Acquisition and Guider (A&G) Box

The Acquisition and Guider box has been designed primarily for use with the Intermediate Dispersion Spectrograph (IDS) and the Faint Object Spectrograph (FOS). Nevertheless, the overall philosophy of the design has been to incorporate a range of facilities which can be used by almost any instrument operating at the Cassegrain focus.


A Westinghouse extended S20 cathode high gain TV camera views a 1.8 * 1.8 arcmin segment of the 20 arcmin diameter Cassegrain field in the ON-AXIS (field viewing)  or OFF-AXIS (slit viewing) positions. The TV is mounted on an linear worm drive X-Y carriage driven by stepper motors and can gain access to any part of the Cassegrain field. For slit viewing, the TV is left in its centred position.

The TV is fixed to an adjustable focus plate on the carriage which allows for re-focusing if the TV is changed or another instument is mounted at CASS which requires a different focus. The TV is nominally focused on the slit jaws of the IDS. A filter wheel is available for the TV camera allowing up to four choices of filters.

To acquire an object, the TV acquistion probe; which is fitted with a FLIP MIRROR, can be in one of two positions:

In front of the TV camera is the photocathode shield. When the comparison arc lamps are used, the comparison mirror passes light onto the slit jaws which could damage the TV camera. For protection, an automatic movement of the photocathode shield blocks off light from reaching the TV.


A Peltier cooled CCD autoguider is available which allows offset guiding using stars anywhere in a 90 square arcmin field. The autoguider is mounted on a similar X-Y carriage as the TV.  As per the TV camera, it is possible to refocus this for the occasions when other instruments with different focal lengths are used at CASS. A filter wheel carrying 4 filters is available for the autoguider. Care must be taken when acquiring a guide star in that the autoguder probe does not vignette the field. A warning message appears on the A&G-IDS mimic display if this happens.

Guide stars are selected these days from X-Y data supplied by the Guide Star Server (GSS).

Comparison Lamps

This unit consists of an integrating sphere with three tubes which holds the comparison arc lamps. The two outer lamps can be removed and changed for a different type. When observing with IDS, the usual setup is with a copper argon (CuAr) and copper neon (CuNe) lamp. The central lamp is tungsten which produces a white light continuum.  This is used for flat-fielding and some other aspects of detector setups.

In front of the integrating sphere output port are two filters wheels known as AFARC and BFARC. The commands that move these being: compfilta and compfiltb respectively. These can be moved either individually or together and provide a range of neutral density filtering. When taking arc exposures, the COMPARISION MIRROR mirror moves into the field and directs light from the output port of the integrating sphere onto the slit jaws of the IDS.

The lamps are driven from circuit boards mounted in a small box on the side of the A&G box. A meter (with a 3 position switch) allows the lamp currents to be measured or adjusted. Depending on the lamps in use, a preset pot on the edge of the board allows the current to be changed. The current of an arc lamp is nominally between 5 and 15mA. A lamp on/off toggle signal (which works when lamp current is sensed) is sent to the MMS crate.

Filter Slides

Two filter slides are fitted at the bottom of the A&G  box. These are the ABOVE SLIT colour and neutral density filters:  ASCF and ASND. These provide light attenuation for bright objects or can filter out wavelengths which are too strong or are not wanted. Some of the colour filters are band pass filters   i.e. Only letting light pass through at a particular wavelength. There is also a clear psoition.

Geneva Mechanisms

The filter trays and comparison mirror use a Geneva mechanism to insert or retract the slide carriage. The stepper motor shaft (driven through a gearbox) carries a cam carrying two roller bearings placed 180o apart. The mirror slide has semi-circular cutouts (scallops) machined along an edge in which the rollers engage sequentially. Thus as the cam turns, the rollers pick up in the scallops and move the slide in or out in a series of steps. Metal flags mounted on the carriage pass through Hettich slotted inductive switches which determine its position.

A&G Box Controller

The A&G box controller is an MMS (Micro-Modular microprocessor System) based around the 8 bit Motorola 6800 processor and was designed at RGO (circa 1982). The software was written in 6800 assembler code and the object code is held in EPROMS on the ROM boards within the crate.

All mechanisms that use stepper motors are controlled by double height eurocard boards (one for each motor) which contains the motor coil phase switching and power (4 power transistors) circuits. A 6821 PIA chip handles the input signals from the mechanism's associated position sensors (Hettich slotted inductive switches) and limit switches and also provides the direction and speed TTL toggles to drive the motor circuits. The ramp up/down of the stepper motors is done using a hardware circuit.

Serial communications to the 6850 ACIA board in the MMS is via an RS232 link (9600 7E1). There are two ports, one for engineering the other for computer control.  Generally only the computer port is used. The RS232 connection runs to a panel mounted on the CASS electronics rack where control can be switched between two ports on a PORTSERVER, port 13 which the ICS computer accesses over the network, or port 11 which a Virtual PC accesses over the network for engineering use. This Virtual PC is a Linux machine running on taurus with a dosbox running the CDSAI Engineering software. The  Virtual PC can be accesed by starting a VNCViewer session on your computer, then connect to: idsconsole:1 with the password: ********  A small box mounted on the panel fitted with a switch selects to where the comms line is routed. See the picture below or  Cassegrain Interconnections drawing for more details.

IDS A&G eng switch

n.b. Although only full I/O control with the Virtual PC is enabled with the switch in the ENG position.  When control is in the ICS position, the Virtual PC can still read incoming data and will show if a mechanism has moved. A useful diagnostic.

Here is an image of the Engineering software user interface:

A&G Eng interface

In case you see the DOS C: prompt instead of the above screen, you can start the program by typing: cdsai <return>. If the screen is present but not updating and no data scrolling then call someone from the CFG group to restart the virtual machine idsconsole:1 ( If the screen is present but the Error Status Window says that there  is no respons and there is no data scrolling in the Data Flow Window, the instrument is switched off. This can be switched on remotely by entering the INT Cass APC webserver.

The original instructions on howto operate the instrument with the CDSAI program can be found here , this document is a scanned version in pdf format, as the original document was lost.

Refer to the INT Technical Manuals, nos:  41  42  43  44   for full details and circuit drawings for the A&G box.
Refer to the INT Technical Manuals: 54 and 55   for the circuit drawings for MMS boards.

Last updated: 27 Apr 2015  rjp