Triplet Apochromats
Triplet apochromats provide additional degrees of freedom through three optical elements and a wider range of material combinations.
Design
CaF₂ and fluorophosphate triplets can be configured as two- or three-medium systems. Where appropriate, oil coupling can reduce the number of glass-air interfaces and improve thermal behavior while protecting internal surfaces.
Published Design Example
The documented CaF₂ Triplet 150/1200 was designed for 436–707 nm. The archived data report a polychromatic Strehl of 0.97 and a maximum focal shift of 53 µm or ±0.002 %.
Strehl Ratio vs. Wavelength – CaF₂ Triplet 150/1200.
Chromatic Focal Shift – CaF₂ Triplet 150/1200.
Example Variants
| Aperture | Focal Length | F-ratio | Strehl | Focal Shift |
|---|---|---|---|---|
| 80 mm | 480 mm | f/6 | 0.96 | ±0.004 % |
| 100 mm | 630 mm | f/6.3 | 0.96 | ±0.003 % |
| 130 mm | 920 mm | f/7.1 | 0.96 | ±0.003 % |
| 150 mm | 1200 mm | f/8 | 0.97 | ±0.002 % |
| 180 mm | 1620 mm | f/9 | 0.97 | ±0.004 % |
| 200 mm | 1800 mm | f/9 | 0.97 | ±0.004 % |