Expert: Jayendra Upadhye - 7/31/2008

Question
Hello. I'm purchasing a reproduction telescope for a library room that's
decorated in an early 18th century style. I'm not an astronomer (forgive my
use of incorrect terminology) and wanted to know if curved/angled eyepieces
were in use on 18th century celestial telescopes. Some eyepieces on early
telescopes were straight, and others were angled or L shaped so that the
viewer could look down into the lense without having to actually look straight
through the telescope. Thanks you!

Answer
Hi Hans,
Though I am not an authority on Early European Telescopes, I do know the following:-

1 - Newton, the father of Early european optics, ground his first
   lenses by his own hands!
2 - The Earliest telescopes were more for military usage (to see what
   the enemy was up to), rather than astronomical usage.
   These were generally the brass tube affairs we see in hand of
   pirates and sea captains looking out for pirates or land!
3 - The L shaped ones are the ones to look out for for their "Value"
   in terms of advancement over the more conventional "all lens"
   refraction telescopes.
   Refraction telescopes suffered from two major flaws:
   (a) - Spherical aberration. As the spherical lenses were BAD
         approximations of the actual parabolic shape required.
         These tended to "focus" different parts of the image at
         different points along the axis.
   (b) - Chromatic aberration. As the refractive index displaces
         differing frequencies of light differently (prism effect)
         The image would split off into separately offest images of
         the primary colors at high magnification.
    the L shaped ones were in fact the reflecting telescopes with
    crude parabolic mirrors, and prisms/mirrors where a larger
    brighter clearer picture was possible without some of the above
    mentioned aberrations.
    The L was the point where gathered light was reflected or   
    diverted at a convinient angle so the user could "look down"
    comfortably into the eyepice rather like in a microscope.

Off hand,  the straight ones would have more antique value on account of age, but the L SHAPED ONES could be valued for their "sophistication & age combined".

Hope that helps!

Please do rate the answer if you find it helpful.

regards
Jayen