Between the A&G unit and the Spectrograph is a mounting spacer, which locates the slit at the correct f/15 focal plane. The spacer is normally mated with the Spectrograph and it houses a simple filter unit. Details of the filters available are given in Appendix 5. Any one of the filters can be inserted in the convergent beam path to the slit. The image viewed by the the TV camera is necessarily filtered also, but a double pass is avoided.
The initial suite of three gratings are designed for first order operation. Thus, when working in first order blue, there may be a need to filter out second order ultra-violet radiation from the comparison arc. When working in first order red, filters will be required to remove second order blue for all spectra. The photographic emulsion will provide the red cut-off.
The neutral density filters should not be required in photographic mode. However, some neutral filters (Kodak ``Wratten'' gelatin) can be inserted in front of the very sensitive TV camera. The filters should be used if objects brighter than m6 are in the field.