Bad weather protocol at CTIO

About this document

The goal of this document is to update the protocols for closure and opening of telescope’s domes when there is bad weather conditions in Cerro Tololo.

This document is based in a section of the Thermal Control Procedures webpage Weather and Thermal Issues.(Internal Access Only)

The Victor M. Blanco 4-m telescope is called Blanco and 4m indistinctly in this document.

This is the translation of the original version in Spanish (intranet pdf document).

Closure and opening conditions during the night

The following rules are necessary to protect the telescopes and instruments of Cerro Tololo InterAmerican (CTIO) Observatory. They apply to the 0.9-m, 1.5m y 4m telescopes. The robotic telescopes in CTIO are completely autonomous.

Cerro Tololo Inter-American Observatory management completely trusts the mountain staff experience to evaluate the weather conditions. They are authorized and they are entrusted to open and close the domes. The observers in disagreement can contact their Support Scientist, and as a last resource, the Director.

The weather conditions referred here are recorded in the internal webpage “TOLOLO Site Environmental Conditions” (VPN needed): (Internal Access Only).

A complement to the previous page is the Las Cumbres Observatory weather Station.

If bad weather conditions are present, the Night Assistant or Observer Support must close RASICAM and create the Emergency-Stop-File in the Seeing Monitor DIMM webpage.


Wind restrictions for Blanco Telescope

Wind transports dust particles that are harmful to the dome interior, particularly to the mirrors. Therefore the limits are:

  • Wind speed > 15 mph (6.7 m/s)

    • Closure of lateral doors.
      This is a recommendation but is a good practice.

    • The Night Assistant or Observer Support informs the Observer the wind speed value, and suggests that they evaluate the image quality. If it has been affected it may be necessary to raise the lower shutter or windscreen if it’s functional. The Night Assistant or Observer Support will suggest not to observe against the wind.

  • Wind speed > 25 mph (11.2 m/s) or wind gusts of 50 mph (22.3 m/s).

    DO NOT observe against the wind.

    The Night Assistant or Observer Support will raise the lower shutter or windscreen, if functional, and they will inform the Observer about this restriction and will monitor the PF camera to assure the image is not blocked.

    The lower shutter can only be open if observation is made in opposition to the wind direction.

ABSOLUTE closure to all telescopes

  • Wind speed > 45 mph (20.1 m/s) average for the last 30 minutes.
    The telescope won’t shake much but the air will transport dust into the domes.

  • Humidity ≥ 85%. However the Night Assistant or Observer Support can close before this limit if he or she detects a risk.

    If the humidity is increasing the Night Assistant will do a visual inspection every 15 or 30 minutes2 of the sky condition, looking for nearby low clouds or evidence of humidity close to the dome.

  • For the 4m: Tmirror - Tdew < 2C.

    Values available in “Blanco ECS: Sensors Diagram” page, updated every 5 minutes. For more information about the dew point “Calculating risk of condensation”. (Internal Access Only)

  • If there is snow in the domes.

In case of absolute closure the Observer Support or 4m Night Assistant will communicate to the other observers in other telescopes the command of closure.


Note: Closure conditions for SOAR and Gemini telescopes

Absolute closure conditions for: SOAR4 are:

Constant wind v ≥ 70 km/h (43.5 mph); wind gusts: v > 75 km/h (46.6 mph); relative humidity >90%, or if there is evident risk of condensation. Besides, the operator will close if there are other risks such as dust in the air or clouds. Dome will not be open in case of snow or ice in the dome.

General closure conditions for Gemini South5 are:
Relative humidity 85%; winds 20 m/s (44.7 mph) or wind gusts of 25 m/s (55.9 mph); or if there is snow or dust. Besides the SOS operator could close before if there are risky conditions. The operator will wait for at least 30 minutes for stable conditions.

Opening the dome for all telescopes

  • Wind ≤ 45 mph (20.1 m/s) average value for the last 30 minutes.

  • Humidity ≤ 70%. Because it’s necessary to make sure that the humidity is decreasing for at least 30 consecutive minutes. In any event the night assistant will check that there is no condensation outside.

  • For the 4m: If Tmirror − Tdew ≥ 2C for 30 consecutive minutes.

  • If it has snowed or rained in previous days, the dome must be dry; no snow nor ice.


In case of opening the dome, the Observer Support or 4m Night Assistant will communicate this to observers in other telescopes. They are free to open or to keep close according to their criteria.


Other closure conditions

The content of this subsection applies to the Blanco telescope and it is extended to other telescopes as described below. The Night Assistant referred here is the Blanco telescope’s Night Assistant.


  • If the sky is completely overcast during the night and there is no other risk such as high humidity or strong winds, the Night Assistant or the Observer Support decide, in conversation with the observer, the closure of the dome, waiting for better conditions.

  • If there are dark (thick) clouds the dome must be closed. The Observer Support or Night Assistant will communicate the decision to other telescopes with observers, and they have to close immediately.

  • Re-open the dome is at the discretion of the Night Assistant and/or Observer Support in conversation with the observer. Usually, if the skies begin to clear, we wait 30 minutes to make sure conditions are stable.

  • If the sky is still overcast 30 minutes before the morning twilight, we declare the end of night.

  • The 0.9m and 1.5m telescopes

    • Both telescopes must be closed if the Blanco has commanded closure.

    • When weather conditions do not qualify for absolute closure, nor 4m has commanded to close as precaution, the observers of the small telescopes have the autonomy to decide if they open or close the dome.

Unexpected precipitation during the night


If the precipitation is minor it can be open again only if the sky is completely clear.


As previously mentioned, if there are thick clouds or complete overcast domes must be closed. In case of precipitations in the middle of the night. The Night Assistant:

  1. Will move the telescope to the NW station.

  2. Will alert the observers in other telescopes.

  3. We declare end of night for all telescopes.

If there is precipitation forecast

Precipitations introduce an extra complication to the telescopes in Cerro Tololo, because some domes are not perfectly sealed. The current official precipitation forecast for Cerro Tololo is made by the DGAC6 Meteorologist Observer7, he sends the information to the Head Civil Works, Maintenance and Utilities8 and then to the Telops Manager, who re-send it to the TelOps group. As a supplement we use other pages such as ventusky and YR.


Important: The procedure to cover and uncover the telescopes, can only be done during the day by trained personnel.


When there is bad weather forecast, the protocol is as follows:

  1. Blanco Telescope

    Before 3pm, the Observer Support, will inform the weather forecast to the TelOps manager, Blanco Scientist and Director. They altogether will decide if the conditions are severe.

    1. If weather conditions are NOT severe and therefore risk is minor. The Observer Support will move the telescope to NW station (dome closed), that way is protected from minor precipitation. The dome can be open later only if there is no precipitation and weather conditions improve.

    2. If severe weather conditions

      1. Trained personnel cover with plastic Blanco telescope during the day.

      2. The dome is moved to point to the north, so the Sun will dry any precipitation.

      3. The observer support declares all domes should remain close, without possibility to open even if conditions improve.

      4. The Observer Support communicates the decision and status of the telescope to the Blanco Observer, Science Support and Mountain Assistant.

  2. Other telescopes, after securing Blanco telescope.

    1. If weather conditions are NOT severe, move the 1.5m, 1.0m and 0.9m telescopes to a secure position, so they don’t get wet in case of minor precipitations. If -and only if- there is no precipitation and weather conditions improve they can open the dome.

    2. If severe weather conditions:

      1. Cover telescopes 1.5m, 1.0m, 0.9m and GONG with plastic.

      2. If applies, move the dome pointing to the north.

      3. The observer support communicates the decision of closure to the observers, that will remain closed even if conditions improve.

If conditions are severe, remember:

Winter operations plan, updated each year, will activate yellow and red alerts. Official information is transmitted by the FOC NOIRLab management or the Mountain Assistant.

  1. TelOps personnel will communicate the Mountain Assistant any threat of bad weather and/or if it has been decided to close the domes.

  2. Visitors and personnel will evacuate the summit before precipitations begin to the dining room/hotel.

  3. Personnel on the mountain, can go to the summit to check on systems following the Winter operations plan.


  1. The frequency of inspections outside will depend on how close of the humidity limit and the humidity increase rate.

  2. Weather, Sky and Monitoring Tools for Cerro Pachón

  3. Visitor’s Guide To Gemini South Telescope - Section “Closing/opening limits and nighttime evacuations”

  4. Dirección General Aeronáutica Civil.

  5. Luis Gerardo Valle Lobos, 2020.

  6. Fabrizzio B. Bruno, 2020.

Authors: Jacqueline Serón, Hernan Tirado, August 2021.

Collaborators: Claudio Aguilera, Rodrigo Hernández, Javier Rojas, Manuel Hernández, Alberto Alvarez David Rojas, Humberto Orrego.

Updated on April 12, 2024, 9:21 am