COS spectra exhibit wavelength gaps due to the physical layout of the detectors
and the optics. The FUV detector consists of two segments whose active areas are separated by a gap approximately 9 mm wide. The optical image of the spectrum is continuous across the segments, but the wavelengths that fall in the gap are not recorded. These wavelengths can be brought onto the active area of the detector by choosing one of the alternate central-wavelength settings listed in Table 5.4
. For the FUV M gratings, the gap (14–18 Å) is about twice the size of the difference in central wavelength shifts (9 Å). To span it we recommend obtaining exposures at two or more FP-POS positions at each of two non-consecutive CENWAVE
settings. For the G140L grating, both CENWAVE
s are needed to obtain a complete spectrum.
For the NUV channel dispersed light from the gratings is imaged onto the detector
by three camera mirrors resulting in three non-contiguous spectral stripes being recorded at once. The gaps between the stripes are approximately 64 Å for the G185M and G225M gratings, 74 Å for G285M, and 700 Å for G230L (Table 5.5
). To acquire a complete medium-resolution spectrum requires six settings with G185M, six with G225M, and eight with G285M. A full spectrum with G230L requires all four CENWAVE
settings. Such a complete spectrum can probably be acquired more efficiently with STIS, but COS may be a better choice when a limited number of specific wavelengths is desired.