When choosing your new telescopes eyepiece, you will find many different types to choose from. It can all be very confusing. You will hear terms you have never heard before like focal length, apparent field of view, actual field of view, and barrel diameters.
The telescope itself comes with it’s own focal length and then you can choose what focal length your eyepiece will have. Focal lengths of eyepieces range from 2.5mm to more than 60mm. Choose your lens depending on what magnification you need or want. It may be in your best interest to have more than one eyepiece in your collection to choose from at any given time.
The term, apparent field of view, refers to the angle required for your eye to move from left to right or up and down in the viewfinder of your telescopes eyepiece. You will find eyepieces with as small as thirty degree apparent field of view to eyepieces with more than 80 degree apparent field of view.
The actual field of view is just the piece of sky you are seeing through the eyepiece before any magnification. Basically, the actual field of view is equal to the apparent field of view divided by the magnification.
Barrel diameter refers only to the diameter of the eyepiece. The most common size being 1 1/4 inches. You will also find bigger and smaller barrel diameters available. Start with the most common diameter so you do not restrict your field of view. As long as you are looking, you may as well see everything you want to see.
Keep in mind that each eyepiece design has a name. The name each has been given refers to who designed it and what lens configuration the eyepiece has. What you may consider good or bad about an eyepiece depends on what you want to see in the first place. Be forewarned that you may not find all the qualities you want in one eyepiece though, and, obviously, some eyepieces cost more than others.
Some good qualities to look for in an eyepiece are the ability to see images well if your telescope has a fast f number. As this is a difficult quality to achieve, having a telescope with a slower f number may allow you to see better images.
A wide field of view is another good quality to look for in an eyepiece, however if you want to combine a fast f number with a wide field of view, be prepared to shell out some serious cash. They do not come cheap.
Finally, if you want an eyepiece to allow you to be able to see fine, low contrast detail on Mars or Jupiter, what is important is the number of air to glass interfaces in the eyepiece. The fewer the better. More air to glass interfaces mean more scattered light and more scattered light means less detail you will see in the object you are looking at.
These are just a few starter tips for anyone who is new at telescoping and trying to figure out how to select their new telescopes eyepiece.