Eyepiece Types
In the following table we have listed some of the most common eyepiece
types with their pros and cons. The graphics at the left side are to be
understood as an example of the eyepiece type, there are a lot of modifications
on the market. As an example, the Kellner design is also available as a
reversed Kellner, which means, that the two-lens element is at the bottom side.
Furthermore one can manufacture any design in a careless way, that you do not
have to think about inherent pros and cons - but for that we do our testing.
Additionally something about the viewing distance (the eye relief): as a
rule of thumb eyepieces have an eye relief of about 2/3 of their focal length.
This is uncomfortable, if the focal length is less than 6 to 10mm, as you have
nearly to touch the eyepiece with your eye. Therefore modern Long Eye Relief
eyepieces combine e.g. a Barlow lens with a Ploessl design, which leads to an
eye relief of about 15 or 20mm for all focal length. However, adding more
lenses will have the effect of more stray light, loss of contrast and generally
image degradation - at least if not very careful manufactured. Such eyepieces
should not be used with an additional Barlow lens.
Design |
Type |
Pros |
Cons |
|
Huygens
Simple, two-lens type |
Cheap |
Apparent field of view is small (ca. 30-35°), not achromatic |
|
Kellner |
Also cheap |
not achromatic, small field of view (about 45°), eyepieces with long focal
length not recommended for optics of F/5 or F/4 |
|
Orthoskopisch 4-lens design |
Nice image quality, achromatic Often used for planetary observations |
Small field of view (about 35-40°) |
|
Ploessl Standard in modern
astronomie |
Relatively cheap to have, very good image quality |
Field of view just about 45°, below 10mm focal length small eye relief |
|
Super Ploessl modified Ploessl, 4- or 5 lenses. Often just Ploessl with an added “Super“ |
Field of view about 52°, if really a modified Ploessl. Contrast and
image quality very good |
A little bit more expensive than Ploessl, eye relief like Ploessl |
|
Erfle Standard design for wide angle eyepieces |
Nice apparent field of view (about 65°), not to expensive |
Near to the edges not good with fast optics (F/5, F/4) |
|
Nagler and Super- or Ultra- Weitwinkel |
Very big apparent field of view (up to 85°). Very good image quality
possible, but depends strongly on the manufacturer. Can be build up for use
in F/4 and F/5 - but expensive in that case. |
Nagler: Very expensive and remarkable distortion Pentax: Very expensive. Others: If cheap: Do not use with fast telescopes. |
Thanks to Skyproject here: I “loaned“ the graphics from there. I would
have asked them for a permission, but there was not contact address given…