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SARG
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Fig 3: SARG measured efficiency as function of the optical derotator position angle. Owing to the small size of the collimated beam in the preslit optics, alignment of the optical and mechanical axis of the derotator is very critical. The alignment procedure, made in the optics farm, provides a very stable rotation axis but with a residual disalignment between rotation and optical axis. This causes some vignetting and a modulated efficency.