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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