Instrument Overview
The Cosmic Origins Spectrograph is an ultraviolet spectrograph that is optimized for high sensitivity and moderate spectral resolution of compact (point like) objects (stars, quasars, etc.). COS has two principal channels, one for Far Ultraviolet (FUV) spectroscopy covering 115–205 nm and one for Near Ultraviolet (NUV) spectroscopy spanning 170–320 nm. 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.
One key technique for achieving high sensitivity in the FUV is minimizing the number of optics. This is done because FUV reflection and transmission efficiencies are typically quite low compared to what is common at visible wavelengths. In accomplishing this, the COS FUV channel uses a single (selectable) optic to diffract the light from HST, correct for the Hubble spherical aberration, focus the diffracted light onto the FUV detector and correct for astigmatism typical of this sort of instrument. Since aberration correction is performed after the light passes into the instrument, the entrance to the spectrograph must be an extended aperture, rather than the traditional narrow entrance slit, in order to allow the entire aberrated HST image from a point source to enter the instrument. The 2.5 arc second diameter entrance aperture allows ≈ 95% of the light from compact sources to enter COS, yielding high sensitivity at the design resolution for compact sources.
| Grating (Channel) | Approximate Useful Wavelength Range | Resolving power (λ/Δλ) |
|---|---|---|
| G130M (FUV) | 115–145 nm | 16,000–21,000 |
| G160M (FUV) | 141–178 nm | 16,000–21,000 |
| G140L (FUV) | <90–205 nm | 1,500–4,000 |
| G185M (NUV) | 170–210 nm | 22,000–28,000 |
| G225M (NUV) | 210–250 nm | 28,000–38,000 |
| G285M (NUV) | 250–320 nm | 30,000–41,000 |
| G230L (NUV) | 170–320 nm | 2,100–3,900 |
| TA1 (target acquisition imager) | 170–320 nm | ~0.05 arc sec. angular resolution |
Post launch performance closely matched expectations. Instrument sensitivity is close to pre-launch calibration values, and detector background is exceptionally low (0.16 counts per resolution element per 1000 seconds for the FUV detector, and 1.7 counts per resolution element per 100 seconds for the NUV detector). FUV resolution is slightly lower than pre-launch predictions due to mid-frequency polishing errors on the HST primary mirror, while NUV resolution exceeds pre-launch values in all modes. In addition, thanks to the minimal number of reflections, the G140L mode can observe light at wavelengths shorter than 90nm despite the very low reflectivity of the MgF2 coated optics at these wavelengths.
Read more about this topic: Cosmic Origins Spectrograph
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