5.5 Spanning the Gap with Multiple CENWAVE Settings

COS spectra exhibit wavelength gaps due to the physical layout of the detectors and the optics (see 4.1 The FUV XDL Detector). 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 (which depend on the CENWAVE selected) 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 5.3. 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, all CENWAVEs offer broad spectral coverage, and users will likely find that they can meet their needs with just one setting. If necessary, the gap in CENWAVE 1280 can be filled with data obtained at one of the other two settings. If users would like to fill the gap in CENWAVE 1291, users will need to include CENWAVE 1222.

Table 5.3: Wavelength Ranges for FUV Gratings for FP-POS=3.

¹ The central wavelength is (approximately) the shortest wavelength recorded on Segment A.
² All wavelength ranges quoted here are approximate, due to uncertainties in the position of the OSM1 mechanism.
³ Segment B of this cenwave is not allowed by the COS 2025 policy.
⁴ G140L spectra are flux calibrated up to 2150 Å. At longer wavelengths, second-order light may be present (see Section 5.1.3).

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 5.4). To acquire a complete medium-resolution spectrum requires six settings with G185M, six with G225M, and eight with G285M (Table 5.5). A full spectrum with G230L requires three CENWAVE settings (Table 5.4). 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.

Table 5.4: Wavelength Ranges for NUV Gratings for FP-POS=3.

¹ The central wavelength setting corresponds to the approximate midpoint of stripe B.
² For central wavelength 2635 Å, the stripe A wavelengths are listed for completeness only (and in case a bright emission line falls onto the detector). The NUV detectorʹs sensitivity at these wavelengths is extremely low. To obtain a low-resolution spectrum at wavelengths below ~1700 Å, we recommend the FUV grating G140L.
³ Their dispersion is twice that of the first-order spectrum, but there is likely some amount of first-order contamination from the source so the flux calibration of Stripe C for these cenwave settings are unreliable (Section 5.1.3).

⁴ Lyman α may be present in second order light.

⁵ Longward of 3200 Å, second-order light may be present. At these wavelengths, the flux calibration applied by CalCOS is unreliable (Section 5.1.3).

Table 5.5: COS modes required to obtain a complete medium-resolution NUV spectrum.

This table gives the NUV medium-resolution grating and central wavelength combinations that are needed to create a continuous COS NUV spectrum from 1670 Å to 3229 Å. The Stripe Wavelength Ranges columns give the wavelength ranges covered by each of the three NUV stripes, using the nominal FP-POS=3 setting, corresponding to each of the grating and central wavelength values on the left. The Cumulative Wavebands columns give the cumulative wavebands covered by the current setting, and all the grating and central wavelength settings above it.