- 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 slit of ISIS, which is tilted by 7.5 degrees, 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. The Autoguider
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 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 percent. The autoguider has an azimuthal scan of
180 degrees, 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.
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
Cu-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. 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
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,
and the cells all carry discrete bar coding for filter identification.
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 and
shutter is provided for use with a CCD.
The field available is 4.5
arcmin and the autoguider may be used.