Roman vs. Hubble

Up early this morning for the successful launch and deployment of the Roman Space Telescope, whose development I have followed for 15 years. Considered a successor to the Hubble Space Telescope, Roman’s 2.4-meter primary mirror is the same size as Hubble’s but is shaped differently, and its shorter focal length provides a much larger field-of-view.

The simplest reflecting telescopes use spherical mirrors, which can naturally be ground by hand, but suffer from spherical aberration (so off-axis light rays focus at different distances). Parabolic mirrors eliminate spherical aberration, but suffer from coma (so off-axis point sources distort to comet shapes) and astigmatism (so light rays in perpendicular planes focus at different points). Exploiting conical geometry, the Hubble and Roman optical designs carefully combine mirrors with hyperbolic or elliptical cross sections to minimize or eliminate such optical aberrations.

Hubble’s two-mirror Ritchey-Chrétien Cassegrain design pairs a concave hyperboloidal primary mirror (with a center hole) and a convex hyperboloidal secondary mirror, as diagrammed.

Hyperbolic (cyan) cross sections of the Hubble Space Telescope’s mirrors.

Roman’s Three-Mirror Anastigmat design combines an ellipsoidal concave primary mirror (with a center hole), a hyperboloidal convex secondary mirror, and an ellipsoidal concave tertiary mirror, as diagrammed. In addition flat fold mirrors between the secondary and tertiary mirrors deflect light off-axis into scientific instruments.

Elliptical (red) and hyperbolic (cyan) cross sections of the Roman Space Telescope’s major mirrors.

Elliptical (red), hyperbolic (cyan), and flat (green) cross sections of the Roman Space Telescope’s mirrors, where the flat mirrors help fold the light rays off-axis into its instruments.

Thanks to newer technology, Roman’s mirrors are smoother, lighter, and stiffer than Hubble’s, although due to its cosmological and exoplanet goals, their reflection coatings are optimized for near-infrared light, while Hubble’s coatings are optimized for visible light, including near-infrared and near-ultraviolet.