“Magnification = the focal length of the telescope / the focal length of the eyepiece”
These days eyepieces come in two barrel sizes, 1.25” and 2”. A 2” eyepiece isn’t necessarily better than a 1.25” eyepiece. Usually 2” eyepieces are used when you want wide true field and sometimes both a wide true field and a wide apparent field.
Some 1.25” eyepieces have inner and outer barrels. This lets them fit into either a 1.25” or 2” eyepiece holder. Often these eyepieces have a relatively wide upper body with wide apparent fields. They often have a built in Barlow that helps the eyepiece have a long eye relief.
True field is the number of degrees or arc minutes of sky that can be seen with a given telescope and eyepiece combination.
Apparent field is how large the hole seems to be when looking through the eyepiece. An eyepiece with a narrow apparent field gives the impression of looking through a soda straw. An eyepiece with a wide apparent field is more like looking through a picture window.
Exit pupil is diameter the beam of light coming out of the eyepiece. You would think a large (7 mm) exit pupil would be best for viewing deep sky objects, however, some very experienced deep sky observers feel a 1.5 mm exit pupil gives the best contrast between a deep sky object and the background sky. When observing planets at high power some of us have issues with a very small exit pupil.
The small diameter beam can be easily interrupted by floaters in our eye. This is where it helps to have a large aperture instrument because it will increase the diameter of the beam of light for a given magnification.
Eyepiece Exit pupil = telescope aperture / magnification A 6” aperture (153mm)telescope running 300x will give a 0.5mm exit pupil. A 14” aperture (355.6mm)telescope running 300x will give a 1.18mm exit pupil.
When observing the planets or tight double stars I don’t mind not seeing the full field of view because the objects of interest usually don’t take up much of the field of view. When observing the moon I very much enjoy an eyepiece with a wide apparent field. This is also true when observing large open clusters or large emission nebulae.
Zoom eyepieces sometimes are more convenient than carrying several fixed focal length eyepiece. However, many of them have an annoying feature, the apparent field changes as you change the amount of zoom. At high zoom the apparent field can be very nice but when zoomed to the low end of the power range the apparent field shrinks dramatically.
Most of the better eyepieces are fully multicoated meaning that every air to glass surface has been multi-coated. A fully multi-coated eyepiece will tend to have better contrast than an eyepiece that is not fully multi-coated or just coated.
Eye relief is measured in millimeters describing how close you need to get your eye to the eyepiece to see the full field of view. Sometimes the eye relief stated would seem to give you plenty of room but some eyepiece designs have the eye lens set down into the upper body of the eyepiece making it difficult to get close enough to see the full field, especially if you wear eye glasses when observing.
The diameter of the eye lens (the lens closest to your eye) can give some idea of the eye relief of the eyepiece. Usually the larger the eye lens the better the eye relief.
Eyepiece designs like orthos and plossls with a long focal length usually have good eye relief, but as you go to shorter focal lengths the eye relief can get very short. One way to have a higher magnification and retain good eye relief is to use a Barlow lens.
Some of the newer eyepiece designs have a number of lens elements but with good glass and being fully multi-coated the amount of light loss and scatter is quite small.
Virtually all 1.25” and 2” eyepieces have barrels that are threaded on the inside to accept a filter and to reduce light scatter. The threads are almost always painted flat black.
I find it is always nice to look through an eyepiece before buying. One benefit of attending star parties is that you can often have a look through a number of different eyepiece designs. I’ve had very few problems with the public looking through my good eyepieces. Today’s multi-coated lenses hold up very well and are easy to clean.
It is good to resist the desire to take apart an eyepiece. If some of the lens elements fall out the chances of getting the lenses back in the right order and with the correct side up is highly unlikely. Most telescope dealers don’t have diagrams of how the lens elements should go in the eyepiece. It is a good idea to put your eyepieces away, out of the reach of little hands. The only eyepiece maker that I know of who will fix one of their eyepieces is Tele Vue.
Eyepieces and other items are sometimes available in the SJAA “For Sale” section in the club’s blog area. See http://sanjoseastronomy.blogspot.com/p/for-sale-eyepieces.html
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