|
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 |
|
 |
|
