The following suite of diagnostic words provide easy access to specific areas within the kernel, via the operator port, to assist in both commissioning and fault finding:


displays the node name and message received by the kernel from the ethernet port. Employed for checking that the message has been passed on by the network layer as well as diagnosing kernel processing of that message.


displays all 6 entries in the monitor-mode request list MON-LIST along with their request type (1 or 2).


displays all 4 entries in the write-only status (immediate and delayed) request list STATREQ.


All the fields in the PRESENTSTATUS table for the specified mechanism are displayed in a tabular form. Each parameter is displayed using its correct numerical base along with its field identification label.


displays the number of resets and communication timeouts for all serial links in a tabular form, labelling each link with its corresponding module name. These values may also be transmitted across ethernet to the VAX System Computer by employing the health monitor status command, HMS200. A history may then be built up for each module, enabling suspect units to be weeded out of the system.


requires the mechanism mnemonic number on the stack prior to executing this word. It displays the work space RAMSLINK for specified serial link. Employed when faults occur on serial link e.g. #ERROR byte field can show framing, parity and overrun errors indicating wrong baudrates etc. See section on communication software.


requires the mechanism mnemonic number on the stack prior to executing this word. It displays the serial communication receive buffer for specified links. See section on communication software.

.MECHS lists all the instrument's mechanism mnemonics. Employed as an aide-memoire.


displays which serial links are faulty. This automatically occurs on powerup/reset of the 4MS but may be requested at any time through the operators port. No output given if all links are OK.

LOCAL Provides the ability to enter a single ethernet message (which may contain several instrument commands) via the operator port. It initially shuts the ethernet tasks down in an orderly fashion to prevent two sources of messages using the same internal buffer resources. The operator is prompted for the required message which is terminated by a carriage return. The kernel then processes the message in exactly the same way as for a message received via the ethernet port. Finally the ethernet tasks are reactivated followed by a NET101 execution to flush the network software layer. Under LOCAL operation status requests (immediate or delayed) will result in status replies being sent to the node who last sent a command via the ethernet port. It is therefore advised to omit all status requests when employing the LOCAL mode.

A shorthand version is available in the from | (pipe).

TRANSPARENT requires the mechanism mnemonic number on the stack prior to executing this word. The SMDM protocol header is displayed, informing the operator which box# and motor# the specified mechanism is attached to. The commands required to enable the echo mode and to terminate the TRANSPARENT mode are also included:

a) Transparent to Smdm C Motor 2

To enable echo : UNSEAL

To return to 4MS : 4MS

Additional mnemonic constant, BCR, is defined to gain access to the barcode reader module ( BCRM ).

The kernel shuts down all status MAINTAIN, JOB and ethernet tasks in an orderly fashion, then revectors the interrupt routine and enables the serial link interrupt ready for transparent operation. This state is indicated by

Application halted

being displayed when commands may now be entered via the operator terminal.

Refer to the corresponding manuals for description of the commands available for each module. On termination of transparent mode the interrupt routine is revectored and all the tasks are reactivated. Resumption of normal operation is indicated by the message

Application restarted

appearing on the operator terminal.

All modules employing the 6303 processor card ( SMDM and BCRM ) must be correctly sealed up by their word '4MS' before returning to normal kernel operation otherwise all subsequent communications with the module will result in continuous timeouts due to unexpected

strings being received.

A shorthand version of TRANSPARENT is available in the form TT.


Two sets of error codes are returned in the status message.

These are command errors and mechanism errors.


These are returned as the first parameter in any status message. They relate to errors in the command received from the network.

The code is returned as two hex digits and has the following meanings.

EPR# Meaning

01 Mechanism already busy

02 out of range parameter

03 Not used

04 Invalid format

05 Table full, cant put you on monitor mode list

06 Invalid function,,

07 Mechanism interlocked

08 Operation cancelled

09 Not on monitor mode list


These are returned in all status messages except 880's.

Mechanism errors relate to faults that occur with the mechanism when an operation ( e.g. a move ) is attempted. The error codes are returned in hex.

ERR# Meaning

01 Mechanical timeout. Mechanism has not reached target position/state in allotted time. Could be a mechanical problem, initialise mechanism to return it to a known state.

02 Communications timeout. Acknowledgement/response not received from SMDM/BCRM in allotted time.

Problem with serial link and/or module.

03 Undefined position. Mechanism has not reached target position. Switch values show target position has not been attained or, if mechanism has encoder, encoder reading outside of tolerance band for target position.

04 Un-zeroed. Mechanism not initialised. Must be initialised before MOVE operation can be executed.

05 Wrong module. Module name ( in SMDM status response ) is incorrect. This could be for one of two reasons.

i) The wrong SMDM is connected to this serial link. Put SMDM'sin correct places or reprogram offending modules.

ii) The SMDM BBRAM is corrupt, therefore the module name and probably stepcounts are also corrupt. In this case reprogram SMDM.

06 Invalid-data. Data returned over serial link has invalid characters. e.g. letters where numbers were expected. Usually refers to SMDM data. Examine data returned ( look at status buffer ) and if necessary reprogram SMDM. Can refer to rotary encoder data. e.g. encoder lamp has failed.

07 Wrong encoder. Not used on A&G.

08 Aborted. A pending operation has been aborted before it has started due to a STOP command being received.

09 Unreliable. Refers to barcode data. of three barcode reads only two concur. Barcode or reader possibly getting dirty.

0A Head absent. Refers to barcode data. Barcode reader head is not connected to BCRM.

0B Missaligned. Refers to barcode data. Reader head and barcode not correctly aligned.

0C SMDM negative acknowledgement #0. Module busy.

0D SMDM negative acknowledgement #1. out of range value or direction

0E SMDM negative acknowledgement #2. Invalid motor no. not 1 - 4

0F SMDM negative acknowledgement #3. Motor not initialised.

10 SMDM negative acknowledgement #4. !+LIMIT set to zero.

11 SMDM negative acknowledgement #5. Limit switch activated.

12 SMDM negative acknowledgement #6. Not set to cycle mode.



Mnemonic Mechanism

AFI Autougider FIlter wheel

AFO Autoguider FOcus

AGR AutoGuider Radial movement

AGT Auto-Guider Theta scan

CFC Comparison Filters, Colour wheel

CFN Comparison Filters, Neutral density wheel

CLP Comparison LamPs

FLF Fibre optic port Large Feed mirror

FOP Fibre Optic Port

FSF Fibre optic port Small Feed mirror

MCB Main Colour Barcodes

MFC Main Filters, Colour tray

MFD Main Filter access Door

MFN Main Filters, Neutral density tray

MNB Main Neutral Barcodes

TAC Television Aquisition/Comparison probe

TFI Television FI1ter wheel

TFO Tevevision FOcus

TRF Television Focal Reducer

TSV Television Slit Viewing probe


Mnemonic Reference

INS INstrument Status

MON MONitor mode

HMS Health Monitor Status (of serial links)

ALL ALL mechanisms



Acquisition and Guider Unit for the Cassegrain Focus of the 4.2m William Herschel Telescope

Peter A Ellis, Richard G Bingham and Susan P Worswick

Royal Greenwich Observatory, Madingley Road, Cambridge CB3 0EZ, England


The design and performance of the Acquisition and Guider Unit for the 4.2-metre William Herschel Telescope are described. We draw attention to the auxiliary foci provided, the field andslit viewing systems, calibration sources and particularly the r-theta scanning CCD autoguider probe. Engineering details are given.


The Acquisition and Guider (A&G) Unit for the William Herschel Telescope (WHT) is the third in a set of dedicated A&G systems, designed and built by the Royal Greenwich Observatory (RGO) for the Isaac Newton Group of telescopes situated at the Roque de los Muchachos on the island of La Palma. Its configuration is markedly different from the unit produced for the 2.5-metre Isaac Newton Telescope, but is similar in layout to that made for the 1.0-metre Jacobus Kapteyn Telescope. Of course, the WHT system is much larger than either and has more facilities.

The design of this piece of instrumentation began in parallel with work on the telescope itself, and a preliminary study of the requirements leading to an outline design was started in April 1980. As the unit was not required until the completion of the telescope and auxiliary instruments, the design was not frozen until April 1985. Prior to that, a draft Functional and Technical Specification was circulated to the astronomical community for comments, and feedback from astronomers was incorporated into the design.

A general view of the instrument is shown in Figure 1.


The A&-G unit has to provide for a number of different instruments at the f/11 Cassegrain focus of the WHT, with fields of up to 15 arc minutes diameter (201mm), and for each instrument theposition of-focus is 150mm below the mechanical interface. All the functions of the A&G unit are remotely controlled. The various facilities included are as follows:

To further increase the sky coverage, the guider probe scans part of the field on an r-theta system. The area scanned covers (half) an annulus with inner and outer radii of 110 and 150mm, and has an angular coverage of 180 degrees centred on the main axis. The extreme edge of this field is .vignetted by about 5 per cent. Thus the sky area scanned equals about .04 square degrees; this usually ensures that at least one star brighter than 13th magnitude is in the field at the Galactic Pole. The r-theta configuration was chosen so that the centroid of the image and the centre line of coma remain constant with respect to the CCD chip, and also the detector stays in focus on an annulus of the curved focal surface. The CCD housing is remotely focussable; this facility is mainly used to inspect out-of-focus images at different field points as an alignment aid. The CCD detector system is described earlier in these proceedings by Thorne et al. The positions of the CCD autoguider probe, the slit viewing system and the comparison lamp optics are shown in Figure 2. The same TV camera provides the acquisition view by means of the 45-degree flat mirror and focal reducing lens, shown in Figure 3. Polarisation calibrators. Four cells (in two pairs) contain types HNP'B and HN22 Polaroids. The cells can be manually set when inserted in the carrier, giving two planes of polarisation for each dichroic type. A further cell containing a calcite block is provided in the carrier as an alternative to the polaroids, e.g. giving broader wavelength coverage. 3. MECHANICAL DESCRIPTION

The basic case of the A&G unit consists of a cylindrical fabricated steel structure 150cm in diameter by 65 cm deep, split into two roughly equal sections, Upper and Lower. The sections are suitably flanged and gussetted for attachment to the telescope instrument rotator above and the instrument below. The upper section contains the Acquisition/Comparison Probe, Slit Viewing Probe, Small Feed Flat Probe, Fibre Optic (auxiliary focus) Feed mirror, Calibration System, TV camera and the focal reducing lenses. The lower section contains the autoguider assembly and the main filter slides. The instrument is shown in Figure 1.

3.1 Upper Section

The Acquisition, Slit Viewing, and Small Feed Flat Probes are of .similar configuration, consisting of a hollow steel quill, vee-grooved longitudinally on its outer surface to run in recirculating ball units. The in-out movement of the quill is accomplished using a recirculating ball nut and screw shaft driven by a stepper motor. A fail-safe magnetic brake is mounted on the motor's extension shaft to prevent the screw shaft running back under gravity load (due to its high efficiency of about 97 per cent). The ball nut floats between disk springs to ensure preload when the quill reaches its mechanical end stops. A steel slug on the floating nut actuates proximity switches at extremes of the quill stroke, disabling the motor and applying the brake.

The main mechanisms in this section are listed and described here.

1. The Fibre Optic Feed Flat carriage runs in recirculating linear bearings mounted on a subplate. This subplate is fixed to the lower plate of the upper section. The drive system is similar to that used for the probes, extremes of stroke being determined by mechanical stops and encoded by proximity switches.

2. The Calibration system is a sub-assembly bolted to the side of the case. It directs comparison light from an integrating sphere via two fixed imaging lenses (fused silica) and retractable flat into the interfaced instrument. Two filter wheels are interposed between the sphere and the lenses. The position of the filter wheels is encoded by three proximity switches per wheel and BCD flags.

3. The TV Camera mounts on a vee bearing slide and is driven by the same type of actuator as the probes. The focussing movement, one centimetre, is monitored by a DC/DC Linear Variable Differential Transformer (LVDT) and limited by proximity switches. Bellows fitted between the camera and the case prevent light leaks into the TV.

4. The two focal reducing lens systems are mounted on a motorised ball slide system, similar to that used for the probes, and positioned in front of the TV Camera. (As explained, these lenses are selected when required to obtain a larger field in the Acquisition or Slit Viewing modes.) A third position of the slide, with no optics, provides direct viewing with the smaller field for both modes. The two extreme positions of the slide are defined by mechanical stops, the central position by counting motor steps from one end.

3.2 Lower Section

This section is a hollow toroid, scaled by a brush between the autoguider "theta" scan bearing and the underside of the lower plate of the Upper section. Filtered air is sucked into this chamber and is exhausted to the dome to remove heat which would otherwise rise into the light path. (Heat is generated by the CCD Peltier cooler, drive motors etc.) A general plan of this section including the autoguider assembly is shown in Figure 4. It contains the autoguider assembly and large filter carriers. These are further described here.

1. The autoguider probe contains the optics and is attached to a cradle which in turn locates the CCD housing. The cradle runs on linear roller bearings which provide the 4mm focussing movement by way of a stepper motor driven screw and anti-backlash nut. Focus position is monitored by a DC/DC LVDT. The assembly described is itself mounted within a steel fork shaped fabrication by another set of linear roller bearings, to provide the 40mm radial displacement. This movement is motor driven and encoded. A six position filter wheel assembly is positioned between the optical train and the CCD.

The assembly which contains the focussing and radial motions is mounted on a further mechanism to provide the 'theta" motion. It is attached to a 130-degree sector of the top surface of a large needle roller radial/thrust bearing. This is a precision bearing and its peak to valley radial runout, or total indicator reading, is 0.006mm. A wormwheel sector is mounted on the remaining 230 degrees. This drive sector engages a worm. driven by a stepper motor. The worm to sector engagement is determined by the worm mounting plate which is pivoted to allow contact adjustment. This mounting plate also serves as the mount for a 17 bit absolute optical encoder and its anti-backlash drive worm wheel. Gear ratios have been chosen such that 300 teeth in 180 degrees of the sector engage the single start worm which in turn engages the 328 tooth encoder drive. Allowing for a possible 1 bit loss on the encoder, one step of the stepper motor is equivalent to 0.035 arc sec on the sky.

2. The other major items in the lower section of the case are the filter slides. Each main filter slide runs on miniature recirculating slideways, and is driven by an Acme screw shaft with a plain phosphor bronze nut. The drive shift is driven by stepper motors. No brake is provided on the drive and the screw to nut friction is sufficient to hold the carriage from running back under gravity. Positional encoding is by step counting from a datum proximity switch at one extreme end of the travel. Bar code reader heads are positioned to decode the bar codes on the filter cells sequentially from the parked position. The relation between the reader and the filter position is, of course, stored in the control computer.


Assembly and testing was first carried out on a telescope simulator at the home base (the Royal Greenwich Observatory). Next, during the period May/June 1989 a comprehensive commissioning programme was undertaken on the telescope. Initially the A&G Unit was tested independently, and subsequently in conjunction with the ISIS spectrograph mentioned above. The main aim was to test the functions and stability of the mechanisms. The mechanisms all functioned as they had done during tests on the telescope simulator at the RGO. This confirmed

our belief that such pre-shipping tests are vital in ascertaining the integrity of mechanics, electronics, optics, and software.

All the extendible-retractable probes except (initially) the slit viewing probe exhibited sub-arc second stability of stellar position on the TV camera or at the auxiliary focus at varying telescope orientations. Problems were found with the slit viewing probe, which has a highly eccentric loading on the support quill. Further investigations were carried out at the first post-commissioning opportunity, and as a result a small modification was made to enable sub-arc second shifts to be obtained on this probe also.

The Unit has been used routinely since commissioning and apart front minor software bugs has exceeded expectations.


The authors wish to thank the many other members of their instrumentation group without whose efforts this project could not have been carried out, and Prof. A. Boksenberg, the director of the Royal Greenwich Observatory, for his interest and encouragement.


1. P. A. Ellis, '2.5-metre Telescope: Cassegrain Aquisition and Guidance Unit', La Palma Technical Manual No. 23, Royal Greenwich Observatory, November 1983

2. P. A. Ellis, 'One-.metre Telescope: Cassegrain Aquisition and Guidance Unit', La Palma Technical Manual No. 31, Royal Greenwich Observatory, November 1983 .

3. D. J. Thorne, N. R. Waltham, G. M. Newton, I.G. van Breda and M. Fisher, 'CCD Guidance Sensors for theWilliam Herschel Telescope',Proc. SPIE Vol 1235, 1990. (Tucson Feb 11-17 1990 - these proceedings).

4. P. A. Charles et al., in preparation 1990.