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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