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.