Early refracting telescopes with non-achromatic objectives were constructed with very long focal lengths to mask the chromatic aberration. An Achromatic telescope uses an achromatic lens to correct for this. An achromatic lens is a compound lenses made with two types of glass with different dispersion.
Building a Copyscope The key component of a Copyscope is an objective lens that must be sourced from a photocopier, preferably with an f-ratio of around f4 to f6. Other parts of a Copyscope include an eyepiece, typically with a barrel diameter of 1 1/4 inches with a focal length of 17 to 20 mm or longer.
A monocular is a miniature, low powered telescope or spotting scope which you hold in your hand like a binocular but use with one eye like a telescope. A monocular shares characteristics with both binoculars and spotting scopes, but is much smaller than either.