The meteorological stations of the 1.5 and 0.84 m telescopes of the OAN: Description and results

Meteorological stations for the 1.5 and 0.84 m telescopes at the Observator io Astronomico Nacional at San Pedro Martir (OAN) are described. The statio ns include devices for measuring temperature, atmospheric pressure, relativ e humidity and wind conditions. All the meteorological variables are monito red continuously and mean values are determined every 5 minutes and written on the hard disk Of the control computer. This information is continuously refreshed in an HTML page that can be accessed from the OAN home page. In this paper we present the results of measurements taken over 2 years. We fi nd that both primary mirrors are substantially warmer than the surrounding air during the night (up to similar to 5 degreesC). This thermal gradient d egrades image quality. For the primary mirror of the 1.5 m telescope this e ffect can be corrected with a temperature control system. For the 0.84 m te lescope, where the primary mirror is located in a closed structure, ventila tors moving air from the dome into the closed tube may suffice. During the night the air is colder within both domes than outside, and we conclude tha t no dome ventilation is necessary in these telescopes.