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The Cassegrain acquisition and guiding (A&G) box

At the rear of the INT primary mirror cell is the Cassegrain turntable which can be rotated by 365 degrees, and onto which the A&G unit is mounted. The Cassegrain A&G Unit of the INT has been designed primarily for use with the IDS and 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. 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.

The following functions are provided:

• Acquisition: Figure 3.2 shows the details of the optical system for viewing the acquisition field. A Westinghouse extended S20 TV camera views a nominal 1.8 1.8 arcmin element of the 20 arcmin diameter unvignetted Cassegrain field (on- or off- axis). The TV display includes labels giving orientation and scale.

  A useful facility available is the option to store the TV images, so as to take advantage of periods of exceptionally good seeing to record, for later use, high spatial resolution images of the objects under investigation; the images (called ``TV archive images'' can also serve as a reference to show the slit position for extended objects. At the user's request, they can be obtained by the Telescope Operator.

  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 blue (BG28), green (BG38), red (RG630) and clear (UBK7). (See Figure 2.5).

• Slit viewing. A simple movement of the flip mirror allows viewing of the light reflected from the slit jaws using the acquisition TV. This facility gives visual confirmation of the position of the target on the entrance aperture of the instrument, as well as an approximate estimate of the seeing profile.

• Autoguider. An autoguider is available which allows offset guiding using stars anywhere in a 90 arcmin field. The area available for guide stars is shown in Figure 3.3. Guide stars should have magnitudes in the range 7 < m < 13.

  

  For blind setting of the telescope, for locating the spectrograph slit relative to a guide star, or for controlling small telescope offsets to make a spectroscopic map of an extended object, guide stars should be selected in advance of the observing run using Figure 3.3. If this is done, and the (, ) of the guide stars is known, then it is possible to save considerable amounts of observing time. 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. The Telescope Operator will normally set up the autoguider.

• Comparison lamps. The A&G Unit includes reference sources for calibrating the wavelength scale and the photometric response of the attached instrument. The lamps are housed in an integrating sphere which can contain up to two hollow-cathode lamps and one tungsten lamp; more than one source at a time can illuminate the slit. Illumination of the slit by the comparison lamps is achieved by movement of a dedicated mirror.

  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. Not all the lamps are well documented. Arc maps are available for the Cu-Ar (La Palma Technical Note 52), Cu-Ne (La Palma Technical Note 35) and Th-Ar lamps (AAO User Manual 4; ESO Scientific Report 6). See Appendix C.

  The comparison lamps have two dedicated ND filter wheels, providing a wide range of ND filtering (La Palma Technical Note 22). These are rarely needed when observing with the CCD.

• Filters.

  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. This is only to be performed by the support staff.

  The colour filters normally available are UG1, BG28, BG38, GG385, WG360, GG495, RG630, RG695 and RG830. The wavelength dependence of transmission of these filters is given in Figure 2.5. 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.