## 3 The \"Rotating Lateral\" ADC (RoL-ADC)  
For the purposes of this paper there is one key point to note from comparing Figures 6 and 7. It is observed that the secondary spectrum residuals of the two optimized designs represented there are of opposite signs. Therefore, it follows that an approach combining lateral offset elements with counter-rotating Amici prisms should be capable of correcting the secondary spectrum. Such a design is produced by replacing element 2 of the CL-ADC design with the Amici prism pair of the baseline C-ADC design, and rebalancing by optimizing with an appropriate merit function. The resultant system has elements of fused silica as well as an Amici prism pair using the same Schott optical glasses as are indicated in Reference 13. Elements 1 and 4 are allowed to decentre and the M2 is decentred, tilted and allowed a small focus, as in the CL-ADC design.  
Figure 5: All fused-silica CL-ADC design proposed as an alternative to the baseline C-ADC design for GMT by [PERSON] et al. Elements 1 to 4 are numbered from left to right.  
Figure 6: (**Top**) Lateral colour vs. field plot for the R-ADC type GMT C-ADC baseline design for maximum (60 degree) zenith distance. (**Bottom**) Spot diagrams, showing the growth of the secondary spectrum residual with increasing zenith distance ranging from zero to 60 degrees. Spots are shown in 0.35 arcsecond circles. Wavelength values are in microns in this and all subsequent figures.  
The results, given below in Figure 8, show that a significant reduction in secondary spectrum is indeed obtained. Reductions of the order of 8\(\times\) in the secondary spectrum of the C-ADC baseline (R-ADC type) design of Figure 6, and 4\(\times\) reduction in the secondary spectrum of the CL-ADC design of Figure 7, are achieved. The significance of this improvement is clear when we note that both the R-ADC-type baseline design and the CL-ADC design were each optimized to minimize the residual secondary spectrum, and so the results presented here for those systems represent the limits achievable in both cases.  
Another point to note is that the new type of ADC, the \"Rotating-Lateral ADC\" or \"RoL-ADC\", by combining actions that generate dispersion, can achieve its total correction  
Figure 8: (**Top**) Lateral colour vs. field plot for the RoL-ADC at maximum (60 degrees) zenith distance. The residual secondary spectrum is of the order of 40 milli arcseconds. (**Bottom**) The spot diagrams show the secondary spectrum is almost eliminated over the whole range of zenith distances.  
Figure 7: (**Top**) Lateral colour vs. field plot for the CL-ADC proposed design for the GMT C-ADC. (**Bottom**) Spot diagrams correspond to those in Figure 6. These plots show that the sign of the residual secondary spectrum is of opposite sign to that shown in Figure 6.  
with smaller magnitude motions, and/or simpler sets of, deflections than the CL-ADC design, and also with significantly smaller prism angles than is the case of the C-ADC baseline design. For example, in the CL-ADC design of Ref. [10], the largest lateral lens motion is 98 mm, whereas in the RoL-ADC design presented here the largest lateral lens motion is 30 mm. This compensates, to some degree, for the overall extra mechanical complexity required by the design.