Posted by Mike Field on October 13, 2001 at 01:43:31 from 203.32.93.6:
In Reply to: Re: Telescopes posted by David Chorley on October 12, 2001 at 20:33:45:
I'm too lazy to look up the reference, David, but astronomical telescopes invert the image -- everything one looks at appears upside-down. With a star-cluster or whatever that hardly matters, but the difficulty of using such a telescope on land becomes obvious. For these situations, an additional rectifying lens is a necessary adjunct to the basic telescope, but was usually available, if at all, only as an optional extra.
Moreover, astronomical telescopes are commonly of powers much greater than are found in terrestrial telescopes or binoculars. A magnification of 25x would be an absolute minimum for an astronomical instrument, even for a most inexpensive one, but this is still enough to make it pretty-well impossible to hold the instrument steady enough to view anything properly -- the image wavers all over the place with the slightest hand/arm tremors. So one needs, at the least, a tripod to mount the telescope on, and then the instrument becomes more unwieldy and the whole exercise of using it except as a stationary fitting it considerably more complicated.
It is usually considered that the ideal size of binoculars for marine use is 7x50, the 7x being the magnification (and the 50 being the diameter in millimetres of the objective or main lens.) This magnification gives sufficient power combined with sufficient movement-tolerance for most uses. On land, the power could be increased to perhaps 10x to 12x, but anything greater would be pretty difficult for hand-held use.