How about a few? Absorption of light through the lens(es). Chromatic aberation, requiring, preferably, a multiple lens like an achromatic or apochromatic lens which increases absorption. Reflection of light off of the objective lens. Maximum exposure to the elements, causing fogging. Maximum diameter of about 40 inches due to gravity. Difficulty of making at least four optical surfaces to reduce chromatiic aberation (and, these must stay aligned for the life of the telescope). Enough?

Frank Twyman has written:

'Prism and lens making'

'The practice of absorption spectrophotometry with Hilger instruments' -- subject(s): Absorption spectra, Spectrophotometer, Spectrophotometry, Spectrum analysis

'Wavelength tables for spectrum analysis'

FEATURES

Red filter

Absorption Screen

Reference temperature Lamp

observer

eyepiece

temperature source

objective lens

outer tube

mulltimeter

rheostat

battery

its absorption