Spectroscopy

Because of its rectangular shape it was decided to mount the grism rotated by 90 degrees such that the grism can be properly aligned without the need of modifying the grism wheel or having problems partly vignetting the beam. The special holder was designed and made at Copenhagen University. When the grism is mounted it gives the dispersion along the rows of the CCD, and can hence only be used with our set of 'vertical' slits. The resolution of the grism is highest of all ALFOSC grisms (including the Echelle grisms), and results in R=10000 for a 0.5 arcsec slit (2.5 detector pixels), with a corresponding dispersion of 0.254$\AA$/pixel. The peak system efficiency, including atmosphere, telescope, instrument and detector, is well above 30% (which is the highest peak efficiency of all ALFOSC grisms), but falls down quite steeply away from the central wavelength.

The Multi Object Spectroscopy mode target acquisition software was modified to allow for use with the 90 degree rotated VPH grism and the software was improved by adding a new analysis task that allows for binned images.