Published online by Cambridge University Press: 12 April 2016
The case for making large mirrors for astronomy with borosilicate glass honeycomb structure has been made in an earlier paper (Angel and Hill, 1981). Honeycomb structure gives rigidity in a lightweight mirror as well as low thermal inertia. Mirrors which are in thermal equilibrium with the air hold the promise of better images, since convection at the mirror is eliminated. Thermal expansion of borosilicate glass is problematical in thick mirrors which are likely to be out of thermal equilibrium and hence distorted. This will not be a problem in honeycomb mirrors with thermal time constants of only a few minutes. We believe a single casting technique holds the greatest promise of making borosilicate honeycomb mirror blanks up to 7 meters in diameter. Our work to develop the technique now involves casting test blanks 60cm in diameter but which have the full thickness (33cm), cell size (15cm) and faceplate thickness (2.5cm) needed for a 1.8m mirror.