1. Atmospheric Dispersion Correctors

2. ZnSe linear ADC Plots are for 60° zenith angle < 10 mas residual dispersion for λ=0.9-2.5μ ZnSe transmission starts rolling off at 0.7 μ (external transmission shown; losses in flat region are almost entirely due to Fresnel reflection) <40nm rms for λ=0.9-2.5μ ~160 mm long, but could be made shorter if necessary Availability larger than 4.5-5” would be problematic right now, but II-VI’s new plant comes online in the next few months

3. ZnSe external transmission

4. ZnSe f/45 linear ADC Better performing and can be made shorter because of slow beam (100 mm long in this example)

5. Two-glass linear ADC Generally better correction over wider band, but at cost of transmission (extra surfaces and perhaps transmission loss in visible and/or K-band, depending on glasses chosen)

6. S-NPH2 / S-BAL42 transmission (excludes coating losses)

7. Visible light (0.7-0.9) f/45 linear ADC Not started, but narrow band, small field, and slow beam should make this not very hard.

8. Summary All linear ADC’s should be possible in 200mm or less length Linear ADC’s over 0.9-2.4 μ (at f/15 or f/45) are straightforward with single glass Linear ADC’s over 0.7-0.9 μ possible with one glass with rolloff at 0.7 μ, or with two glasses (perhaps with rolloff at K-band); other glass choices should be investigated Linear ADC over 0.7-0.9 μ not started, but should be straightforward