The Intermediate dispersion Spectrograph and Imaging System (ISIS) is mounted at the Cassegrain focus of the 4.2m William Herschel Telescope
ISIS is a high-efficiency, double-armed, medium-resolution (8-121 Å/mm) spectrograph, capable of
long-slit work up to ~4' slit length and ~22" slit width. Use of dichroic filters permits
simultaneous observing in the blue and red arms, with a ~200Å wavelength range of reduced throughput at the dichroic crossover from reflection to transmission. Both arms are optimised for their respective wavelength ranges.
Linear and circular spectropolarimetry, and imaging polarimetry modes are also
The CCD currently in use on the blue arm of ISIS is a
thinned, blue-sensitive EEV12, an array of 4096×2048 (13.5 micron) pixels.
This is the default device for the ISIS blue arm. The default detector for the ISIS red
arm is RED+.
It is a red-sensitive array of 4096×2048 (15.0 micron) pixels with almost no fringing.
An Image Slicer was commissioned in semester 2014A, and is currently offered on a shared risks basis. The image slicer limits loss
of light at the entrance slit of the spectrograph, allowing the maximum spectral resolution of the spectrograph to be attained with greater throughput in modest seeing conditions.
For fast or faint-object spectroscopy, two EMCCDs
available for both arms of ISIS. They provide high-speed spectroscopic
capability (to about 10Hz) with very little dead time and essentially zero
read noise. EMCCDs have distinctive features and optimising their use depends
critically on the scientific goals.
In 2012 the ISIS 4MS controllers were replaced by a PLC
(Programmable Logic Controller) system which offers improved reliability and
faster mechanism movements.