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The RGO Spectrograph

This spectrograph was designed and built by the Royal Greenwich Observatory (RGO) and is mounted at the f/8 Cassegrain focus of the AAT, where the scale is 6.7"/mm (i.e. 1"=150um, so a 300um slit corresponds to 2" on the sky). There are two cameras: a 25cm f/1.67 Cassegrain-Maksutov system and an 82cm focal length f/5.5 spherical mirror. A parabolic collimator is used with the 25cm camera and a specially figured collimator is used with the 82cm camera. For most purposes the 82cm camera has been superseded by the UCLES spectrograph. Light can be fed into the spectrograph either through a slit or via a fibre optic system which can collect light from up to 60 objects over a 40' field of view. The fibre input systems (Autofib and FOCAP) will shortly be superseded by the 2dF spectrograph. A number of extra functions are available on request. Time-resolved observations are supported. Low resolution observations of the full optical spectrum can be obtained by observing simultaneously with FORS. Spectropolarimetric observations are made using the Polarimeter Waveplate Module. Imaging with a small field is available using the auxiliary focus.

The spectrograph can give dispersions in the range 2-140Å/mm, and operates over a wavelength range of 3100Å to 11,000Å. Twelve different diffraction gratings are currently available, and Table 5.1 lists their basic properties. The 300B grating has very similar properties to the 250B grating, and slightly better efficiency. With the 25cm camera, gratings may be used either blaze to collimator or blaze to camera. Blaze to collimator results in slightly larger wavelength coverage and much better slit projection factor. Blaze to camera provides better dispersion but requires a narrow slit for high resolution. The program RGOANG, available on the telescope and Epping VAX computers and at Starlink nodes in the UK, calculates the grating angle, dispersion and slit projection factor for any specified wavelength. Note that significant vignetting occurs for the 25cm camera for grating angles less than 10 degrees.

 
Figure 5.1: The RGO spectrograph

A brief overview of the spectrograph is given below and in Figure 5.1.

For further details of the spectrograph and its operation, see AAO UM 2: The RGO spectrograph.

 
Table 5.1: Gratings available with the RGO spectrograph



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