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William Herschel Telescope

ISIS: Intermediate dispersion Spectrograph and Imaging System

— Low- and medium-resolution slit spectrograph —

First light (at ING): Autumn 1989.

Designed and built by: It was produced by the RGO and Oxford University in a collaborative effort with the design and construction carried out dominantly at the RGO.

Description: This versatile Cassegrain instrument consists of two intermediate dispersion spectrographs which can be operated one at a time or simultaneously, and are separately optimized for the blue and red spectral regions.

The main goals are to provide: a range of dispersions between 130 and 16 Å/mm in first order, corresponding to a range in resolving power respectively between 800 and 7000 when using a narrow (0.5 arc sec) slitwidth; maximum possible throughput; high operating efficiency in both the setting-up and observing modes, with the capability of operating the blue and red spectrographs as a single unit; rapid interchange between ISIS and the Faint Object Spectrograph, which share the same slit assembly; long slit (4 arc min), multi-slit and fibre optic feed capabilities; a cross-dispersed mode capable of recording the wavelength range from 3000 to 1100 nm with a resolving power of about 2000; facilities for spectro-polarimetry; and full capability of being remotely operated.

One of the ISIS capabilities is to convert the spectrograph into a spectropolarimeter, capable of measuring linear and circular polarization. The polarimetric capability became available for testing in the spring of 1990, and as a common-user option (for linear polarization) by the end of that year.

ISIS can also operate with fibres. First commissioning run was in April 1992. Two different bundles are used: a 61-fibre multiobject 400 micron bundle and a 125-fibre fixed 200 micron array. In either case the output end of the fibres is arranged in a linear array at the multislit position of ISIS.

Circular spectropolarimetry (using the quarter-wave plate) and imaging polarimetry with ISIS were first commissioned in 1993.

The ISIS multislit facility was withdrawn in 1993.

A new dichroic filter replaced the former one in 2005.

First use of an L3CCD on ISIS in 2007.

Time allocations:
Nights scheduled since semester 2007B

Instrument information:

Research impact:

Scientific highlights (65)

Public outreach:

Media releases (37)


Photo archive (10)

Video archive (2)

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Last modified: 08 November 2023