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]
(Å)		 Wavelength
Coverage
(Å)		 Dispersion

(Å/pixel)		 Notes
G2501584000114313.75
G4001588000114313.752
G5103001000059992.012,3
G181316750057081.91
KPGL2316440057081.91
KPGL3527550034171.16
G420600800029811.023
KPGL1632420028720.95
KPGLF632820028720.95
G4506321100028720.95
KPGLD790850022900.75
KPGLG8601100021010.68
G3801200800015630.483

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 4000Å.
  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
   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 (mkeane@noao.edu)
Jack Baldwin (jbaldwin@noao.edu)