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, 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.


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

GratingCentral wavelength setting (Å)1Recorded wavelengths2

Segment BSegment A

G130M

1055

899–1040

1055–1196

1096

940–1080

1096–1236

1222

1067–1207

1223–1363


 

1291

1134–1274

1291–1431

1300

1144–12833

1300–1441

1309

1154–12943

1309–1450

1318

1163–13033

1319–1460

1327

1172–13133

1328–1469

G160M

1533

1342–1515

1533–1707


 

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

800

HV OFF

815–1948


 

1105

HV OFF

1118–22514

1280

<900–1165

1280–23914

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.

GratingCentral wavelength setting (Å)1Recorded wavelengths
Stripe AStripe BStripe 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 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 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 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.

GratingCentral 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

2751–2792

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.