Acquisition and Guider Unit for the Cassegrain Focus for 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 and slit 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 andbuilt 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 the position 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 andthe 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.