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From: chris@leadsv.UUCP
Newsgroups: net.astro.expert
Subject: Re: Eyepieces
Message-ID: <468@leadsv.UUCP>
Date: Wed, 5-Jun-85 19:17:52 EDT
Article-I.D.: leadsv.468
Posted: Wed Jun  5 19:17:52 1985
Date-Received: Mon, 10-Jun-85 08:32:56 EDT
Organization: LMSC-LEADS, Sunnyvale, Ca.
Lines: 41
Summary: Telescope eyepieces - the choices

In article <2470@decwrl.UUCP>, liss@psyche.DEC (Frederick R. Liss 237-3649) writes:
> today is the advertisements for telescope eyepieces. When I got my first
> telescope there were only a few types of eyepieces. I'm not sure of the
> spelling or the exact names, but I believe they were the Orthoscopic (?)
> and Hyugens (?) eyepieces (correct me if I'm wrong) . Today I see adds
> for Nagler, Wide Field, Plossl, and Kelner eyepieces. 

	What you are seeing is a variation in the complexity of the
construction of the lenses.  The cheapest lenses (such as Huygens) are
made with one or two simple lens.  Orthoscopic, an expensive brand, is
made of several lenses packed together.  The newer lenses fill in the
gap in between.

	The reasons for different lenses are several:

	1. To offer the customer a variety of price ranges.
	2. To offer different degrees of solution to the various
	   problems associated with lenses.

	One problem is chromatism, or the tendency for the lense to
act like a prism and break up the light of the image, particularly
near the edges.  Some types of lenses are limited in their focusing
power.  All the brands should come in a variety of magnifications
and allow the option for a filter to be installed in the base of
the eyepiece.  Some lenses are better built (shock proof, etc.)
but cost more. 

	As far as lenses being matched to a particular type of
telescope, I believe that eyepieces with wider fields are best
used with shorter focal lengths.  There is a maximum practical
eyepiece magnification for a given telescope.  The manual that
comes with it should mention this.  (E.g., a 60mm refractor shouldn't
be used with anything smaller than a 12.5 mm eyepiece. )

	An excellent source for information on telescopes is
Edmund Scientific Co. in New Jersey.  They sell everything associated
with the hobby through a mail order catalog that they put out.

			- Christopher Salander
			Lockheed Space Systems Div.
			Silicon Valley