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 (8-120 Å/mm) spectrograph, capable of
long-slit work up to ~4' slit length and ~22" slit width. Use of dichroic slides permits
simultaneous observing in both blue and red arms,
which are optimised for their respective wavelength ranges.
Linear and circular spectropolarimetry and imaging polarimetry are also available, as well as
a drift-scan mode for high-time resolution monitoring programmes.
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 chip for the ISIS red
arm is RED+.
It is a red-sensitive array of 4096×2048 (15.0 micron) pixels with almost no fringing.
For fast or faint-object spectroscopy, two EMCCDs
available for both arms of ISIS.
In February 2012 ISIS 4MS controllers were replaced by new PLC
(Programmable logic controller) system that is more reliable, increases a
speed of all mechanism movements and offers continued maintenance
and better transparency in identifying problems.