A simple and inexpensive optical system is described which can be used to focus a laser beam to provide a very fast f# diffraction-limited point source, for use in optical testing, experiments, etc. The design is characterized by an extremely low sensitivity of its performance to uniform temperature changes (thermal soaks). A temperature change of several hundred degrees centigrade - from cryogenic temperatures up to near the melting point of the optics, has essentially no effect on the predicted performance. Since the system has no aspheric surfaces, it is substantially less expensive than a fast f# parabolic mirror, which would also provide a point source. The system is also less alignment sensitive than a parabola of the same f#.
David Shafer, David Shafer,
"Laser Focusing Optics With Extremely Insensitive Performance/Temperature Dependency", Proc. SPIE 0216, Optics in Adverse Environments II, (5 August 1980); doi: 10.1117/12.958456; https://doi.org/10.1117/12.958456