The COS NUV channel employs a Czerny-Turner design (see Fig. 2). This channel provides excellent sensitivity for moderate resolution spectroscopy of faint UV targets in the 1700 - 3200 Å region. It serves partially to back up the STIS NUV spectroscopic modes and also will restore capability to observe faint targets that has been mitigated by the high background of the STIS NUV MAMA detector. The COS NUV channel is fed by a mirror (called NCM1) on the Optics Select Mechanism 1 (OSM1). The beam is collimated by a second optic (NCM2) and sent to the Optics Select Mechanism 2 (OSM2), which contains several flat, first-order gratings and a mirror used for imaging during instrument test, on-orbit alignment, and target acquisition.

Three medium-dispersion gratings (G185M, G225M, and G285M) deliver resolutions R≥ 16,000 over the wavelength range 1750 - 3200 Å. The dispersed light from the gratings is imaged onto a CsTe MAMA detector by three camera optics (NCM3a,b,c). The spectra appear as three non-contiguous 35-41 Å strips on the MAMA detector, allowing 105-123 Å wavelength coverage per exposure. The gratings can be scanned to cover the entire NUV wavelength band. A low-dispersion grating, G230L, delivers 400Å coverage per exposure with a resolution of 1.2Å. A fifth optic on OSM2 is a flat mirror (TA1) that images a few arcsec field of view. Because the optics correct aberrations at a point rather than over a large field (like STIS or ACS), unaberrated imaging is constrained to the central arcsecond of the aperture. The COS NUV spectroscopic modes are summarized in Table 2.

Figure 2: Schematic of the COS NUV channel. The optical beam is received from the HST OTA through the aperture (shown at lower right), and is magnified by a mirror on the optics select mechanism 1 (shown at lower left). The light is directed to a second optic which collimates the beam and sends it to the optics select mechanism 2 containing several flat gratings and a mirror. The dispersed spectrum is imaged by three camera optics onto a CsTe MAMA detector (shown in orange near the middle-left) adapted from STIS.

Table 2: COS NUV spectroscopic modes. Grating Wavelength Range Coverage per Exposure Dispersion (Å / pixel) Resolving Power (λ/Δλ) G185M 1700 - 2100 Å 3 × 35 Å ~0.0342 16,000 - 20,000 G225M 2100 - 2500 Å 3 × 35 Å ~0.0342 20,000 - 24,000 G285M 2500 - 3200 Å 3 × 41 Å ~0.0400 20,000 - 24,000 G230L 1700 - 3200 Å 3 × 400 Å ~0.3887 1550 - 2900