At TopTenREVIEWS We Do the Research So You Don’t Have To.™ In this case we rated and weighed the pros and cons of various excellent telescope eyepieces, but there are some things that can only be decided by you based on your specific telescope and needs.
We looked mostly at eyepieces that will offer you a stunningly wide field of view, in some cases up to 100 degrees, because of the potential wow factor. We looked at instruments that had a variety of focal lengths, selecting ones that would fit into the same basic price range for comparative purposes.
The topic of individual selection offers plenty of room for discussion, however, as you consider which eyepiece you ultimately purchase.
Appropriate focal length is worth some thought. Each piece of equipment we reviewed comes from a family of eyepieces that include many different focal lengths of the same design. The one that is best for you may be a “sibling” to one in our lineup – an eyepiece with the same design but a different focal length.
The largest focal length eyepiece your telescope can benefit from can be calculated by anyone. Just multiply the focal ratio of your telescope by 7. That will give you the outer limit of focal length you would want in the eyepiece you purchase. A telescope with a focal ratio (focal length divided by aperture) of f/5 should not use an eyepiece with a focal length larger than 35mm, for example.
Beyond that limit you can make your decision about focal length based on certain guidelines. These guidelines are general and there will be some variation between manufacturers, but you can expect to get the best results when you consider the following:
1. Eyepieces in the 2mm-4.9mm range offer high magnification and will work best with long focal length refractors and standard Schmidt-Cassegrains.
2. Eyepieces in the 5mm-6.9mm range are great for finding details in planetary viewing tasks for long focal length telescopes. They will also work in shorter focal length telescopes if there are good seeing conditions.
3. Eyepieces in the 7mm-9.9mm range will offer great results for high magnification for short focal length telescopes, helping you see detailed views of the Moon and planets.
4. Eyepieces in the 10mm-13.9mm range will provide good views through telescopes of a wide range of focal lengths. You will be able to see nebula, galaxies, planets and the details of the Moon.
5. Eyepieces in the 14mm -17.9mm range are known for offering good mid-range magnification viewing experiences for all focal lengths.
6. Eyepieces in the 18mm-24.9mm range work well with long focal length telescopes for a look at a wider field. With shorter focal length telescope these eyepieces can help you see mid-range magnification for things like galaxy clusters.
7. Eyepieces in the 25mm-30.9mm range will offer views of open clusters and nebula with a longer focal length telescope. With a shorter focal length telescope you will be able to look at large open clusters and large objects like the Orion nebula.
8. Eyepieces in the 31mm-39.9mm range are best with shorter focal length telescopes for extended views.
9. An eyepiece in the 40mm or more range really only work with shorter focal length telescopes for looking at large vistas.
You may also want to consider calculating the true field of view before making an eyepiece selection. The apparent field of view is the one advertized by eyepiece makers. That number represents the swath of sky you would theoretically be able to see with their telescope eyepiece, or the amount of sky you will feel like you are seeing. The true field of view is the swath of sky you will actually be looking at.
The true field of view can be calculated by dividing the apparent field of view by the eyepiece’s magnification. There are also several calculators online that can do the math for you.
A fair part of eyepiece selection is based on gut reaction. Networking by attending star parties will be of great benefit to you. See which telescope eyepieces provide the most wonderful new perspective on the Universe.