Melting Glass...

[GPS]

Panta reis! (Heraclitus)

[James E Galvin]

Telescope lenses are limited to 40 inches or less for several reasons: the difficulty of making a large piece of optical quality glass, the difficulty of strain free support for the weight, the loss of light from absorption through the considerable thickness, and secondary spectrum (the compensation for chromatic aberration is not perfect.) In my reading, secondary spectrum is the main problem. I have looked through the 36 inch Lick telescope, there is a considerable blue-violet glow around a star (secondary spectrum). Secondary spectrum gets worse as the lens size is increased. The only cure (that is before ED glass) is to increase the f#. But the telescope is already very long at f/19, 57 feet. For glass flow, do a Yahoo on