6.2 Sensitivity

When used in imaging mode, COS concentrates the targetʹs NUV flux into a diffraction-limited image rather than dispersing the light. The local count rate limit for COS/NUV, 50 count/s/pixel (Table 10.1), is easily reached, even for fairly faint objects. Observers should use the COS Exposure Time Calculator (ETC) to get an accurate estimate of expected count rates, but the following values will provide a guide. These have been calculated for a flat-spectrum source (flux independent of wavelength). The approximate flux levels where the limiting count rate is reached are listed in Table 6.1.

Table 6.1: Approximate Flux Limits for a Flat Spectrum.

Aperture and Mirror

Flux Limit (erg cm–2 s–1 Å–1)


2 × 1015


4 × 1013


4 × 1014


7 × 1012

When MIRRORA is used for imaging observations, sensitivity extends from about 1600 to 3300 Å, peaking at ~2300 Å (Figure 6.3). Both MIRRORA and MIRRORB have sensitivity tails that extend to about 8000 Å (see COS ISR 2010-10, Goudfrooij 2010), so care must be taken when observing cool stars (Teff < 5000 K) and other red objects, as high count rates at long wavelengths could damage the detector. When MIRRORB is employed, its use of a first-surface reflection allows short-wavelength light to reach the detector. Sensitivity extends to the NUV detector cut-off, about 1150 Å, for the primary image.

Figure 6.3: Effective Area for COS NUV Imaging with the PSA.