4mRC Gratings
203×254 mm Gratings for Use in the 4.0-m R-C Spectrograph
The 4.0-m R-C spectrograph has a fixed 46° angle between the optical axes of the collimator & camera. For sufficiently large grating tilts e.g., observing at high dispersion, the beam coming from the collimator can overfill the grating resulting in the loss of a small amount of light. For the 4.0-m R-C spectrograph, the collimated beam begins to overfill the grating for tilts > ~30.3° (readout < ~36.6°).
The amount of light which may be lost due to overfilling the grating is almost always neglible, typically a few percent. In practice, this small loss would be more than offset by the abliity to observe using a wider slit while preserving spectral resolution that results from the anamorphic demagnification at large grating tilts.
At present, there are thirteen 203×254 mm gratings available. Their nominal specifications are listed below, along with dispersion, and coverage in 1st order available with the BAS + L3K.
| 1st Order | |||||
|---|---|---|---|---|---|
| Grating | l/mm |
Blaze [1] (A) |
Wavelength Coverage (A) |
Dispersion
(A/pixel) |
Notes |
| G250 | 158 | 4000 | 11431 | 3.75 | |
| G400 | 158 | 8000 | 11431 | 3.75 | 2 |
| G510 | 300 | 10000 | 5999 | 2.01 | 2,3 |
| G181 | 316 | 7500 | 5708 | 1.91 | |
| KPGL2 | 316 | 4400 | 5708 | 1.91 | |
| KPGL3 | 527 | 5500 | 3417 | 1.16 | |
| G420 | 600 | 8000 | 2981 | 1.02 | 3 |
| KPGL1 | 632 | 4200 | 2872 | 0.95 | |
| KPGLF | 632 | 8200 | 2872 | 0.95 | |
| G450 | 632 | 11000 | 2872 | 0.95 | |
| KPGLD | 790 | 8500 | 2290 | 0.75 | |
| KPGLG | 860 | 11000 | 2101 | 0.68 | |
| G380 | 1200 | 8000 | 1563 | 0.48 | 3 |
Notes:
- Blaze wavelengths are given for Littrow configuration. Multiply by 0.92 to correct for the actual configuration in the R-C spectrograph.
- Gratings G400 and G510 are silver-coated and thus do not reflect much light below about 4000A.
- Gratings G510, G420 and G380 are not very efficient when used in second order.
- KPGL1 should always be used with a blocking filter, EVEN in first order. Tests done by P. Massey and K. Olsen in March 2005 demonstrated that a few percent of the light in the red appears in the blue at half the wavelength (not to be confused with second order diffraction, which goes the other way). The problem was likely introduced by the machine that ruled the grating. None of the other gratings have been found to have this problem.
R-C Spectrograph
Grating Relative Efficiencies
Grating: 250 400 510 181 KPGL2 KPGL3 420 KPGL1 KPGLF 450 KPGLG KPGLD 380
Lines/mm: 158 158 300 316 316 527 600 632 632 632 860 790 1200
Blaze:4000 8000 10000 7500 4400 5500 8000 4200 8200 11000 11000 8500 8000
Wavelength: Blue-blazed gratings relative to grating 250
3250 1.00 1.13 0.77 3250
3500 1.00 0.04 1.14 0.38 0.76II 0.78 3500
3750 1.00 0.10 0.64II 1.14 0.52 0.71II 0.83 3750
4000 1.00 0.26 0.79II 1.17 0.70 0.73II 0.97 4000
4250 1.00 0.45 0.83II 1.20 0.86 0.64II 1.04 0.73II 4250
4500 1.00 0.66 0.81II 1.17 1.08 0.58II 1.11 0.71II 4500
4750 1.00 0.89 0.76II 1.25 1.29 0.43II 1.21 0.81II 4750
5000 1.00 1.16 0.72II 1.24 1.42 0.34II 1.29 0.98II 0.67II 5000
5250 1.00 1.40 0.62II 1.28 1.58 0.27II 1.34 5250
5500 1.00 1.64 0.66II 1.31 1.66 1.40 5500
5750 1.00 1.92 0.57II 1.25 1.71 1.36 5750
6000 1.00 2.14 0.58II 1.29 1.88 1.44 6000
6250 1.00 2.47 0.49II 1.18 2.02 1.47 6250
6500 1.00 2.65 0.36II 1.14 2.11 6500
6750 1.00 2.85 0.38II 1.12 2.15 6750
7000 1.00 3.18 0.35II 1.20 2.31 7000
Grating: 250 400 510 181 KPGL2 KPGL3 420 KPGL1 KPGLF 450 KPGLG KPGLD 380
II signifies second order.
Grating: 250 400 510 181 KPGL2 KPGL3 420 KPGL1 KPGLF 450 KPGLG KPGLD 380
Lines/mm: 158 158 300 316 316 527 600 632 632 632 860 790 1200
Blaze:4000 8000 10000 7500 4400 5500 8000 4200 8200 11000 11000 8500 8000
Wavelength: Red-blazed gratings relative to grating 400
5000 0.86 1.00 0.62II 1.06 1.22 0.37II 1.11 5000
5250 0.71 1.00 0.45II 0.91 1.13 0.24II 0.96 5250
5500 0.61 1.00 0.40II 0.80 1.02 0.17II 0.86 5500
5750 0.52 1.00 0.30II 0.65 0.89 0.71 0.69II 5750
6000 0.47 1.00 0.27II 0.60 0.88 0.67 0.62II 6000
6250 0.41 1.00 0.20II 0.48 0.82 1.05 0.60 1.05 0.54II 0.74 6250
6500 0.38 1.00 0.14II 1.03 0.43 0.80 1.04 1.01 0.45II 0.75 0.46 6500
6750 0.35 1.00 0.14II 0.96 0.40 0.76 0.99 0.97 0.39II 0.77 0.50 6750
7000 0.32 1.00 0.11II 0.90 0.38 0.73 0.89 0.93 0.38II 0.77 0.48 7000
7250 0.30 1.00 0.91 0.91 0.35 0.70 1.01 0.90 0.89 7250
7500 0.27 1.00 1.00 0.90 0.32 0.67 1.04 0.86 0.90 7500
7750 0.27 1.00 1.00 0.86 0.32 0.69 1.01 0.84 0.95 7750
8000 0.26 1.00 1.05 0.89 0.67 1.00 0.82 1.02 8000
8250 0.24 1.00 1.14 0.90 0.64 1.02 0.81 0.46 1.09 8250
8500 0.26 1.00 1.13 0.90 1.03 0.81 0.54 8500
8750 0.23 1.00 1.14 0.84 0.60 8750
9000 0.25 1.00 0.99 0.92 9000
9250 0.27 1.00 0.98 1.08 9250
9500 1.00 1.09 1.11 9500
9750 1.00 1.20 1.16 9750
10000 1.00 10000
Grating: 250 400 510 181 KPGL2 KPGL3 420 KPGL1 KPGLF 450 KPGLG KPGLD 380
II signifies second order.
Michael Keane (mkeaneATnoao.edu)
Jack Baldwin (jbaldwinATnoao.edu)
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