Additional Instrument Design Details
COS has two channels, the Far Ultraviolet (FUV) covering 115–205 nm and the Near Ultraviolet (NUV) covering 170–320 nm. All COS optics are reflective (except for the bright object aperture filter and NUV order sorters) to maximize efficiency and to avoid chromatic aberration. Principal COS observing modes are summarized in table 1.
Light from the Hubble Space Telescope enters the instrument through either the Primary Science Aperture (PSA) or the Bright Object Aperture (BOA). 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). Both apertures are oversized (2.5 arc second clear aperture) permitting more than 95% of the light from a point source to enter the spectrograph.
After passing through the PSA or BOA the light travels to one of the optics on the first of two optic select wheels, either one of the three FUV diffraction gratings or the first of the NUV collimation mirrors (table 1), depending on whether an FUV, NUV, or target acquisition channel is selected. All optics on the first wheel have an aspheric profile to correct for the Hubble spherical aberration.
The FUV channel has two medium and one low resolution spectroscopy modes. 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 diffracted light is focused onto a 170x10 mm cross delay line microchannel plate detector. 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 NUV channel has three medium and one low resolution spectroscopy modes as well as an imaging mode with an approximately 1.0 arc second unvignetted field of view. The NUV channels utilize a modified Czerny-Turner design in which collimated light is fed to the selected grating, followed by three camera mirrors that direct the diffracted light onto three separate stripes on a 25×25 mm Multi Anode Microchannel Array (MAMA) detector. The imaging mode is primarily intended for target acquisition.
Read more about this topic: Cosmic Origins Spectrograph
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