Cassegrain acquisition & guider Unit

The layout of the Cassegrain A&G Unit is shown in Figure . It is described in some detail in La Palma Technical Note 56. Briefly, the unit has a full field of 15 arcminute diameter at the nominal telecope focus, 15 cm below the A&G to instrument interface. The following functions are provided

  
[ TIFF ]

[ TIFF ]

[ TIFF ]

Figure: Three views of the Cassegrain acquisition and guider unit for the WHT. (a) Plan, showing the autoguider, cable wrap and filter slides. (b) Elevation, showing TV, autoguider and comparison system. (c) Elevation, showing the large and small feed mirrors and the auxiliary port.

• Acquisition. This is carried out via an extendable probe carrying a mirror feeding a Westinghouse ISEC TV camera. When used direct, this provides a 1.5 arcmin field at the telescope scale of 4.51 arcsec mm. It is possible to interpose a focal reducing system, which provides a larger field of 4 arcmin at a scale of 12 arcsec mm. The TV camera is provided with a filter wheel with six filter positions. The filters normally mounted are B (BG28), V (BG 38), R (RG 630), CLEAR (UBK7) and EMPTY. The B, V, R, and CLEAR filters all have the same thickness, and the TV can be focussed independently to compensate for different filter thicknesses.

• Slit Viewing. The ISIS slit, which is tilted by 7.5, can be imaged via a one to one transfer lens and flat into the same TV camera used for acquisition, with field sizes identical to those provided by the acquisition system. Note however that the Autoguider (see below) can vignette the return beam from the slit under some circumstances.

• Autoguider. The autoguider consists of a CCD detector head fed by a right angled prism, and focal reducing optical system with a field diameter of 1.8 arcmin. The present CCD has 385x288 22 micron pixels covering a field of 1.5x1.1 arcmin at a scale of 0.23 arcsec/pixel. The centre of the autoguider field rotates about the centre of the main field at a radius of 110 to 150 mm (8.2 to 11.2 arcmin) and the entire probe assembly has a radial displacement of 40 mm. The extreme edge of the autoguider field is partially vignetted, but only by about 5%. The autoguider has an azimuthal scan of 180, so the total area scanned at a field scale of 4.51 arcsec mm equals 152 square arcmin or 0.04 square degrees. This gives a good chance of finding a star brighter than 11th magnitude at the galactic equator or 13th magnitude at the galactic pole (C.W.Allen, Astrophysical Quantities, publ. Athlone Press, 1976). The first 35 of the range of travel of the autoguider probe causes vignetting of the slit-viewing optics and is not normally used.

  The autoguider is provided with a filter wheel with six filter positions. CLEAR (UBK7), B (BG28), V (BG38), I (RG630), OPAQUE and EMPTY. The B, V, I and CLEAR filters all have the same thickness. The autoguider can be focussed independently to compensate for different filter thicknesses.

• Comparison lamps. A calibration system is provided consisting of an integrating sphere into which light is fed directly from two hollow cathode lamps (Cu-Ar and Cu-Ne), and light from a further six lamps (a choice from Cs-Ne, Fe-Ar, Fe-Ne, Th-Ar, Al/Ca/Mg-Ne, Na/K-Ne, and Deuterium) imaged via fused silica lenses onto 3 mm diameter fused silica light guides. Any combination of lamps may be used simultaneously, but the throughput of the light guides is very low (about 10%). The exit pupil of the integrating sphere is fitted with an obscuring disk to simulate the telescope entrance aperture obscuration, i.e. the secondary mirror structure. The reverse side of the acquisition mirror is used to feed the calibration light to the instrument. This enables simultaneous object acquisition and spectral calibration.

  The light guides incur losses of about a factor of ten, so the lamps which are fed directly are very much brighter. Lamps can be interchanged by technical staff upon request.

  Two eight-position filter wheels are provided for the comparison system. ND and colour filters similar to the set used on the INT A&G comparison system are provided to give a range of ND from 0 to 5. Two filter positions in each wheel are available for colour filters, BG24, GG375 and GG495 filters are normally mounted.

• Filters. Two filter slides, situated below the autoguider assembly, provide colour and ND filtering. Each slide carries five filters in cells. The filter cell carrier may be removed and alternative cells fitted. The filters have a maximum diameter of 85 mm. Neutral filters ND0.3, ND0.9, ND1.2, ND1.8 and ND3.0; and colour filters UG1, BG38, GG495, RG630 and WG320 are normally mounted, but others are available mounted in spare cells, and can be fitted on request. The cells for the two filter slides are identical.

• Polarisation calibration. For polarisation module calibration, a special double cell containing two dichroic polymer filters (i.e. Polaroid) with their polarizing axes mutually at right angles, may be fitted to the carrier.

• Auxiliary focus. A large mirror may be inserted to feed to a focal plane at right angles to the main telescope beam, and outside the A&G box, to enable the use of fibre-optic aperture plates and fibres to feed a spectrograph or other instrument. The full 15 arcmin field is available. Guiding in this configuration is not possible with the internal autoguider due to obscuration by the feed mirror, so coherent fibre bundles will have to be used for guiding.

  An alternative mirror (small feed flat), may be extended from the opposite side of the case, coplanar with the fibre-optic feed mirror, to direct light to a CCD camera or other small instrument at the auxiliary focus. A six-position filter wheel for 50mm filters and a shutter are provided for use with a CCD. The field available is 4.5 arcmin and the autoguider may be used.

The recommended method for finding guide stars is to select them from the Space Telescope Guide Star Catalogue (GSC) using the GSS software described in section . The coordinate conversions between position and probe coordinates are given in ``The Guide Star Search Software: Preliminary User Guide, by R.A. Laing.