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✅ About Eyepieces: Different Eyepiece Designs
There are a flood of eyepieces of various designs on the amateur astronomical supply market which you can use for your own observing. You will hear strange-sounding names, terms such as field of view, focal length, exit pupil and then still not really know which eyepiece is it the right one for your observing. In order to remedy this somewhat, I have listed the various designs with their advantages and disadvantages below.
Basically, eyepieces are like a magnifying glass used to magnify the view of the intermediate image produced by the telescope itself. In principle, such an 'eyepiece magnifier' could consist of just a single lens e lement. Since however different eyepieces also want to produce different fields of view, an eyepiece has a combination of lens e lements at various fixed spacings. But achieving longer eye relief and the correction of image aberrations are also reasons for this. The 'holder' which keeps all these lens e lements in place is also known as the eyepiece 'barrel'.
Huygens eyepieces
These eyepieces are a simple two lens e lement design which provides a relatively small apparent field of view. The lenses are not cemented together and therefore well suited for solar projection through a telescope. These eyepieces are among the oldest designs and are only rarely found among telescopes accessories. Their field of view is about 40°.
Kellner eyepieces
Kellner eyepieces are composed of three lens e lements and have a field of view of around 45°. Since the two eye lenses are cemented and constitute an achromatic doublet, only a little chromatic aberration is produced. Kellner eyepieces can be used for higher magnifications with telescopes having an aperture ratio of up to 1:10. The limit with Newtonian reflectors is at an aperture ratio of 1:5, and here it is better to resort to Ploessl eyepieces.
Orthoscopic eyepieces
These eyepieces have four lens e lements, two of which are biconvex and one is biconcave. These eyepieces provide a high level of sharpness both at the centre as well as at the edge of the field of view. They are therefore also of interest for planetary and double star observing. They feature a flat image field. The low absorption exhibited by these lenses due to their use of only a few lens e lements and accordingly good lens coatings is another plus for these eyepieces. Their field of view is about 40° to 45°.
Ploessl eyepieces
Ploessl eyepieces are the astronomical standard lens and anyone who does not want to spend a lot can afford these eyepieces. Often one finds these Ploessl design lenses in complete accessories kits for beginners.
These eyepieces always consist of four lens e lements in two pairs. These lens e lement pairs are cemented together and each form an achromatic doublet meaning there is virtually no chromatic aberration. But short focal length Ploessl eyepieces have a problem with their eye relief. This means that the eye lens e lements are so small that you have to position your eye very closely to the eyepiece. Your eye is glued to the eyepiece, so to speak. Other eyepiece designs are therefore preferable at short focal lengths.
Their apparent field of view is about 50°.
Long eye relief eyepieces
These eyepieces have become especially popular in recent years. If you look at the eyepieces used by any amateur astronomer, you will probably find at least one of this type. These eyepieces cannot be assigned to any one particular design, but rather have long eye relief as a dominant feature.
They always have long eye relief of around 16-20mm, even at short focal lengths, and are hence especially comfortable to use.
These eyepieces are ideal for spectacle wearers, but also people who do not wear glasses appreciate the b enefits
Nagler
Nagler eyepieces are an in-house development from the eyepiece manufacturer TeleVue. The main e lements of these eyepieces consist of a number cemented doublets. Most of these eyepieces have seven lens e lements, but there are also variations using fewer e lements. These eyepieces you provide one with a fantastic impression of the night sky. They almost make you feel as if you are floating in space. This is due largely to their enormous apparent field of view of 80°.
These eyepieces also reduce image errors such as coma and distortion. In practice, this means that you observe sharp stars right out to the edge of the fields of view, even in 'fast' telescopes.
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