Cosmic Origins Spectrograph

The Cosmic Origins Spectrograph (COS) is a science instrument that was installed on the Hubble Space Telescope during Servicing Mission 4 (STS-125) in May 2009.

The FUV channel can work with one of three diffraction gratings, the NUV with one of four, providing both low and medium resolution spectra (table 1).

In addition, COS has a narrow field of view NUV imaging mode intended for target acquisition.

Our understanding of the origin and evolution of the cosmos critically depends on our ability to make quantitative measurements of physical parameters such as the total mass, distribution, motions, temperatures, and composition of matter in the Universe.

For distant objects, some of these properties (e.g., motions and composition) can only be measured through spectroscopy.Ultraviolet (UV) spectroscopy provides some of the most fundamental diagnostic data necessary for discerning the physical characteristics of planets, stars, galaxies, and interstellar and intergalactic matter.

By determining the redshift and line width of the intervening absorbers, COS will be able to map out the temperature, density and composition of dark baryonic matter in the Cosmic Web.

All COS optics are reflective (except for the bright object aperture filter and NUV order sorters) to maximize efficiency and to avoid chromatic aberration.

The BOA introduces a neutral density filter to the optical path that attenuates the light by approximately a factor of one hundred (five astronomical magnitudes).

The FUV channels are modified Rowland Circle spectrographs in which the single holographically ruled aspheric concave diffraction grating simultaneously focuses and diffracts the incoming light and corrects both for the HST spherical aberration and for aberrations introduced by the extreme off-Rowland layout.

The FUV detector active area is curved to match the spectrograph's focal surface and is divided into two physically distinct segments separated by a small gap.

The Cosmic Origins Spectrograph on its handling cart in the Spacecraft Systems Development Facility cleanroom at the Goddard Space Flight Center
COS Optical Layout. FUV and NUV channels initially share a common path. The first optic is either a concave, holographically ruled diffraction grating that directs light to the FUV detector (red) or a concave mirror directing light to the NUV gratings and the NUV detector (purple). The green colored ray packets represent the FUV optical paths, and blue colored ray packets represent the NUV optical paths. A wavelength reference and flat field delivery system is shown at top left (orange ray packets) and can provide simultaneous wavelength reference spectra during science observations.