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Grisms and Filters

LDSS-2 has four grisms, of which 3 can be mounted in the grism wheel at the any one time. These grisms give a choice of three spectral resolutions as listed in Table 2. The normal configuration is to have the Low, Medium/Blue and High dispersion grisms in the wheel, with the remaining 3 places taken by the Hartmann masks HU and HD, and a clear position.

Up to 7 filters can be mounted in the filter wheel at once. Three broad-band filters (the BVR Harris set as used by NOAO) are supplied for direct imaging. For information on the Harris prescription BVR filter set, see NOAO Newsletters 11, 12 and 14. Three other filters are available which have been designed for use in spectroscopic mode for spectroscopic order selection and wavelength isolation. All these filters have been antireflection coated, and are specified to be flat to 2 with surfaces parallel to 1 arcmin. Their overall thickness is held to +0.1mm, -0.2mm. These are detailed in Table 3, and the throughputs plotted in Appendix B.

Additional filters in the A&G unit can be used using the ICL commands MAINFILTC and MAINFILTND, but these give an unvignetted field of only 9 arcmins. RGO also have some neutral density filters which can be installed in the LDSS-2 filter wheel. Note that separate neutral density filters are provided in the comparison lamp system.

Observers may supply their own filters. The specification is that the diameter must be 100mm (0.1mm) and the thickness less than 8mm. In the case of user-supplied filters, staff should be notified in advance to give time for any additional spacers to be manufactured.