- 14 Mar 2013
Mirror Ventilation Operating Guidelines
The purpose of MV is to uniformly control the primary mirror temperature such that the temperature of the whole mirror is driven to the target temperature defined in the PMC GUI. Normally we desire the mean mirror temperature to be driven to that of the current ambient conditions. However, if the enclosure is closed we drive the mean mirror temperature to the value we anticipate the ambient temperature to be at the time the enclosure is to be opened.
The trick here is to predict the desired mirror temperature an hour or two before you intend to open the enclosure. You can use the weather forcast and the current ambient conditions as a guide, but this requires experience. It is best to error on the side of lower temperatures for two reasons: 1. the temperature normally falls after dusk, and 2. mirror seeing is worse when the mirror is two degrees above that of ambient, vs. two degrees below that of ambient. If the mirror temperature is known to be significantly higher than the predicted ambient temperature at dusk, it may be necessary for the day crew to start thermal conditioning the mirrors before the OSA is awake.
In automatic mode (PMC GUI), MV drives the mean temperature of each mirror to the value detected by a number of temperature sensors located near the mirrors, and thus the mirrors are driven to the chamber temperature. If the enclosure is open, this is probably the mode you would like to use. However, if the enclosure is closed, you can use manual mode (PMCGUI->Ventilation subGUI: set "Target" temperature, click "Manual", click "Update") to define a fixed target temperature that differs from the chamber temperature. It is safe to set a target temperature that differs from that of the the chamber by as much as 10 degC. In manual mode, it is necessary to pay close attention to MV to ensure the mirror is safe.
When the enclosure is open, the mean temperature of each mirror cannot differ from that of ambient by more than 10 degC. Furthermore, if the mean temperature of a mirror is more than 5 degC from that of ambient, MV must be operated.
Each PMC GUI is capable of displaying the mirror temperature distribution. Under ideal conditions, the peak-to-valley temperature range should not exceed 0.2 degC and differences higher than 5 degC are considered unsafe. Under these conditions, MV should be turned off and the enclosure must be closed.
MV starts to become ineffective when the ambient temperature falls below ~ -10 degC. It is recommended that the MV continue to be operated, though you can expect the mirror seeing to deteriorate as the temperature error grows. MV will continue to ensure uniform mirror temperature, even when the mean temperature error is large.
Dew Point and Rain/Snow/Fog
The operating temperature of MV can approach to within 2 degC of the dew point. However, the performance begins to diminishes when the operating temperature approaches within 10 degC of the dew point. These limits apply to the duct temperature rather that that of the chamber.
It is recommended that a one-hour defrost cycle be conducted each afternoon, to remove accumulated ice before nightly operations begin. This can be performed by the day crew before the telescope is handed over for nightly operations. Unfortunately it is not obvious how to induce a defrost cycle, go to ECSGUI->Mirror Ventilation->Settings Panel and change the "Maximum Pressure" to 0. Once MV starts to defrost, you can set it back to the nominal value of 0.4 hPa.
Under normal operations, MV will handle defrosting without intervention. During the defrost cycle, the blower will be shut off for 30 minutes, while the heat exchangers are thawed out. Normal operations will resume automatically when the defrost cycle is completed.
MV draws its air from the chamber, rather than from outside. However, there are still cross-ventilation blowers that draw air from the outside. For the time being, these blowers use paper filters that can be damaged by water. Thus, please do not operate MV when it is actually raining or snowing. You are free to operate MV in fog.
There is some concern about the impact of MV induced vibrations on AO science operations. On those nights, MV should be operated until the actual science observations begin.
For the first few hours after starting MV, ECS normally reports frequent "supply temperature out of range" warnings. It is necessary to clear these warnings before they become alarms and shut the system off. The state should be checked often. Note that these messages are not currently logged in LSS.
LBC is particularly sensitive to the thermal condition of the primary mirrors. It is important to start MV early when LBC is to be used at the start of the night. Again, this requires a manual target to be set in the PMC GUI.