## 2 Correcting ADCs for GMT Direct Gregorian Focus  
The two-mirror design for the GMT (without corrector) is that of an aplanatic Gregorian telescope using seven-segmented primary and secondary mirrors (see Ref. [13]). The effective focal length for the telescope is 203 m (f/8.0) with the focus located 5.5 m below the primary mirror vertex. The focal length of 203 m means that the plate scale at the f/8 focus is 1 arcsecond per millimetre, to within 1.5%.  
The aplanatic image is produced on a curved surface, and the dominant residual aberration is third-order astigmatism (the best focus is found at the surface lying half-way between the tangential and sagittal focal surfaces). The telescope can operate with or without a \"pick-off\" tertiary mirror (M3). In the \"direct Gregorian mode\" the M3 is retracted and an unvignetted field of -26 arcminutes diameter is available, though astigmatism limits the seeing limited field (assuming best seeing of 0.25\") to about 10 arcminutes in diameter (see Figure 2).  
For wider fields of view, a field corrector is required to ensure that the delivered image quality from the telescope does not degrade best seeing. For GMT's wide-field mode a field-corrector incorporating an R-ADC type ADC is envisaged (Figure 3). The project refers to this as the \"Correcting-ADC\" or \"C-ADC\" subsystem.  
As shown in Figure 4, the uncorrected lateral colour is reduced from approximately 3 mm (corresponding to 3 arcseconds) to about 0.35 mm, or 0.35 arcseconds.  
Ref. [10] describes in detail a proposed alternative GMT C-ADC design of the CL-ADC type. Note that this CL-ADC design is a proposal that is not adopted by the project at this stage, and the R-ADC type design described in Ref. [13] is the baseline system for the GMT C-ADC. In the CL-ADC design, shown in Figure 5 below, the elements are all of fused silica. Lens elements 1 and 3 are decent  
Figure 2: Spot diagram for GMT in 1 arcminute radial field increments from on-axis to 10 arcminutes off axis. The circle size is 0.25 arcseconds. The increase in image size with field is almost purely quadratic, and due to third-order astigmatism. The seeing limit can be seen to be reached at a field radius of 0.08333 degrees, corresponding to the 10 arc-min field diameter described above.  
compensate atmospheric dispersion while maintaining image quality, and focal surface and pupil positions. M2 tilt and decentre are also included as degrees of freedom.  
Figure 4: (**Top**) Modelled atmospheric dispersion lateral at the GMT site in Chile for a zenith distance of 60 degrees. (**Bottom**) Residual secondary spectrum after C-ADC correction. The horizontal scale in microns is also very close to the exact value for the angular lateral colour in milliarcseconds.  
Figure 3: Baseline “GMT Correcting-ADC” (C-ADC) corrector design, incorporating an Amici prism (R-ADC), as given in Figures 6–10 of Ref. [13]. The first element (far left) and last element (far right) are of fused silica. Element 1 is over 1.5 m diameter The Amici prism elements use optical glasses (see Table 6-3 and associated discussion on page 6–13 of Ref. [13]). In this paper “optical glasses” refers to glasses other than pure fused silica. Fused silica is of course a potential crown element for the Amici prisms but the performance in that case is not as good as can be achieved with other crowns.  
Both corrector designs meet current GMT requirements for the C-ADC, and their relative merits are discussed in the references given.