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Hubble Space Telescope
COS Gratings

The COS FUV gratings are concave and have holographically-generated grooves to provide dispersion and correct for astigmatism. The gratings have aspherical surfaces to correct for HST’s spherical aberration. The FUV “M” gratings have been ion etched to produce triangular groove profiles for better efficiency. The G140L grating has grooves with a laminar profile. All FUV gratings are coated with MgF2 over aluminum.

The NUV gratings are flat and were not constructed holographically. The NUV MAMA has low but measurable sensitivity at FUV wavelengths, and with some gratings second-order light could contaminate the spectrum. To minimize this effect, the optics are optimized for wavelengths above 1600 Å. Even so, it is possible for second-order light to appear in G230L spectra, especially in the long-wavelength stripe.

Further details can be obtained in Chapter 5 of the COS Instrument Handbook.

COS Grating Parameters

Grating Normal wavelength range (Å) Bandpass per exposure (Å)1 Resolving
power R = λ/Δλ
Dispersion
(mÅ pixel-1)
FUV Channel
G130M 9002-1450 300 16,000 - 21,0003 9.97
G160M 1405-1775 370 16,000 - 21,000 12.23
G140L 900 - 21503 >1,100 1,500 - 4,000 80.3
NUV Channel
G185M 1700 - 2100 3 X 35 16,000 - 20,000 34
G225M 2100 - 2500 3 X 35 20,000 - 24,000 34
G285M 2500 - 3200 3 X 41 20,000 - 24,000 40
G230L 1700 - 3200 (1 or 2) X 400 2,100-3,200 390
  1. Normal wavelength ranges are for the primary (default) central wavelength setting.
  2. Modes covering wavelengths below 1130 are expected to have a much reduced resolving power of about 2000.
  3. Second order contamination possible longward of 2150 .

Wavelength Ranges for FUV Gratings

Grating Central Wavelength (Å) Recorded Wavelengths
Segment B Segment A
G130M 1055 1 900 - 1041 1055 - 1197
1096 1 940 - 1081 1096 - 1237
1222 1 1068 - 1208 1223 - 1364
1291 1132 - 1274 1291 - 1433
1300 1141 - 1283 1300 - 1442
1309 1150 - 1292 1309 - 1451
1318 1159 - 1301 1318 - 1460
1327 1168 - 1310 1327 - 1469
G140L 2 1105 No value for Seg B 3 1105 - 2253
1230 4 < 300 - 1095 1230 - 2378
1280 < 500 - 1165 1280 - 2405
G160M 1577 1382 - 1556 1577 - 1752
1589 1394 - 1568 1589 - 1764
1600 1405 - 1579 1600 - 1775
1611 1416 - 1590 1611 - 1786
1623 1428 - 1602 1623 - 1798
  1. These are new modes that are currently being characterized, so these numbers are preliminary. 1055 and 1096 central wavelength settings have been offered starting from Cycle 19 and 1222 central wavelength mode starting from Cycle 20.
  2. It is not yet clear how much of the G140L segment B short wavelength (< 900 ) ranges will be available due to uncertainties in the HST OTA throughput. This is currently being investigated.
  3. The G140L grating and 1105 central wavelength setting moves the zero-order image onto segment B. Therefore, only segment A is available for this setting.
  4. Beginning in Cycle 18, the G140L 1230 setting was replaced by the 1280 setting.

Wavelength Ranges for NUV Gratings

Grating Central Wavelength (Å) 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 - 1733 1 2435 - 2834 1768 - 1967 2
2950 1650 - 2050 2750 - 3150 1900 - 21002
3000 1700 - 2100 2800 - 3200 1950 - 2150 2
3360 2059 - 2458 3161 - 35603 2164 - 23612
  1. The wavelengths listed for central wavelength 2635 in stripe A are for completeness only and also in case a bright emission line falls onto the detector. Note that the NUV detector’s sensitivity at these wavelengths is extremely low. To obtain a low-resolution spectrum at wavelengths below about 1700 we recommend G140L and the FUV channel.
  2. The values in shaded cells are wavelength ranges as seen in second-order light. In these cases the achieved dispersion is twice that for first-order mode. Note, however, that some first order light may contaminate the spectrum depending on the SED of the target.
  3. The spectrum longward of 3400 may be contaminated by second order spectrum, depending on the SED of the target.