10 The Primary Mirror Air-Conditioning System
The primary mirror is cooled to reduce the effect of "mirror seeing". When the mirror is warmer than the ambient air tube currents can deteriorate the seeing, adding 0.4" for every 1 C. The mirror should ideally be slightly cooler than the ambient air but when this condition cannot be met it is best to have the mirror considerably cooler than ambient. The best seeing is obtained when the dome air is continuously flushed with the ambient air. Make sure that the louvers and garage door are fully open and that the passageway doors are closed.
The temperature of the telescope and the ambient dome air is monitored by seven T-type thermocouples. These are either glued to the glass mirrors, bolted to the telescope structure, or, in the case of the ambient dome temperature sensor, placed on a long probe midway between the primary and secondary mirrors. In addition, the relative humidity inside the dome is monitored by an Omega HX92-V temperature-compensated relative humidity transmitter which employs a thin film polymer capacitor. Information from the sensors is read by an Omega 12-bit interface card.
The following points are monitored:
The relative humidity of the dome near the east declination bearing
The air-conditioning unit is located outside to the south-east corner of the dome. This unit cools the refrigerant, turning it into a liquid. The cold liquid is then pumped to a series of coils located under the dome floor. A fan draws ambient dome air in through a floor register (located to the east of the telescope), passing across the cold coils, and reaches the primary mirror through a flexible insulated hose. A deflector at the end of the hose prevents ``cold spots'' from forming on the mirror. The air at the mirror-end of the hose is approximately 12 degrees C cooler than ambient during the summer months, and only 4 degrees cooler than ambient during the coldest winter months.
The air conditioning system is controlled by a GATEWAY2000 4DX2-66 PC located in the computer room, and a switch panel which displays the system status.
10.3.1 Turning on the cooling system.
The computer is normally left on at all times. In the event of a computer-crash hold down the Ctrl and Alt keys and press the Del key (on the numeric keypad). This will re-boot the machine. Follow the instructions on the monitor to start the software.
The program takes about 30 seconds to load, and starts by displaying the temperature and humidity readings in a digital form.
The following MENU OPTIONS are available:
ESC - press the ESC key to get the main menu
1 - digital output
2 - graphical output
F - focus tool
H - toggle heater control on/off
I - set A/C cooling coil minimum temperature
S - set safety margin for cooling near the dewpoint
T - set the primary mirror temperature
X - exit the program
? - help
Please leave the system in menu 1 or 2, which are the only menus in which the data logging can take place. All temperatures are in degrees C. You can directly access a menu option by pressing the menu key while in one of the display modes.
10.3.2 Menu Option 1: Digital Output
The analog inputs are read every second and are displayed on the monitor (Figure 10.1). Seven temperatures, the relative humidity, and the dewpoint are monitored. The desired temperature of the primary is also shown. You can change this using menu T. The system will attempt to maintain the desired temperature UNLESS one of the following conditions exists:
The desired temperature of the primary is shown next to the current temperature and can be changed with menu option T.
The current dewpoint safety margin is shown next to the relative humidity, and can be changed with menu option S.
The telescope focus changes with temperature of the truss arms. The focus tool is shown next to the truss arms and is turned on or off with menu option F which calculates the focus as the temperature changes.
The safety margin for cooling the mirror is shown next to the dewpoint. It can be changed through menu option S.
The cooling coils of the A/C unit can freeze over. When they do the ice blocks the airflow and the output air temperature is closer to the ambient than to the coil temperature. To stop this from happening the unit is set to turn off COILDELAY seconds after reaching MINCOIL degrees, as displayed to the side of the A/C cooling coils. Change it with menu option I (for Ice over!.... be careful). It then idles for 180 seconds before restarting.
Use the relative humidity sensor to keep track of the conditions during the night. When the relative humidity exceeds 85% a warning is issued to close the mirror covers . When it exceeds approximately 90% another warning is issued and the status of the heater is reported. If the heater function is enabled (through menu option H) it is used to warm the primary.
A complete description of each menu option follows (in alphabetical order).
10.3.3 Menu Option 2: Graphical Output
Shows the ambient dome temperature (red), mirror temperature (green), and desired set point temperature of the primary (blue), over a 3 day period (max).
Use the graphs at the end of each night to determine a set point for the next night. PLEASE BE CONSIDERATE AND DO THIS EVEN IF IT IS YOUR LAST NIGHT.
New data points appear at 15 minute intervals starting on the hour.
Under ideal conditions the set point will equal the desired temperature of the primary. However, when the relative humidity is high the set point may be temporarily forced to a higher value.
Note that when observing starts in the early evening the mirror warms up as it is exposed to the evening air, whose temperature is falling. When set correctly, the primary should remain below the dome ambient for the whole night, being about 0.5 degrees below at the end of the night.
10.3.4 Menu Option F: Focus Tool
Calculates the telescope focus as the temperature of the truss arm changes. Bring a star into sharp focus for a given filter and then enter the value of the telescope focus displayed on the TELESCOPE MONITOR. As the temperature of the truss arms fall the value to set the telescope for best focus is reported.
10.3.5 Menu Option H: Toggle Heater Control On/Off
A heater can be activated when the mirror temperature is below the dewpoint. It keeps the mirror approximately 1 degree above the dewpoint. The normal mode is to have the heater OFF. When it is toggled to ON the safety margin for cooling (menu S) is forced to 2 degrees to stop the heater and air-conditioner from fighting each other. The current status of the heater is reported in the main display (Menu Option 1) when the relative humidity exceeds 85%.
10.3.6 Menu Option I: Set A/C Cooling Coil Minimum Temperature
The air-conditioning unit was optimally designed to cool a house to around 20 C on a warm summer day. We are using it to cool the mirror to near freezing on a cold winter's day. As a result, the cooling coils can ice over. When this occurs the air cannot pass the coils and the mirror starts to warm up in the ambient air. To prevent the coils from freeing the outside compressor shuts off COILDELAY seconds (default 120) after reaching MINCOIL degrees C (default 0 C), but the inside fan remains on to warm them up again (deadband 1 C).
Why have a timer and a minimum temperature? We would like to obtain a cold air output of 0 C, and to achieve this the coils must be at about -2 or -3 C. If you simply set MINCOIL to -3 the coils may be below freezing for sufficient time before reaching MINCOIL thereby allow freeze-up to occur. For this reason, it is NOT recommended to set MINCOIL below 0 C. Rather, alter the value of COILDELAY. The coils take about 1 minute to warm up from -4 to +1, during which time the compressor is disabled. During this time the COILS and FAN lights should be on.
10.3.7 Menu Option S: Set Safety Margin for Cooling near Dewpoint
To stop condensation from forming on the mirror you should keep it above the dewpoint temperature. The current value of the dewpoint is given in the main display (Menu Option 1). It can change by many degrees in only a few minutes and so we recommend that you do not cool the primary to closer than 2 C of the dewpoint. You can alter this safety margin, but take note of the precautions listed when this menu is called.
10.3.8 Menu Option T: Set Primary Mirror Temperature
First use menu option 2 to examine the temperature trends over the last three days. At night, the mirror should be marginally below the ambient dome temperature. The system is not active once the mirror covers are open for observing and the mirror will start to warm up in the early evening air, while the ambient air starts to cool. You should therefore cool the mirror to 2 to 3 C cooler than the expected ambient temperature at the start of the night. Listen to the weather forecast for any major cooling or heating trends which might affect your decision.
Note the current temperature difference between the ambient and the mirror. This will give you a vertical scale for the graphs. New points appear on the graph at 15 minute intervals, starting on the hour.
If you are unsure what temperature to aim for, err on the cold side. However, a mirror which is substantially colder than the ambient is also likely to introduce poor seeing.
During times of high relative humidity you can specify a desired temperature which is below the dewpoint. The system reports whether the primary is being cooled, is idling below the safest minimum temperature, or is being heated. The system tracks the dewpoint and when conditions improve the mirror will be cooled and the message changes to ATTEMPTING TO MAINTAIN PRIMARY AT tt.t Degrees C. The desired temperature of the primary is shown next to the current temperature of the primary.
10.3.9 Menu Option X: Exit
You should normally leave the program running, unless bad lightening storms are in the area and you are forced to close down all the computers. When the computer responds with the OK prompt enter the command SYSTEM followed by a <CR>. Turn the computer and the monitor off. To restart the system turn the monitor and computer on and then follow the instructions on the screen.
10.3.10 Menu Option ?: Help
Displays the online help utility. Additional guidance is also given within each menu option.
10.4 THE SWITCH CONTROL PANEL
The switch control panel (Figure 10.3) reports the current status of the system.
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