# Air-spaced doublet

> Two lenses almost touching, with a thin air gap instead of glue.

**Joseph von Fraunhofer** · c. 1817 — Fraunhofer’s air-spaced objectives

Leave a thin air gap and each surface can have its own curve. Fraunhofer used that extra freedom in early-nineteenth-century objectives, along with better glass than Dollond had. The pair is no longer one glued pill like the [cemented achromat](https://www.loomrume.com/examples/achromat-doublet.md).

Cemented is sturdier; air-spaced is slower to make and easier to get wrong. On the board the gap is the whole difference — same two glasses, one more radius to play with.

## Sources

- [Achromatic lens](https://en.wikipedia.org/wiki/Achromatic_lens)
- [Joseph von Fraunhofer](https://en.wikipedia.org/wiki/Joseph_von_Fraunhofer)

## Links

- [HTML](https://www.loomrume.com/examples/airspaced-doublet)
- [Open in Studio](https://www.loomrume.com/studio?template=airspaced-doublet)
- [llms.txt](https://www.loomrume.com/llms.txt)