At the rear of the INT primary mirror cell is the Cassegrain turntable which
can be rotated by 365 degrees, and onto which an A&G
unit is mounted. The Cassegrain A&G Unit of the INT has been
designed primarily for use with the Cassegrain Intermediate Dispersion
Spectrograph (IDS) and Faint Object Spectrograph (FOS-1). Nevertheless,
the overall philosophy of the
design has been to incorporate a range of facilities which
would be required by most instruments operating at the Cassegrain
focus. Figure shows the layout of the A&G unit.
The full field is 20 arcmin in diameter, but the
comparison lamps and filters are only useful over a more restricted field
of 4 arcmin, matching that of the IDS.
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Figure: Three
views of the Cassegrain acquisition and guider unit of the
INT, showing the autoguider, TV camera, comparison system and
filters. (a) and (b) Elevation, (c) Plan
The following functions are provided:
A filter wheel is available for the TV camera allowing up to four circular filters to be loaded. These filters are not immediately accessible, as it is envisaged that most users would not require them to be replaced. The filters currently available are BG28, BG38, RG630 and clear (UBK7).
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Figure: The
optical system for viewing the acquisition field at the
Cassegrain focus of the INT.
Ideally guide stars should be selected in advance of the observing
run using GSS (section ).
Alternatively, an overlay showing the IDS
slit and the area within
which guide stars may be selected can be superimposed on the TV image
of the finder field. A guide star can then be selected visually,
and the A&G Unit TV and/or autoguider probes can be directed to
that position.
Care must be taken to avoid selecting a guide star
so positioned in the field that the autoguider probe obstructs the on-axis
star-beam.
A filter wheel is available for the autoguider beam allowing up to four circular filters to be loaded. These filters are not immediately accessible, as it is envisaged that most users would not require them to be replaced. The filters currently available are BG28, BG38, RG630 and clear (UBK7).
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Figure: The
area within which guide stars may be chosen for use with
the autoguider at the Cassegrain focus of the INT. Stars
may be selected in the shaded area. If the autoguider probe
is moved closer to the centre of the field, then the slit is
obscured. Partial obscuration occurs in the area shaded with
dashed lines. The field outside the circle
is heavily vignetted, but bright stars may
be usable. The orientation of the diagram on the sky is
given for the instrument rotator in PA = 0. At other PA's,
the diagram must be rotated anticlockwise by the PA.
The standard set-up consists of a tungsten lamp as continuum source, and copper-argon and copper-neon discharge lamps for wavelength calibration. A further selection of the following discharge lamps is offered: Th-Ar, deuterium Fe-Ne, Fe-Ar, Al/Ca/Mg-Ne, Na/K-Ne, Cu-He, and helium. Arc maps are available for the Cu-Ar (La Palma Technical Note 52), Cu-Ne (La Palma Technical Note 35) and Th-Ar lamps (La Palma Technical Note 91).
The comparison lamps have two dedicated ND filter wheels, providing a wide range of ND filtering (La Palma Technical Note 22).
Two main trays are available for ND and colour filters common
to the star and lamp beams. The trays can be
loaded with up to 5 filters each (plus a clear position).
The filters are 45 mm 60 mm
3 mm in size.
The two main filter trays are easily
accessible, allowing rapid replacement of the trays'
contents. Care must be exercised when removing the filter trays, since
the two trays (colour and ND) are not interchangeable.
The colour filters normally available are UG1, BG28, BG38, GG385, GG395,
GG495, RG630, RG695 and RG830. The wavelength dependence of transmission
of these filters is given in Appendix . The following ND filters
are available; ND = 0, 0.3, 0.6, 0.9, 1.0, 1.2, 1.5, 1.8, 2.0 and 3.0.