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SARG - Cross Dispersing Grisms Characteristics

In SARG spectrograph the dispersion is made by means of an R4 echelle located in the first pupil position created by the spectrograph collimator. A choice of cross dispersing grisms may be located on the second (white) pupil image. Up to four grisms may be located on the grism wheel. One additional position is left free to be used for long slit observations. In this case an interference filter must be positioned in the Filter Wheel, to avoid order confusion.

All grisms use regular resin transmission gratings which are replicas of masters in the Milton Roy catalogue.

A summary of cross disperser grism characteristics are listed in Table 1; constructive parameters of the grisms are listed in the Table 2.

In Table 1 the Peak Efficiency was evaluated for the whole system Detector + Spectrograph + Telescope+ Atmosphere (Z=1).

Table 1
CD1
Blue Grism
CD2
Green Grism
CD3
Yellow Grism
CD4
Red Grism
Spectral Range (nm) 360 - 514 414 - 566 462 - 792 496 - 1011
Peak Efficiency (%) 9.0 10.0 13.5 12.0
Minimum order separation (arcsec) 10.7 13.7 8.0 6.0

Table 2
CD1
Blue Grism
CD2
Green Grism
CD3
Yellow Grism
CD4
Red Grism
Glass LF-7 LF-7 SF-10 SF-10
External Size (mm) 108X121 108X125 108X112 108X110
Thickness (mm) 31.9 36.4 19.4 16.6
Groove (gr/mm) 600 600 300 200
Blaze Angle (Deg.) 28.7 34.0 17.5 15.0
Grism Wav. (nm) 430 480 610 740
Dispersion (Å/mm) 27.2 27.1 61.0 95.3
RS Prod. 1928 2165 1209 938
Blaze Wav. (nm) 483.7 560.0 588.9 751.7

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