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 [1] 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.

Notes:

1. Blaze wavelengths are given for Littrow configuration. Multiply by 0.92 to correct for the actual configuration in the R-C spectrograph.
2. Gratings G400 and G510 are silver-coated and thus do not reflect much light below about 4000A.
3. Gratings G510, G420 and G380 are not very efficient when used in second order.
4. 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 [2]
   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 [3]
   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)