1. Blanco Primary Mirror Cooling Protocol
Alistair Walker
Ver 3*
September
* Minor changes by Tim Abbott

2. Requirements
The temperature of the surface of the Blanco mirror during observing hours should be within the range 0C to -2C (i.e. equal, or slightly cooler) compared to the surrounding air
The air in the dome (“low dome”) should be very close to the outside temperature, i.e. well-mixed during observations.

3. How do we achieve this? We cool the whole dome during the daytime.
We use the two Air Handling Units (AHU) on the C-floor, and the Helicopter Fan on the M-floor to mix the air.
This is to prevent the dome air from warming during the day, which otherwise means that the lower surfaces of the dome warm up and radiate through the night
We cool the primary mirror during (part of) the day, and suck out air from near the primary at other times, and at night.
We run the glycol colder in winter (~ -2C) than in summer (~ +3C).
We open the lateral doors to have the dome well-vented at night time.
If the wind is too strong (> 15m/s), they are closed.

4. Protocol
Soon after lunchtime each day (~ 2 pm) Observer Support will decide whether to change the mirror control from cooling to sucking at: (i) right away (ii) 4:00 pm, (iii) when opening the dome for observing.
The next few slides show how to make the choice. We use as input:
The height of the zero degree isotherm for the present and future days, from the weather forecast. This is used as a proxy for night time temperature
The 2 pm outside air temperature and the 2 pm mirror surface temperature (PMAS), and how these temperatures have changed recently.

5. From Meteochile http://www.meteochile.gob.cl/pronostico_cordillera.php
The “isoterma cero” tells you whether the next day is going to be hotter or colder than the present day, We see here that the temperature is going to be constant until Sunday (i.e. Saturday night probably).

6. Approximate Temperature Calibration
Isoterma Cero meters
Night time temperature C
2200
2700 4
3200 7
3700 11
4200 14
4700 17
5200 20
This will tell you whether the night time temperature is going to go up, or down, or stay the same.

7. To view the recent temperature history of the mirror surface and outside Use SISPI database,
Select environmental data, display for 2-3 days, tick the “outside temperature” and “PMAS temperature” or just follow this link* *

8. Mirror getting too cool
Sucker on
@ 2 pm
Sucker on
@ 5 pm
Sucker on
@ 3pm
Sucker on
@ 4 pm

9. Primary Mirror at Surface (PMAS)
Remember, the PMAS temperature is a surface temperature only – it is very hard to change the temperature of the whole primary by more than 1-2C per day. That is why it jumps up when the cooling is turned to sucking.
BUT, the fact that the temperature of the mirror varies inside to outside does not matter since it is zero-expansion (CERVIT). It is the surface temperature that is important – if too hot, then you get a thin layer of high turbulence.
Why don’t we use low dome temperature? Because that doesn’t tell us what is happening outside. And we are trying hard to make low dome = outside!

10. How to decide whether to switch?
Rule: Night temperature predicted to be colder? COOL MIRROR ALL DAY
Rule: Night temperature predicted to be much the same? SWITCH AT 4 PM
Rule: Night temperature predicted to be warmer: SWITCH AT 2 PM
Rule: If the previous night did not work out well, ADJUST THE ABOVE RULES
Other advice:
If the weather is very cold (e.g. below 8C) then you will probably need to cool more than the above rules indicate.
If the weather is very hot (e.g. above 15C) then you will probably need to cool less than the above rules indicate,
Particularly in winter time, look ahead for large predicted temperature changes. You may need to cool more (if getting colder) or less (if getting warmer).

11. Final Thoughts
We will try to automate this in future, but let’s see how well this works
Suggestions and modifications to the protocol are welcome – send to Alistair Walker and Tim Abbott