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Two grisms are available to copy the full spectral response of the CCD chip. They are made from UBK7 glass 10mm thick and their relative efficiencies are given in Fig 5. It is not possible to image both the zero order and first order images or the first and second order images on the chip at the same time. For the blue grism the object to be observed must be offset 60 arc seconds to the west if the first order image is to centred on the chip. For the red grism the image should be offset 90 arc seconds to the west. Both the RCA2 and GEC2 chips have been accurately aligned so that the dispersion is parallel to the columns of the chips. D.H.P.Jones has determined the calibration of row number versus wavelength for both the grisms (Figure 7). Figure 6 shows examples of spectra of HM Sge, a well-known symbiotic star taken through each grism.
The dispersions of the blue and red grism are respectively 10.21 and 10.18 per pixel. Filters are needed to remove second order blue light. These order suppressing filters are located in CCD filter wheel B which contains the following filters:
The telescope focus is automatically corrected for the thickness of the grism when it is inserted into the beam and similarly the autoguider filters will also make a suitable correction to keep the autoguider in focus.
Zero order objects outside the CCD field may still be seen.