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tenbyfifty

Orthoscopic vs plossl etc

8 posts in this topic

Can anybody tell me the difference between orthoscopic eps and 'regular' plossls etc

I know they are used with planets but what is the difference in terms of their design?

I'm after a 6mm to get a closer look at Saturn and Jupiter (when she shows up) so should

I go with an ortho?

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Basically Orthos have less glass in their design. Bottom line (and this is a generalization!!) they have less FOV than Plossels but give better contrast.

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Heres a useful link about eps among other things: http://www.actonastro.com/eyepieces.htm

A pretty cool diagram showing the designs of different eps comes from: http://members.shaw.ca/quadibloc/science/opt04.htm

eyevar27.gif

From that same website:

The Orthoscopic eyepiece is a design due to the noted optician Ernst Abbé. Unlike many other eyepieces, the elements of crown glass, shown in blue, are on the outside of the eyepiece. Crown glass is harder than flint glass (shown in green) and is basically the same as the ordinary glass used for most purposes, although optical glass is made more carefully to be of a higher quality; flint glass has a higher index of refraction, but it also has dispersion that is not only larger than that of crown glass, but is larger proportionately than its index of refraction. The simplest forms of flint glass are similar in composition to lead crystal, (but usually have an even higher proportion of lead oxide) the "sparkle" of which is a consequence of the additional dispersion. Thus, flint glass elements normally are of the type opposite to the function performed by the lens, as they serve to correct chromatic aberration.

An unusual thing about the Orthoscopic is that the eye lens has only one element, but the field lens has three. Since, like most eyepieces, the field lens is actually located somewhat past (to the left of, in this diagram) the image plane, it is still possible for corrections made in the field lens to have an effect on the image of the eyepiece.

Orthoscopic eyepieces tend to be made in high powers, as they are chiefly used for observing the planets at high magnification. The special feature of this design is that it causes almost no distortion of shapes, such as barrel or pincushion distortion.

Because the crown elements are on the outside, the Orthoscopic eyepiece has proven to be more popular than the other designs with this property.

Heres some info about the history of eps: http://www.brayebrookobservatory.org/BrayObsWebSite/BOOKS/EVOLUTIONofEYEPIECES.pdf

Hope that helps :lol:

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Anyone got any TOLLES, they don't want Field of view around 25% Planet heaven

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For planetary work you can't beat an Ortho, but a well deigned Plossl will also be a good eyepiece.

As mentioned on another thread a S/H Ortho is fantastice value for money.

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I prefer the "volcano tops" they make it easy to get close enough to the lens. The other designs are a bit more tricky.

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Gaz

I know what you mean. My 3 Orthos are volcano tops and I would not swap them for any similar eyepiece :lol:

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