Hi Bob,
I think one reason for so many elements in a long eye relief
eyepiece like the Lanthanum is because they have built in
barlows. At least in the medium and short focal lengths.
Rich
>I was looking through Orion's latest catalog. They have some new
>eyepieces, including an "EPIC ED-2" series. According to the catalog,
>the ED stands for "Extra-low Dispersion" optical glass. It goes on to
>say that they feature a "...six-element lens design in which two of the
>elements are crafted from ED glass...".
>
>Now, I'm no optical expert, but I wondered about this. If the point of
>the special glass is to avoid unwanted light dispersion, then how much
>good does it do to make only 2 of the 6 elements out of the special
>glass?
>
>Later in the same catalog, I read about their "Lanthanum" series
>eyepieces that "One of the six to eight lens elements is made from
>Lanthanum, a rare earth element that provides virtually total freedom
>from visual abberations". Again, why have 1/6 or 1/8 of the optical
>train free from visual abberations and then have 5/6 or 7/8 of it made
>out of lower-quality glass with (presumably) significant visual
>abberations?
>
>Full marks to Orion for being up-front and not trying to imply that all
>optical elements are ED or Lanthanum in these two cases.
>
>But the question remains: is this just a gimmick or is there some real
>technical reason why using the special glass in only a fraction of the
>lens elements makes sense? And is this a common practice in other
>eyepiece designs?
>
>Thanks, Bob J.
>
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