Hi,
"Extra-low Dispersion" does not refer to extra-low scattering. The
dispersion is a function on how the refractive coefficient of the
media (glass) changes as a function of the light wavelength. For
building an achromatic/apochromatic lens one needs glasses with different
dispersion. Having one of the elements with "extra-low dispersion"
simplifies the design and provides better color correction.
Here is a more detailed description:
http://www.scopecity.com/Products/binoculr/minox/ed.htm
Regards,
- Alex
Richard Ozer wrote:
> Scattering is, among other things, a function of the angle of incidence in the light path. Certain elements in the design are
> likely to be more prone to scattering than others because of their position in the stack. If this glass is expensive, the idea is
> to get as much bang as possible for the buck. They want to be able to sell these things. The same goes for special coatings.
> There are also certain positions in the stack where the coatings offer the best performance improvement. Lanthanum coatings also
> scratch easily, so keeping them away from the exposed surfaces is a good design.
>
> ----- Original Message -----
> From: "Bob Jardine" <rljtac@No-Spam>
> To: "The Astronomy Connection" <sf-bay-tac@No-Spam>
> Sent: Friday, February 07, 2003 11:19 AM
> Subject: [TAC] Eyepiece question(s)
>
>
>
>>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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>
>
>