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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 6 of the COS Instrument Handbook.
COS Grating Parameters
| Grating |
Useful wavelength range (Å)1 |
Bandpass per exposure (Å) |
Inferred resolving
power2 R = λ/Δλ |
Dispersion
(mÅ pixel-1) |
| FUV Channel |
| G130M |
1150 -1450 |
2923 |
16,000 - 21,000 |
9.97 |
G160M |
1405 - 1775 |
3604 |
16,000 - 21,000 |
12.23 |
| G140L |
<900 - 2050 |
>1150 |
1,500 - 4,000 |
80.3 |
| NUV Channel |
| G185M |
1700 - 2100 |
3 × 35 |
22,000 - 28,000 |
37 |
| G225M |
2100 - 2500 |
3 × 35 |
28,000 - 38,000 |
33 |
| G285M |
2500 - 3200 |
3 × 41 |
30,000 - 41,000 |
40 |
| G230L |
1650 - 32005 |
(1 or 2) × 398 |
2,100 - 3,900 |
390 |
- The useful wavelength range is the expected usable range realized in each grating mode.
- Empirically-determined FWHM of the LSF through the PSA, which is
not Gaussian, for observations through the PSA. The lesser value of R is realized for the
low-wavelength end of the useful range, and R increases roughly linearly with wavelength.
- The inter-segment gap misses 14.3 Å.
- The inter-segment gap misses 18.1 Å.
- Some shorter wavelengths are recorded in second-order light.
Wavelength Ranges for FUV Gratings
| Grating |
Central Wavelength (Å)1 |
FP-POS |
Recorded Wavelengths |
| Segment B |
Segment A |
| Wavelength Ranges for G140L |
| G140L |
1105 |
3 |
No value for Seg B |
1120 - 2246 2 |
| 1250 3 |
3 |
<900 - 1155 |
1260 - 2385 2 |
| 1280 |
3 |
900 - 1165 |
1280 - 2405 2 |
| 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 |
- The Central Wavelength is (approximately) the shortest wavelength recorded on segment A.
- The G140L grating is flux calibrated up to 2150 Å. At longer wavelengths, second-order light may be present.
- Pre-launch, central wavelength 1250 was referred to as 1230. In Cycle 18, central wavelength 1250 will be replaced with 1280 to move the zero-order image off the detector.
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