Eyepieces
The amateur astronomy market is awash with all different sorts of eyepieces. Amongst these you'll hear foreign sounding names, along with terms like field of view, focal length, exit pupils. It can all seem a little confusing at first, and it’s not always easy to establish which eyepiece is the right one for you.
To help you out, we've outlined some of the pros and cons of the different varieties.
To put it simply, magnifying lenses are there so that you can see, in a larger size, the second image which is produced by the telescope. In theory, a magnifying eyepiece like this could be made up of just one lens. But different eyepieces are made to be suitable for different fields of view, for example, and this is why an eyepiece has to be made out of a combination of definite lenses set a certain distance apart from each other. A larger distance between the eye and the eyepiece, as well as aberration correction can be desirable features. You can get a type of mount that holds the lenses – this is also known as eyepiece casing.
Huygens
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These eyepieces are made out of two lenses which each have a relatively small field of view. The lenses are not cemented together and are well designed for projecting the sun with the telescope. The field of view is around 40%, and aberration will occur.
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Kellner eyepieces are made up of three lenses. Because the lenses are cemented together and therefore form an achromat, aberration is low although still present. They can be used with telescopes with magnifications of up to 1:10. The limit for Newton reflectors is an aperture ratio of 1:5, otherwise you should go for a Plössl eyepiece. Field of view up to 45%
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Orthoscopic eyepieces
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These eyepieces have four lenses – two bi-convex, one bi-concave and the other is the ocular lens. These eyepieces offer a shaper image in the middle and also towards the edges of the image. Therefore they're good for observing planets or binary stars and have an even field of view. As there are just four lenses and they are coated well, these eyepieces don't have a high absorption, which is another plus. Its field of view is around 40-45%.
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Plössl eyepieces are one of the most common standard eyepieces. It is usually this type supplied when you buy a complete telescope. There are eyepieces which are made from two pairs of four lenses. These pairs of lenses are cemented together and each produce an achromat – meaning they don't show so much aberration. As Plössl eyepieces have small focal lengths, the distance between the eye and the eyepiece can be too small, which means that you have to get very close to it and the eyepiece “sticks” to the eye. Other eyepieces can be better in this sense. The field of view is around 50°.
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Erfle
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Erfle eyepieces have five lenses and a field of view up to 70°. It’s best to get them with a large focal length and a wide-angled eyepiece. Smaller focal lengths don’t always give you a good view and under 20mm is not recommended.
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LV-eyepieces
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LV eyepieces have a special design which means that even with smaller focal lengths there can be a large distance between the eyepiece and the eye, and they therefore offer a good view. These eyepieces are particularly well-designed for glasses wearers, but even those who don't wear glasses will notice the advantages. Amongst these eyepieces are the LV by Vixen, as well as the Long Eye Eyepiece and Ultra Wide Angle by Skywatcher (66° field of view). There are also 2'' eyepieces from Vixen's LV eyepiece range.
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Nagler
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Naglerokulare bestehen aus verschiedenen Linsenpaare, die miteinander verkittet sind. Meist verfügen diese Okulare über sieben Linsen, es gibt aber auch Abwandlungen mit weniger Linsen. Mit diesem Okularen hat man einen gigantischen Himmelseindruck. Man glaubt fast in den Himmel einzutauchen. Das liegt zum großen Teil an den gigantischen Gesichtsfeldern von etwa 80°. |
Large angle eyepieces have the disadvantage that their fields of view are curved, as with the Plössl or the orthoscopic eyepieces, for example. This means that they’re not necessarily good for photographic purposes.
2" eyepieces
Instead of a 1 ¼ inch eyepiece, you might choose a 2'' eyepieces. These eyepieces have the advantage that they have a significantly better imaging ability and a larger field of view than most 1 ¼ inch eyepieces. An important factor is that the 2'' eyepieces have a much larger field aperture than the 1.25'' eyepieces. Many people report that you really feel as if you're plunging into the sky, but they are only really suitable for higher focal lengths as wide angled eyepieces.
Field of view
The field of view that an eyepiece offers will be a deciding factor. It can vary hugely, ranging from 30° to 80°. Each field of view corresponds to the angle by which the eyepiece is labelled and recognised. But wider eyepieces can sometimes be misleading, because the field of view is not the actual field that you are able to see when you look at the sky.
Something which largely affects the field of view is the telescope itself. The actual field of view that you’ll see is also decided by the different magnifications. But by taking the field of view that's labelled on the eyepiece, you'll be able to work out the actual field that you are able to see.
First, the magnification:
M = Lens focal length / Eyepiece focal lengt
e.g. 1000:10 = 100 fach
Now the actual field of view:
Actual field of view = supposed field of view / Magnification
For example, let's take the Super Plössl eyepiece with 52° field of view.
Actual field of view = 52° : 100x = 0,52° = 30’
The field of the view would therefore have a size of 0.5°, or 30 arc minutes.
For comparison, here is a table with all the different fields of view:
Eyepiece |
Field of view |
Magnification |
True field of view |
Kellner |
40° |
120x |
0,3° |
Plössl |
50° |
120x |
0,4° |
Super Plössl |
52° |
120x |
0,43° |
Ultra Wide Angle |
66° |
120x |
0,55° |
Panoptic |
68° |
120x |
0,56° |
Nagler |
82° |
120x |
0,68° |
How do you work out the true field of view when you think the details aren't correct, or they aren't labelled?
This is what you can do:
Measure the aperture from the underside of the eyepiece outwards (unscrew the socket first so that it’s easier to measure). The second value that you need is the focal length which is labelled on the eyepiece. The field of view can be calculated by the following inverse tangent function:
Apparent field of view = half the aperture of the field of view / focal length of the eyepiece tan-1
This figure is not whole field of view is not given, rather half of the field of view – so multiply the result by two.
Example:
Plössl
Measured aperture of field of view = 12mm, focal length = 12.5mm
6mm/12.5mm tan-1 = 25.6 x 2 = 51°
Which eyepiece suits my telescope?
18.05.2012
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