The standard met system is the Casella system at both the WHT and INT and is shown on the displays in the control rooms. In case of problems with the Casella met systems, the Vaisala met system can also be used.


The INT should be warned as soon as possible if you are closing the WHT for bad weather, although the limits may be reached earlier or later there.



James wrote a script which checks the weather status. You can run it when the weather is over the limits and it sounds an alarm when everything is OK again. It can be run in any terminal:

python jmcc/Scripts/



The dome should be closed when:


* the outside humidity reaches 90% OR

* when the mirror temperature is within 2 degrees of the dewpoint


It should not be opened again until:


* the humidity has dropped to 80% AND

* the humidity has been below 90% for 20 mins AND

* it appears moderately stable


If the humidity reaches 75% or 90%, an alarm will sound in the control room. The alarm can be set using a 3-positoin switch: 75%-OFF-90%. This is very useful as a warning.


The Casella humidity meter is currently not perfectly calibrated and probably won't go above 97% even if it's very wet. The Vaisala humidity meter is also not perfectly calibrated so is even less accurate at high humidities, often only going up to 95% and usually reading ~5% lower than the Casella meter at high humidities, so if you are relying on the Vaisala meter you may need to close before it reaches 90%. You can use a handheld humidity meter to check, but this must be at ambient temperature for a correct reading so, if you take it out into the cold, leave it a few minutes before taking the measurement. Calibrating all these meters perfectly is a difficult (impossible?) task so we need to be aware of their idiosyncrasies.


To protect the dome shutters, as well as the telescope, from possible damage by wind or airborne dust, the dome should be closed at the INT or WHT if:


* the wind speed exceeds 80 km/h OR

* if it gusts many times for more than 10 sec above 80 km/h OR

* if you see any gusts above 100 km/h


However, these limits are slightly dependent on the wind direction relative to the telescope position. If you are pointing into the wind, these limits must be obeyed, but if you are pointing away from the wind, then it may be safe to exceed the wind speed limit by ~10%, but you should close for any gusts over 100 km/h whatever the wind direction. Also bear in mind that both the Casella and Vaisala meters are in a position which is shielded from westerly winds. The Wind Gust value on the WHT/INT met systems is taken from the JKT every minute, but remember that it is more exposed here than at the INT or WHT. The Vaisala meter measures both maximum wind speed and average wind speed.


If the met system fails, use a hand-held anemometer to check the wind speed.



This is difficult to quantify. A rough guide is that if you are unable to guide with a guide star of magnitude 11 with a 5 second exposure then the cloud is very thick and there is a moderate risk of rain. If the  cloud is bad enough that observations aren't useful, but not so thick that the risk of rain is significant, just close the petals and re-open them again when it looks better.


The TNG has the only dust monitor at ORM. While this can be used as a guide, we donít have a dust limit. However, if you can see a lot of dust in the beam of a torch, or if it looks terrible, then close the dome.



If there is ice, snow or rain and very cold temperatures, try to rotate the dome frequently so that it doesn't get frozen in a fixed position, which will make it harder to get any ice to melt later. If the dome shows any resistance to movement due to ice, either when opening or rotating, don't force it. Don't try to rotate the dome for more than 5 seconds if it doesn't start to move straightaway and never try to chip ice free. If you can see ice or snow on the top of the dome (it can be difficult to tell, but see how much ice or snow is on the roof and decide based on that if you canít see clearly), do not open the dome because there is a danger of ice or snow coming free and falling in onto the telescope.


In case of ice/snow, the domes should be left pointed as follows:


* WHT 156 degrees

* INT 212 degrees




In wet weather, the domes should be rotated at the end of the night so that the shutters point into the wind. e.g. if the rain is coming from the South, do USER> dome 180 or rotate it at local control.

Revised 22/02/16 by Fiona Riddick