Chapter 1. The RGO Spectrograph

Sections: General Description | Available Detectors | Special Observing Modes

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1.1 General Description

The Royal Greenwich Observatory (RGO) Spectrograph is mounted at the Cassegrain (f/8) focus of the AAT, where the plate scale is 6.7 arcsec/mm. It has two cameras; an 82 cm focal length f/5.5 spherical mirror and a 25 cm f/1.67 Cassegrain-Maksutov system. The Newtonian collimators are of 120 cm focal length and give a 15 cm beam. A specially figured collimator is used with the 82 cm camera and a parabolic collimator with the 25 cm camera. A schematic diagram of the system is presented in Figure 1.1. The light path is shown in Figure 1.2. The spectrograph can be rotated to any angle.

The spectrograph is capable of giving plate dispersions in the range 2-140 Å/mm over a wavelength range of 3100 Å to 11000 Å. At present ten different gratings are available and are interchangeable during the night. The normal mode of operation of the RGO spectrograph is with the 25 cm camera. Once the instrument of choice for high resolution work, the 82 cm camera has been superseded for most purposes by the UCLES spectrograph (AAO UM 25.3: The UCL and UHRF echelle spectrographs ). However, for some projects the higher spatial resolution of the 82 cm camera make it the more appropriate instrument.

In normal operation the slit width is continuously adjustable from 10 µm to 2.5 mm, equivalent to 0.067 to 16.75 arcsec on the sky. A variety of slit lengths can be selected using a series of dekkers above the slit. The maximum slit length is 38 mm (4.2 arcmin).

A variety of neutral density and colour filters are available both above and below the slit. Remotely controlled Hartmann masks in front of the collimator are used to focus the spectrograph. Continuum lamps and a variety of emission line sources  (most commonly CuAr) are available for calibration purposes. These are located both within the spectrograph and in the telescope's central baffle tube, locally referred to as the chimney, where they illuminate an optical diffuser. Control over all normal spectrograph functions is carried out remotely using the RGO console in the control room. Acquisition can be made using a camera in direct view and slit view. Autoguiding is used for long exposures.

Note that it is planned to decommission the RGO spectrograph at the end of Semester 2003B.

Figure 1.1 Schematic diagram of the RGO Spectrograph, as viewed from beneath.
Schematic of RGO

Figure 1.2 Light path through the RGO spectrograph.
Light Path of RGO

1.2 Available Detectors

The RGO uses the current portable detectors also in use at Coude and Cass. At the time of writing, the detectors used with the RGO spectrograph are charge-coupled devices (CCDs). A range of CCDs are available:
Device Format Pixel Sensitivity Comments
EEV2  2k x 4k  13.5 µm  Blue  Best in blue, heavy fringing in red 
MITLL2A  2k x 4k  15 µm  Red, and some blue  Best for general optical coverage 
MITLL3  2k x 4k  15 µm  Red  Best in red.
Not currently available.
Tektronix  1k x 1k  24 µm  Blue and some red  Better in blue than MITLL2A, less fringing than EEV2 

New detectors are regularly introduced, so check the instrumentation pages for information on the current suite of detectors.

1.3 Special Observing Modes for the RGO

There are two additional modes which are available for use with the RGO spectrograph.

1) Time Series

Time-resolved spectra can be obtained using a special mode of the observing and CCD control software designed to minimise readout time. CCD overheads can be set at a fraction of a second with minimal spatial information.

2) Spectropolarimetry

The Polarimeter Waveplate Module can be used with the RGO to measure linear or circular spectropolarimetry. The module is used in conjunction with a calcite block, located below the RGO slit, which acts as the polarisation analyser. The spectropolarimeter is described in detail in a separate manual (AAO UM 24: Spectropolarimetry at the AAT), and will not be discussed further here. 

Sections: General Description | Available Detectors | Special Observing Modes
Previous: Title Page | Next: Planning the Observing Run | CONTENTS

Ray Stathakis