COS Instrument Handbook for Cycle 24

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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. 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, both CENWAVEs are needed to obtain a complete spectrum.

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

Grating	Central wavelength setting (å)1	Recorded wavelengths2
		Segment B	Segment A
G130M	1055	899 – 1040	1055 – 1196
	1096	940 – 1080	1096 – 1236
	1222	1067 – 1207	1223 – 1363
	1291	1134 – 1274	1291 – 1431
	1300	1144 – 1283	1300 – 1441
	1309	1154 – 1294	1309 – 1450
	1318	1163 – 1303	1319 – 1460
	1327	1172 – 1313	1328 – 1469
G160M	1577	1386 – 1559	1577 – 1751
	1589	1397 – 1571	1589 – 1762
	1600	1409 – 1581	1601 – 1774
	1611	1420 – 1594	1612 – 1786
	1623	1432 – 1606	1625 – 1798
G140L	1105	HV OFF	1118 – 22513
	1280	<900 – 1165	1280 – 23913

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

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

Grating	Central wavelength setting (å)1	Recorded wavelengths
		Stripe A	Stripe B	Stripe C
G185M	1786	1670 – 1705	1769 – 1804	1868 – 1903
	1817	1701 – 1736	1800 – 1835	1899 – 1934
	1835	1719 – 1754	1818 – 1853	1916 – 1951
	1850	1734 – 1769	1833 – 1868	1931 – 1966
	1864	1748 – 1783	1847 – 1882	1945 – 1980
	1882	1766 – 1801	1865 – 1900	1964 – 1999
	1890	1774 – 1809	1872 – 1907	1971 – 2006
	1900	1783 – 1818	1882 – 1917	1981 – 2016
	1913	1796 – 1831	1895 – 1930	1993 – 2028
	1921	1804 – 1839	1903 – 1938	2002 – 2037
	1941	1825 – 1860	1924 – 1959	2023 – 2058
	1953	1837 – 1872	1936 – 1971	2034 – 2069
	1971	1854 – 1889	1953 – 1988	2052 – 2087
	1986	1870 – 1905	1969 – 2004	2068 – 2103
	2010	1894 – 1929	1993 – 2028	2092 – 2127
G225M	2186	2070 – 2105	2169 – 2204	2268 – 2303
	2217	2101 – 2136	2200 – 2235	2299 – 2334
	2233	2117 – 2152	2215 – 2250	2314 – 2349
	2250	2134 – 2169	2233 – 2268	2332 – 2367
	2268	2152 – 2187	2251 – 2286	2350 – 2385
	2283	2167 – 2202	2266 – 2301	2364 – 2399
	2306	2190 – 2225	2288 – 2323	2387 – 2422
	2325	2208 – 2243	2307 – 2342	2406 – 2441
	2339	2223 – 2258	2322 – 2357	2421 – 2456
	2357	2241 – 2276	2340 – 2375	2439 – 2474
	2373	2256 – 2291	2355 – 2390	2454 – 2489
	2390	2274 – 2309	2373 – 2408	2472 – 2507
	2410	2294 – 2329	2393 – 2428	2492 – 2527
G285M	2617	2480 – 2521	2596 – 2637	2711 – 2752
	2637	2500 – 2541	2616 – 2657	2731 – 2772
	2657	2520 – 2561	2636 – 2677	2751 – 2792
	2676	2539 – 2580	2655 – 2696	2770 – 2811
	2695	2558 – 2599	2674 – 2715	2789 – 2830
	2709	2572 – 2613	2688 – 2729	2803 – 2844
	2719	2582 – 2623	2698 – 2739	2813 – 2854
	2739	2602 – 2643	2718 – 2763	2837 – 2878
	2850	2714 – 2755	2829 – 2870	2945 – 2986
	2952	2815 – 2856	2931 – 2972	3046 – 3087
	2979	2842 – 2883	2958 – 2999	3073 – 3114
	2996	2859 – 2900	2975 – 3016	3090 – 3131
	3018	2881 – 2922	2997 – 3038	3112 – 3153
	3035	2898 – 2939	3014 – 3055	3129 – 3170
	3057	2920 – 2961	3036 – 3077	3151 – 3192
	3074	2937 – 2978	3053 – 3094	3168 – 3209
	3094	2957 – 2998	3073 – 3114	3188 – 3229
G230L	2635	1334 – 17332	2435 – 2834	1768 – 19673,5
	2950	1650 – 2050	2750 – 3150	1900 – 21005
	3000	1700 – 2100	2800 – 3200	1950 – 21505
	3360	2059 – 24584	3161 – 35605	2164 – 23615

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 detector’s 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

The values in shaded cells are wavelength ranges observed in second order light. Their dispersion is twice that of the first-order spectrum. First-order flux, from wavelengths twice those of the listed range, will be present at the ~ 5% level.

4

Lyman-α may be present in second order light.

5

Longward of 3200 å, second-order light may be present. At these wavlengths, 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

Grating	Central Wavelength (å)	Stripe Wavelength Ranges (å)			Cumulative Wavebands (å)
G185M	1786	1670 – 1705	1769 – 1804	1868 – 1903	1670 – 1705	1769 – 1804	1868 – 1903
	1817	1701 – 1736	1800 – 1835	1899 – 1934	1670 – 1736	1769 – 1835	1868 – 1934
	1850	1734 – 1769	1833 – 1868	1931 – 1966	1670 – 1966
	1953	1837 – 1872	1936 – 1971	2034 – 2069	1670 – 1971	2034 – 2069
	1986	1870 – 1905	1969 – 2004	2068 – 2103	1670 – 2004	2034 – 2103
	1921	1804 – 1839	1903 – 1938	2002 – 2037	1670 – 2103
G225M	2217	2101 – 2136	2200 – 2235	2299 – 2334	1670 – 2136	2200 – 2235	2299 – 2334
	2250	2134 – 2169	2233 – 2268	2332 – 2367	1670 – 2169	2200 – 2268	2299 – 2367
	2283	2167 – 2202	2266 – 2301	2364 – 2399	1670 – 2399
	2410	2294 – 2329	2394 – 2428	2492 – 2527	1670 – 2428	2492 – 2527
	2339	2223 – 2258	2322 – 2357	2421 – 2456	1670 – 2456	2492 – 2527
	2373	2256 – 2291	2355 – 2390	2454 – 2489	1670 – 2489	2492 – 2527
G285M	2617	2480 – 2521	2596 – 2637	2711 – 2752	1670 – 2527	2596 – 2637	2711 – 2752
	2657	2520 – 2561	2636 – 2677	2751 – 279>	1670 – 2561	2596 – 2677	2711 – 2792
	2695	2558 – 2599	2674 – 2715	2789 – 2830	1670 – 2830
	2952	2815 – 2856	2931 – 2972	3046 – 3087	1670 – 2856	2931 – 2972	3046 – 3087
	2979	2842 – 2883	2958 – 2999	3073 – 3114	1670 – 2883	2931– 2999	3046 – 3114
	3018	2881 – 2922	2997 – 3038	3112 – 3153	1670 – 2922	2931 – 3038	3046 – 3153
	3057	2920 – 2961	3036 – 3077	3151 – 3192	1670 – 3192
	3094	2957 – 2998	3073 – 3114	3188 – 3229	1670 – 3229

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.