The HUT primary mirror, seen here in a lab photo during the construction of the telescope, is 0.9-m (36 inches) in diameter and is made of CERVIT (a ceramic-based material with very good thermal properties). For Astro-1, the mirror was coated with a thin layer of the element iridium, which was the best material available for reflecting far and extreme ultraviolet light at that time. For Astro-2, the mirror was recoated with a new material, silicon carbide, which has about a factor of two better reflectivity over the HUT primary wavelength range (840 - 1840 Angstroms). This permitted HUT to see fainter objects during Astro-2 and get higher quality data on bright objects.
