referent temperature channel” and minimum permitted positive temperature difference with respect to it. Referent channel can be any row, composite or forecasted temperature channel, or dew point channel. Relative limits are useful for preventing dew formation as well as for maximizing time available for scientific work.
Forecasted temperature is a numerical value (in °C) that telescope operator is expected to enter periodically, which tells the device the predicted air temperature for the following night, so that ETH-2 can maintain air temperature inside the dome during the daylight close to that value, eliminating the necessity of temperature equalisation between air inside and outside the dome every evening. This maximizes time that can be used for scientific measurements, but is not essential or mandatory because ETH2 firmware automatically invalidates FT value if it has not been refreshed over the three previous days.
Relative humidity sensors
ETH-2 has two analog relative humidity input channels which measure relative water content in air in 5% to 100% range with precision and accuracy of ±1%, in temperature range between -35°C and +85°C. Sensors are protected from condensation by GortexTM filters. Because sensitive surface of any type of rH sensor has to be directly exposed to air, atmospheric trace gasses such as H2S can influence its accuracy over several years of operation, so periodic re-calibration might be required. ETH-2 firmware provides a quick and easy procedure for this.
Dew point channels
One of the most valuable information in telescope maintenance is knowing the dew point of air i.e. temperature at which, if cooled, air with current temperature and relative humidity would start to precipitate water in the form of liquid condensate. For each of its two rH input channels, ETH-2 calculates dew points. This data is available in the form of two independent "virtual" measurement channels that can be displayed to the operator and used as reference values for automatic air conditioning functions.
For calculating the dew points, ETH-2 uses Wanielista-Kersten numerical model. Taking into account characteristics of temperature and relative air humidity measurements described above, accuracy and precision is better than ±0.2°C. This is more than sufficient for the purpose of dome air conditioning and prevention of dew formation.
