30 min before sunset:

1. Check temp4m and weat
2. Orient the shutter eastward to make sure no direct sunlight will enter the dome.
3. Open the dome (before opening mirror cover!)
4. Open the mirror cover
5. Turn OFF mirror cooler.
6. Turn ON sucker & extractors
7. Turn OFF dome floor stirring fan
8. Make sure all passageway doors are closed, especially the glass door in front of the lift at GR floor, the cryocooler compressor and pump rooms.
9. Turn on CORR (See why [1])

Just after sunset:

• Open the lateral sliding doors.
  Note: if wind is greater than 15 mph on average wind buffet on the telescope may produce elongated images in which case the sliding doors should be closed. Under any circumstances, the lateral doors must be closed if the wind speed is greater than 25 mph.
• If working at f/8:
  • Turn ON the chimney fans
  • Turn F/8 OFF [Ed: WHY?]
  • Check collimation [2]
• If working at Prime:
  • Nothing special for Prime

Blanco 4-m Daily Procedures - During the Night

• Watch the weather, make sure the bad weather rules [3] are followed.
• Check the status of the mirror cooler and the extractor if the seeing is very poor.
  A 1° temperature difference between the mirror surface and the ambient air will create additional mirror-seeing (typically up to 0.5" extra for 1° difference). Thus our goal is to maintain the mirror temperature within 1° of the predicted temperature of the following night.
• If wind speed is greater than 15 mph on average wind buffet on the telescope may produce elongated images in which case the sliding doors should be closed.
• Occasionally do a temp4m and check that:
  • All for mirror sensors are within 0.5-1°C of each other
  • Mirror temperature, dome temperature and outside temperature are within 1°C of each other
  • R.A. bearing hydraulic oil cooling is working (if not, the oil temperature will rise quickly to around 30°)
• Report any problems.

Blanco 4-m Daily Procedures - End of the Night

1. Turn OFF the sucker and extractors
2. Turn on the mirror cooler
3. Turn ON dome floor stirring fan
4. If working at Cass, turn off the chimney fans
5. Close the mirror cover (before closing dome!)
6. Close the dome
7. Close the lateral sliding doors
8. Make sure an End-of-night report has been filed.
9. If this is the last night of a run, remind the observer to complete an End-of-run report.

Blanco 4-m Daily Procedures - Environmental Control

• The mirror cooling should NOT be turned off for instrument change except for safety reasons. That is, if the noise or temperature is such that you are likely to make a mistake. If ever you do turn it off, DON'T FORGET to turn it back on!
   
• The temperature control system automatically controls mirror temperature according to the following logic:
  1. T_mirror < T_goal - 2.5°: glycol valve closed (but ambient air is still blown onto the mirror)
  2. T_goal + 1.7° < T_mirror < T_goal -2.5°: glycol valve partially opened
  3. T_mirror > T_goal + 1.7°: glycol valve fully opened
  4. If T_mirror < T_{dew_point} + 7°: the glycol valve will close (safe limit to prevent condensation)
   
• The cooling of the C floor and M floor in the dome is always ON. It is currently regulated manually.
   
• The large stirring fan on the M floor (inside the dome) should always be ON when the dome is closed and OFF when the dome is opened. This fan improves greatly the air circulation inside the dome. It can be turned off during instrument change or any other works around the telescope if the noise bothers you. But turn it back on when you are done!
   
• Oil Cooling: The cooling of the oil circulating under the R.A. rear and front pads is activated automatically when the pumps are turned ON by Observer support at sunset. The oil glycol valve is controlled (range is 1-2.25 V) by an equation involving the low dome and oil glycol temperature sensors. It is important that the oil cooling be working: if it is not, you can diagnostic it in "temp4m" (the "before pad" and "glycol" entries on the OIL line will quickly indicate around 30°).
   
• The Chimney Fans are located inside the chimney at the level of the primary mirror and suck ambient air into the mirror cell so as to form a laminar downward flow in the chimney. This is to prevent Cass cage heat sources from creating convection in the light path in the chimney. At all Cassegrain foci (f/8), Observer Support is to turn ON the chimney fans before sunset. The switch is on the power supply on the old console room desk. The night assistant must turn it OFF at the end of the night.
   
• Air conditioning - Always leave on. Do not use heat sources at level M, MZ and C. Always keep passageway doors closed, especially the glass oor in front of the lift at GR floor, the cryocooler compressor and pump rooms). In general, respect the signs in the building.

Calculating the risk of condensation

Each material has its own radiation cooling property (which is not easy to calculate by the way). Just know that if there is no wind, the radiation cooling is low and surfaces will not equilibrate thermally with the ambient temperature very quick. Also, a surface, even with moderate ability to radiate, looking into a "cold" sky can cool down several degrees below ambient temperature. Fortunately, a mirror radiates very little. Nevertheless, when cooling the mirror during the day, the mirror temperature is usually several degrees below ambient temperature and there are risk of condensation. The control loop will turn off the cooling when the difference between the mirror temperature and the dew point shrinks to only 2 degrees.

Knowing the Relative Humidity (RH in %) and the ambient temperature (T_a) of the air, one can calculate the dew point temperature (T_d) and determine whether a surface at temperature T_s can become wet or not :

• if T_s<T_d: surface becomes wet
• if T_s>T_d: surface keeps dry

I think "dew point" refers only to the case where RH=100%; when RH<100% we might have to talk about condensation point. We won't do that distinction anymore in the text.

How to use the table?

From your ambient temperature on the X-axis, go up to intersect the curve corresponding to the RH, then move horizontally onto the left to read the dew point on the Y-axis. You can then check if your surface is hotter or colder than this dew point value.

Example: Ta is 10°C, RH is 80%, Td is 6.5°C. So a 5°C mirror would be wet!

[4]

Calculation of dew point:

• The dew point is the temperature at which the partial pressure of ambient air (PA) is equal to the partial pressure of saturated water vapour (PS).
• The pressure of saturated water vapour is tabulated versus temperature at sea level in thermodynamic books. Numbers from -10 to +35°C are used to determine empirically the relation PS = f(T) and adjust it for Tololo's elevation.

PS = (5.10^-7.T³ + 10^-5.T² + 5.10^-4.T + 0.0061) / 1.294

•  PV = RH * PS where RH is between 0 and 1
• We can then empirically determine T_d = g(PV) for different values of RH with the following equation:

T_d = 6.10⁸.PV⁵ - 10⁸.PV⁴ + 6.10⁶.PV³ - 193789.PV² + 3957.9.PV -14.911

•   The fit is quite good, yielding accuracy for Td of less then 0.3°between 0 and 20°C.