All of the dispersive elements in COS are holographically-ruled, ion-etched diffraction gratings with excellent scattered light properties and reflectivities. The gratings have Al+MgF2 coatings, except the G225M and G285M gratings, which have bare Al coatings.

Further details can be obtained in sections 3.2.3, 5.11, and 13.3 of the COS Instrument Handbook.

Grating
Useful wavelength range (Å)1
Bandpass per exposure (Å)
Resolving
Power2 R = λ/Δλ
Dispersion
(mÅ pixel-1)
FUV Channel
G130M
1150 -1450
2923
20,000 - 24,000
9.97
G160M
1405 - 1775
3604
20,000 - 24,000
12.23
G140L
1230 - 2050
>820
2,000 - 5,000
80.3
NUV Channel
G185M
1700 - 2100
3 × 35
16,000 - 20,000
37
G225M
2100 - 2500
3 × 35
20,000 - 24,000
33
G285M
2500 - 3200
3 × 41
20,000 - 24,000
40
G230L
1650 - 32005
(1 or 2) × 398
1,550 - 2,900
390
1The useful wavelength range is the expected usable range realized in each grating mode. Note that G140L is set so that Lyman-alpha falls in the gap between the two micro-channel plates to minimize the effects of geocoronal glow. With G140L, one half records 1230-2050 Å. The other half records whatever spectrum is detected below 1100 Å, but that is expected to be very little in most cases, hence the 820 Å nominal bandpass. The response of COS below Lyman-alpha will be evaluated after launch.
2The lesser value of R is realized for the low-wavelength end of the useful range, and R increases roughly linearly with wavelength.
3The inter-segment gap misses 14.3 Å.
4The inter-segment gap misses 18.1 Å.
5Some shorter wavelengths are recorded in second-order light.
Grating Central Wavelength (Å) FP-POS Recorded Wavelengths
Segment B Segment A
Wavelength Ranges for G140L
G140L 1105 3 No value for Seg B 1120 - 2246
1250 3 <900 - 1155 1260 - 2385
Wavelength Ranges for G130M
G130M 1291 3 1134 - 1274 1290 - 1430
1300 3 1144 - 1284 1300 - 1400
1309 3 1153 - 1294 1309 - 1449
1318 3 1163 - 1303 1319 - 1459
1327 3 1173 - 1313 1328 - 1468
Wavelength Ranges for G160M
G160M 1577 3 1386 - 1559 1577 - 1749
1589 3 1398 - 1571 1589 - 1761
1600 3 1410 - 1581 1601 - 1772
1611 3 1421 - 1594 1612 - 1784
1623 3 1433 - 1606 1625 - 1796