Telescope Dome Ventilation

The 0.9-m at sunset with 7 of the 11 ventilation louvers visible.

In August 1994, dome vents were added to the 0.9-m dome to provide much improved flow-through ventilation as part of an image quality improvement project. The 11 individually controllable vents are located opposite the dome slit to allow the dome air to flush several times per minute under typical wind conditions. Rapid flushing helps eliminate warm air pockets from forming in the dome, which in turn, reduces convection that would otherwise compromise image quality (that is, "seeing").

Normally, some complement of vents will be opened at the beginning of the night, around sunset, and closed when the telescope is closed. Under typical observing conditions, this is done in the "automatic" mode described below. If the dome is iced up, the vents may need to be broken free.

Based on limited testing, the dome flushes adequately with about half the vents open during mild wind conditions. The current default configuration is for vents 2,3,4,9,10,11 to be open.

Under the following conditions the observer should restrict the air flow by closing some or all of the vents.

High gusting winds that cause the telescope to bounce.
The dome shutter is opened during the day and the outside temperature is higher than inside.

Selective operation of the vents is done by setting the switches on the control panel located near vent #7, opposite the dome shutter, as described in the "detailed operation" section below.

If the air is very calm, less than about 5 mph, you may wish to open additional vents.

Automatic Operation of the Vents

The 6 default vents will open (or close) automatically when the dome shutter is opened (or closed) if the control panel switch is set to "Automatic". This is the usual mode of operation and will be ideal most of the time. The 6 desired toggle switches in the "Together-Off-Separate" switch row must be set to "Together".

Disabling Operation of the Vents

There may be times when the extra air flow is undesirable. If you want the vents to remain closed when the shutter is open, the control panel main switch ("Auto-Off-Manual") should be set to "Off".

Detailed Manual Operation of the Vents

In this mode, one can open or close any or all of the 11 vents individually. The vent control panel consists of four types of switches as described below. The term "ensemble of vents" refers to those vents whose "Together-Off-Separate" switches are set to "Together".

Auto-Off-Manual - this is the main control switch at the top of the control box.
- Auto - Enables automatic open/close of the ensemble of vents when the dome shutter is opened/closed. This disables manual open/close operations from the "Open-Close" main switch (see next major bullet).
- Off - Disables both automatic and manual open/close operation of the ensemble of vents.
- Manual - Disables automatic open/close of the ensemble of vents when the dome shutter is opened/closed. Enables manual open/close of the ensemble of vents with the "Open-Close" main switch.
Open-Close - This switch operates on the ensemble of vents. It is a "momentary" switch which must be held while the vents continue to open or close.
Together-Off-Separate - This is a row of 11 switches, each having 3 positions, to control the detailed operation of an individual vent.
- Together - Include this vent as part of the ensemble.
- Off - Remove this vent from the ensemble and disable it from manual open/close operations.
- Separate - Remove this vent from the ensemble and enable it to be opened/closed manually by its individual open/close manual switch (see next major bullet).
Open-Close (row of 11) - If the individual vents "Together-Off-Separate" switch is set to Separate, then it can be opened/closed by its individual open/close switch from this row (towards the bottom of the control panel).