Introduction

The MaxDome II controller system provides complete automated control of a telescope dome and associated shutter. For lower-cost installations, rotation-only operation is also supported.

 

Rotation

The MaxDome II Rotation Controller card directly drives a 12V DC motor through a set of electromechanical relays. The recommended power source is a trickle-charged 12V Gell Cell, which will allow safe automatic closure of the dome in the event of a power loss.

 

The Rotation Controller connects directly to a Windows-based Personal Computer via a serial RS-232 link. The MaxDome driver in the computer handles all communications to the controller card.

 

The Rotation Sensor counts rotations of the motor shaft. The "tics" from this sensor are used to detect when the dome has rotated to the correct position. One side of the shaft is colored black, the other is white; in this way, the sensor will count tics twice per rotation. This is sufficient to position the dome very accurately. Some installations have used black and white stripes around the perimeter of the dome or other schemes; the main criteria is that tics are cleanly produced and occur no faster than about once a second during rotation.

 

In order to set the dome to a specific azimuth, a home sensor must be installed.  The home sensor is a simple magnetic switch similar to those used in home burglar alarms. Whenever the home position sensor is detected, the dome position is reset to the user-supplied azimuth value on the next tic.

 

Manual operation of the dome rotation is supported through an external hand paddle switch (not included). When the dome is operated manually through this switch, the dome keeps track of the current position, eliminating the need to re-home the dome after manual movement.

Shutter

A second identical card, the Shutter Controller, is installed on the dome itself for controlling the shutter motors. This card is powered by a solar-charged battery, usually a 12V Gel Cell, and a wireless communications link is provided to the rotation card. This means that there are no electrical connections to the rotating dome, eliminating a major reliability concern.

 

One or two shutters may be controlled, depending on the dome configuration. If two shutters are used, they are sequenced so that they can overlap when closed. Usually the "upper" shutter overlaps the "lower" shutter, to provide a better weather seal. If desired, the MaxDome software driver can be configured to open just the upper shutter, for special situations (high winds, light pollution).

 

Each shutter must have limit switches at each end of the travel. These limit switches connect to the controller card, and go open circuit when the shutter hits the end of travel. The card will automatically deactivate the control relay when the end of travel is detected.

 

Communications between the two controller cards is provided via a magnetic induction link. Every two seconds messages are passed back and forth between the cards. When the PC commands a shutter operation via its serial connection to the rotation card, a message is passed to the shutter card to initiate shutter movement. The shutter card reports back the current shutter status at all times.

 

For observatories where there is some risk of physical collision between the telescope and the shutter mechanism, MaxDome II can be configured to automatically Park the dome prior to operating the shutter. The dome will only be allowed to rotate again under computer control once the shutter is fully closed or fully open.

Remote Operation

Additional features are provided to protect your telescope equipment against failures and power outages, to help reduce the risk of weather damage to the observatory equipment in remote observing situations.

 

Auxiliary close inputs are available on both cards; a contact closure to ground will initiate shutter closure. This allows for direct connection of cloud and/or rain sensors, such as a Boltwood Cloud Sensor.

 

If the communications link between the rotation and shutter cards fails for more than ten minutes (perhaps due to a power failure), the shutters will automatically close. Because the Shutter Controller cannot know whether the dome is parked or not when the link is down, this feature can be optionally disabled.

 

If communications are lost between the Rotation Controller and the PC, the dome will automatically close after ten minutes. This protects the observatory in the event of a power outage or computer failure. If so configured, the dome will automatically park before the shutter is moved, which is important if there is any risk of a collision between the telescope and the shutter actuator.

 

It is recommended that the rotation controller also be powered by a battery, either AC charged or solar charged. This ensures that the dome will be safely closed automatically in the event of a complete power failure, without the need for an external UPS unit.