Faint Object Spectrograph (FOS-1)

Last commissioning:

Designed and built by: Durham University and RGO.

Description: The Faint Object Spectrograph (FOS) is an efficient, fixed-format spectrograph with a CCD detector used for low resolution (15-20 Å FWHM) spectrophotometry over the wavelength range 4000-10000 Å. The optical design is a Schmidt camera without a collimator in the diverging beam from the Cassegrain focus. The dispersion is provided by a transmission grating and a cross-dispersing prism, which give a two-order format. The principal advantage of the FOS is its high throughput - 12% for the combination of atmosphere, telescope and FOS at 7000 Å - some three times larger than that of the IDS with the same detector. It is highly appropiate for the study of emission-line objects such as quasars, radio galaxies, HII regions and planetary nebulae, but less useful for absorption-line objects (particularly those with narrow features) owing to its low resolution. There are four operating modes: beamswitching for point sources; beamswitching with a larger throw for extended objects; long slit (first order only) and 25 arc sec slit (both orders).

Some scientific highlights:

• High redshift quasars and very faint stars (1991)
• Spectrally resolved eclipse maps of an accretion disk (1994)

More information: The Faint Object Spectrograph for the 2.5-m Isaac Newton Telescope (MNRAS, 227,909)

More photos of this instrument: http://www.ing.iac.es/PR/archive/int/instruments.html