The optical system has required minor changes to match the spectrum imaging plane to the CCD elements within the cryostat. In the original design, the focal plane was located 12.5,mm from outer surface of the main instrument casing. The CCD surface is located some 15.5,mm (RCA) or 14.5,mm (GEC) from the cryostat faceplate. In addition, the window on the cryostats is 4,mm thick which must be included in the optical design.
A new field flattener has been provided which images over a 30,mm field at the CCD plane. One surface of the field flattener is within a mm or so of the cryostat window, although fouling is mechanically impossible if everything is installed correctly. Relative to the 56,mm x 90,mm aperture in the main casing plate (through which the field flattener protrudes), the optical centre, for both the spectrograph and cryostat is located 20,mm from the edge nearest the camera and is in the 90,mm dimension. (The 20,mm dimension represents a 2,mm shift of the optical centre).
The field flattener has been coated with single layer MgF anti-reflection coatings optimised for the CCD red response. The coatings were conducted by David Jackson at the RGO.
A new low dispersion grating has been added to the existing suite of three. The new grating has 300 l/mm and provides a dispersion of approximately 160 A/mm. (Grating is Milton Roy (ex Bausch and Lomb) type 35-53-06-180 with blaze of 6.5for 7500 A). Details of wavelength ranges and dispersions for all gratings are given in Section 5. These tables assume that the detector format covers 15,mm of spectrum.