Design

In the following description the telescope is assumed to be parked pointing to the zenith, with the spectrograph mounted. Figure shows schematically the main optical components of the spectrograph. These are:

• Filter slide. A filter slide common to both star and calibration beams is available above the slit. The slide is operated manually and is located within a simple spacer between the spectrograph and the A&G Unit.

  The following Schott glass colour filters are available: BG28, BG38, UG1, GG385, GG395, GG495, RG630, RG695, RG830. Filters are 25 mm in diameter and 2.5 mm thick.

  Two ND filters, 25 mm in diameter and 2 mm thick, are available: ND=1.42, 3.0.

• Dekker and slit assemblies. At the entrance to the spectrograph are a dekker mask, the slit assembly and a shutter located in front of the slit. The shutter is controlled via the CCD software. The dekker has a star slot 1.7 mm (23.5 arcsec) long, projecting to 0.475 mm at the detector, an `arc' position and normal long slit (out).

  The slit assembly is interchangeable so as to allow the use of a number of slits of fixed width. Two slits are available, of widths 73 m and 110 m (1.0 and 1.5 arcsec respectively). For a typical slit-to-detector reduction factor of about 4.1 (weakly dependent on the angle of incidence on the grating) these slit widths project to approximately 18 m and 27 m at the detector. Access to the slit assembly and optical components is by removing the side-panel. The slit position angle can only be set manually by rotating the instrument turntable with the telescope at zenith park.

• Collimator. The collimator, an off-axis paraboloid, is located at the base of the pipe projecting from the bottom of the spectrograph. The focal length is 750 mm, giving a beam of diameter 50 mm. The collimator is interchangeable and is offered with two coating options which maximize reflectivity in the blue and red spectral regions respectively.

• Gratings. There is a choice of four first-order gratings, all working blaze-to-collimator. The most important characteristics are summarized in Table below. Gratings are 57 mm 57 mm 10 mm in size. The gratings are mounted with three M3 bolts onto a turntable, and a tangent screw on each grating assembly ensures precise location of the grating relative to the turntable. Grating angle is selected by manual rotation of the turntable using a calibrated knob and a scale with vernier. The ``Richardson-Brealey Spectrograph Manual'' (La Palma User Manula XI) gives details of the dependence of central wavelength, dispersion and slit-to-detector reduction factor on grating angle (Note - the new tables should be used as given in the supplement).

    
  Table: Characteristics of gratings for the Richardson Brealey Spectrograph
  

• Camera. The camera is an f/4 off-axis Maksutov system, comprising a two-element corrector, and spherical mirror and a field flattener with adjacent prism to direct light to the focus on the front face of the spectrograph (refer to Figure ). The focal length is 208 mm. Like the collimator, the camera mirror and two correcting elements are interchangeable and two sets are available with coatings which maximise efficiency in the blue and red spectral regions respectively. The red set of optics makes better use of the red sensitivity of the CCD and can be used to the blue cut-off with some loss of efficiency. Focussing of the spectrograph is achieved by movement of the camera mirror, controlled by a micrometer screw on the camera mirror assembly. A Hartmann mask, located on the slide in front of the collimator, allows the best focal position to be set with an accuracy of 50 m.

• Detector. The spectrograph has been designed to work in conjunction with a CCD camera. Any chip available generally for the JKT may be used.

  
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Figure: The JKT Richardson-Brealey Spectrograph