Doublet Apochromats
Two-element apochromats are compact systems but place high demands on material pairing, centering and tolerances.
Design
For a high level of chromatic correction, a specialty medium such as an ED glass or CaF₂ is typically combined with a suitable partner glass. In air-spaced systems, the lens separation provides an additional design variable while demanding precise mechanical centering.
Published Design Example
A documented DGE CaF₂ doublet with 150 mm clear aperture and 1350 mm focal length was designed for 436–707 nm. The archived data report a polychromatic Strehl of 0.97 and a maximum focal shift of 84 µm or ±0.003 %.
Strehl Ratio vs. Wavelength – CaF₂ Doublet 150/1350.
Chromatic Focal Shift – CaF₂ Doublet 150/1350.
Example Variants
| Aperture | Focal Length | F-ratio | Strehl | Focal Shift |
|---|---|---|---|---|
| 80 mm | 570 mm | f/7.1 | 0.98 | ±0.004 % |
| 100 mm | 800 mm | f/8 | 0.98 | ±0.003 % |
| 130 mm | 1170 mm | f/9 | 0.98 | ±0.003 % |
| 150 mm | 1350 mm | f/9 | 0.97 | ±0.003 % |