Weather Guidelines for Opening/Closing the LBT Enclosure

A Critical Note about Weather Closing and the Authority to Close

Please be aware that the below are GUIDELINES; opening or closing of the telescope enclosure is at the sole discretion of the Observing Support Associate (OSA/TO). His/her decision may encompass many mitigating and complex weather factors. For the safety of the telescope, enclosure, and personnel, OSA authority regarding weather closure is final and absolute during nighttime operations.

Main Shutter Doors

  • Stay closed unless Temperature Difference between Primary Mirror glass therms and WS ambient is less than 10 degrees C. (Open vent doors and run mirror ventilation to bring mirrors into better equilibrium if other conditions permit.) Always have mirror ventilation running before opening shutter doors if a temperature difference of more than 5 degrees C exists.

  • Close if Maximum sustained Wind Speed exceeds 20 m/s (45 MPH) or gusts to 22 m/s (50 MPH). (The present wind limits are based on telescope shake, blowing oil droplets, adaptive secondary safety and building survival.) Close if you are getting more than 1 gust above the limit in 6 minutes. Reopen if there have been no gusts above the limit for 18 minutes. Also monitor the enclosure-telescope encoder values on mcspu for excessive fluctuations; > ~0.5 deg which indicate wind shake of the enclosure that means it is time to close.
    • For the purposes of these guidelines, you may assume that the higher of the two anemometer readings from the roof of the enclosure represents the true wind speed.

  • Close when Humidity exceeds 95%, and reopen when Humidity is less than 90%. To open, the exterior surface of the rotating building should be dry and not dripping water. The railings on the patio make a fair substitute hygrometer (if the railing is wet, the shutter doors should be closed). Turn to AZ=120.6 to line up the drains if it is raining.

  • Close if there are any rain showers or thunderstorms within 30 miles (48 km).

  • Close if there is blowing snow, rain, dust, ash or pollen. These particles can be detected in the beam of a normal flashlight. See also ParticleMonitor .

  • Close unless some stars visible to the naked eye. (Clouds have not more than 6 magnitudes of visual extinction).

  • Remain closed unless there is no substantial amount of snow or ice on shutter door joints/seals which could fall or blow into the telescope. (Stable ice on the shutter door panels is OK if the seals are clear.)

  • Ambient temperature must be above -20 degC. (Don't even drive the closed enclosure around if temperature is below -20 degC, and leave the telescope parked at zenith.)

Ventilation Doors

  • No snow accumulation on vent door sills before opening them.

  • No blowing snow, rain, dust, ash or pollen.

  • Close ventilation doors to 1/4 when wind exceeds 6 m/s. Close ventilation doors when wind exceeds 10 m/s. (These wind limits assume the main shutter doors are open. If main shutters are closed, then the ventilation doors may follow the main shutter wind limits.)

  • Do not open side vent doors if the there is any chance of dripping water from the roof drain on the main shutter doors.

  • Ambient temperature must be above -15 degC. (Pending a fix to the vent door drives that will allow them to operate like the shutter doors.)

  • Keep vent doors closed before sunset in situations where the telescope is tracking. Close all vent doors at 18-deg morning twilight regardless of telescope tracking state.

Sun Avoidance

  • Obviously, we don't want to point the telescope anywhere near the sun - neither on purpose, nor accidentally.

  • Morning sky flats should only be in the western part of the sky. Sky flats before sunset or after sunrise are acceptable by prior arrangement with the telescope operator. If the sun is above the horizon, the telescope azimuth drive should be safely disabled during sky flats to avoid accidental presets.

  • Restrict observations in the following sky positions after morning 18-deg twilight: (sunriseAZ - 60deg) through (sunriseAZ + 90deg) at elevations below 45 deg all elevations. This is quite similar to saying "don't observe within 60 degrees of the sun in the morning". This restriction doesn't remove all risk, but it minimizes the likely failure scenarios (such as a pre-dawn power failure with the telescope pointing east).

  • Critical observations (facing away from the sun) are possible while the sun is less than 30 degrees above the horizon. Permission for these observations, or an excemption from the above morning restrictions, should be obtained from the observatory director at least 1 week in advance so that appropriate other risk mitigations may be organized.

Level 4 Stealth Exhaust Dampers

  • Keep closed if the relative humidity is over 80%

-- JohnHill - 20 Jan 2010; edited by SciOps - 10-Feb-2014; added sun restrictions 22-Jul-2022

-- SA/20240426 sun restriction edit/clarification
Topic revision: r26 - 26 Apr 2024, SteveAllanson
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