6.3 Sensitivity
Figure 6.1 shows the sensitivity of COS for NUV imaging. Please note the following:
- The flux over all wavelengths must be integrated to get the total number of counts per second.
- That total count rate is for the entire image; the PSF shown in Figure 6.2 indicates how much ends up in the peak pixel (about 14%).
Figure 6.1: Sensitivity Curve for COS NUV Imaging with the PSA.
COS is very sensitive, and the imaging mode concentrates an object's NUV flux into a diffraction-limited image rather than dispersing the light. The local count rate limit for COS/NUV is 75 counts per pixel per second, and that limit is easily reached, even for fairly faint objects. Observers should use the COS ETC (see Chapter 10) 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), and the limiting count rate is reached for the following approximate flux levels:
- PSA + MIRRORA: 2 FEFU;
- BOA + MIRRORA: 400 FEFU;
- PSA + MIRRORB: 30 FEFU;
- BOA + MIRRORB: 6,000 FEFU.
