The Telescope

Light collection in the ultraviolet is not as simple as in the optical, where you can get away with very steep bounces and relatively flat mirrors.

If you shoot a UV photon straight at a mirror, chances are high that instead of reflecting back at you, it will get absorbed by the material it hits. And as you move to the X-ray regime, the photons become high enough energy that they may pass straight through your mirror!

Thus, in the UV, grazing-incidence reflectivity is an important tool to allow for multiply bounces without totally devastating the overall efficiency of the system. The shallower the bounce, the more reflective your typical surface is, and the more science you can do with a given telescope size.

Source: wikipedia.org

Wolter II Telescopes

A design that utilizes this grazing incidence is the Wolter-II telescope. Wolter II's utilize a parabolic primary mirror that for all intensive purposes looks like a gradually-thinning cylinder with a reflective interior. The primary reflects light at a shallow graze angle onto a hyperbolic, spike-like secondary that extends the focal point of the primary further down field.

An early Python raytrace I made of the DEUCE telescope, a Wolter-II design. Light enters from the right, hitting the inside of the grey, parabolic primary mirror. It then hits the outside of the purple, hyperbolic secondary mirrror, on its way to the focus on the left.

The DEUCE telescope

The DEUCE telescope is a Wolter-II made from 6061-T6 aluminum, with the inside coated in a layer of polished nickel, with a surface roughness of about 20 angstroms. The secondary spike is coated with around 250 angstroms of Silicon Carbide. (see Wilkinson 1993)

It weights about 220 pounds, and was made by diamond turning. It has extensive flight heritage, having flown on as many as six missions going back as far as 1987. That makes it easily older than I am. It's a tank. To prevent thermal expansion of the telescope during flight as the rocket skins heat up, the telescope never actually contacts the metal bulkhead; instead, it is insulated by spacers made of low thermal conductivity plastic.

The telescope was rush-built in order to observe the time-sensitive Supernova 1987a, meaning its optical quality is not what it could be, with a spot of about a milimeter. Because of the Wolter II design, the spot is in the shape of an annulus. Fortunately, the telescope has a rather forgiving depth of field, due to its long focal length. This makes placement in the payload and focusing less crucial than it would otherwise be. See below for pictures of the telescope, including primary and secondary.