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PRIME FOCUS CAGE

The `cage' is in fact a solid-walled, slightly oval-sectioned cylinder, approximately 0.72m in diameter and 2m deep, mounted on its own interchangeable top end. The cage is open ended so the observer can enjoy the sky. The most obvious feature within the cage is the prime focus (PF) camera head and pedestal, (see separate section) mounted centrally , and a chair mounted on the wall. Less apparent are several important switches and controls which are also mounted on the wall and are identified in this section.

Cage Rotation

This aspect of observing at prime focus can cause problems for the unprepared. It is remarkably easy to become disoriented in semi-darkness as the telescope slews, the dome rotates, and the cage can be in motion, while the only other obvious reference point, the camera head, can be in any one of four positions.

When the telescope is slewed to the PF access platform where the observer can climb aboard it has a ZD of about 77° degrees and is pointed towards the north. The chair is normally on the north side of the cage with its backrest at the lowest point on the cage wall. This is the `park' position. Slip inside the cage and ask for the telescope to be driven to zenith. On the right-hand wall of the cage is a handset with three buttons marked +, PARK and -. The + button drives the cage clockwise from the northern PARK position around the optical axis through east and south into the west, 270°  from the park position, BUT NOT BEYOND. The - button drives the cage a little more than 90°  into the west from park, but again, not beyond. The PARK button returns the cage to its northern parked location from any position.

NOTE :  The telescope will normally slew from pointing in the south west to PF access directly; the cage however must move through S and E on its way to the northern park position, an operation which takes much longer than the telescope slew. It is normal and safe practice to ask the night assistant to slew to prime focus access from the south west or west via the zenith.

The cage can be rotated during an exposure without seriously upsetting telescope guiding or tracking, but as a precaution the shutter should be closed during rotation. The height of the chair above the cage floor can be adjusted by a short lever on the left of the squab. The angle of the squab itself is adjustable by a wheel on its right; however be careful to avoid rapid movements in the cage as these can be detected on the autoguider readout and may affect image quality.

Guide/Focus Handset

A handset with a 4 - position selector switch and 4 buttons is attached to the cage wall, though it can be hand-held if required. The selector switch can be set to SET, GUIDE, OFFSET, FOCUS.

Appropriate rates for SET, and GUIDE and angles for OFFSET are loaded into the telescope control computer from the control console by the night assistant, and by pressing the buttons the observer can move the telescope in the selected mode. The relative orientation of the N, S, E and W buttons is appropriate for a direct view of the sky. When viewing the field in the focal plane or through the prime focus guide eyepiece, there is an odd number of reflections, so the view is reversed. If manual guiding is required, it may be helpful to hold the handset with its face away from you so the sense of the buttons is consistent with the movement they actuate.

In the FOCUS position TWO buttons must be pushed simultaneously to move telescope focus in or out, fast or slow. To focus, the observer must request that the night assistant sets the console buttons to FOCUS MANUAL.

Intercom

This two-way system is normally left open so that the observer can speak to the console without touching the microphone. At the console, the night assistant can only reply by pressing a button on his microphone which overides the observer's conversation. Three-way conversations with people at coudé or in the Cassegrain cage are possible but confusing.

Telephone

Can be used to dial anywhere on the mountain top and to STD numbers within Australia, not beyond. A list of emergency numbers is provided.

Slew Inhibit

A pre-panic button which stops fast large-scale motion of the telescope.

Emergency Stop

A panic button causing immediate and sudden shutdown of the telescope. Recovery from this shutdown can be lengthy and this control is to be used ONLY if life or limb is in danger.

Autoguider ON-OFF

Applies power to the autoguider electronics but does not turn on the EHT to the phototube unless the AUTO/MANUAL switch on the camera is selected to AUTO. This switch is usually left on all night when the autoguider is in use.

Light Switch

Operates a dim light within the cage, which may be used with the autoguider active but not during an exposure. The brightness of the light can be controlled by rotatable polarising filters in the lamphouse.

Filter Holder Box

Takes four filters mounted in 10 x 10 inch ( i.e. standard) holders.

Wall Pockets

Designed to carry four (or with difficulty six) 10 x 10 inch plate-holders, including the knife-edge plate. There are also numerous smaller pockets for flash lights, eye-pieces, gloves etc.

Music Speaker

Connected to Console ``hi-fi''system which consits of a cassette and CD player and radio. Bring your own tapes or discs; as with choice of object, so with music selection; the observer selects, but it is the night assistant who presses the buttons.

Nitrogen Flow

Nitrogen is piped into the cage (and thence to the space between plate and filter). An adjustable flowmeter in the line should be opened and the flow checked. The normal flow rate is 2 litres/min. The flexible tubing carrying the nitrogen and flowmeter is loose in the cage and can become snagged on projections as the cage is rotated.

Prime Focus Checklist

When the PF top end has been fitted to the telescope the following must be checked either by staff members or by the observer. The items to be checked by the observer are marked with an asterisk. Some of the items in this list are described in detail in subsequent sections.

It is also prudent to know where the following are stored:

NOTE:  Smoking and drinking are not permitted in the prime focus cage.



next up previous contents
Next: THE PRIME FOCUS Up: No Title Previous: OPTICS

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