Telescope Eyepieces


Telescope Eyepieces are types of lens that are attached to a variety of optical telescope devices. When looking through the device, the lens that is closest to the eye that is considered the eyepiece. It is the job of the objective lens to collect light and focus it to create an image. it is ideal to place the eyepiece near the focal point of the objective lens to magnify the collected image; however, the amount of magnification depends on the focal length of the eyepiece.

By switching the eyepiece, observers can adjust what is viewed in the telescope in many different ways. One example of a reason to change an eyepiece will be to increase or decrease the telescope magnification. Differences in eyepieces offer a variety of information such as field of view, differing degrees of eye relief for the viewer, and magnification.

Eye Relief

The term eye relief refers to the distance from the eyepiece to the observer’s eye. This is an important aspect of many eyepieces because it helps determine how easy it is to use said eyepiece.  The shorter this distance, the more difficult it can be to observe, which can cause trouble if the observer is wearing eyeglasses. Long-focal-length eyepieces, which are usually lower powered, have long eye relief and do not need to be specially designed to increase eye relief.  However, short-focal-length eyepieces do not have long eye relief and must be specially designed to make them easier to use.

Magnification/Focal Length

Written on the side of every eyepiece in millimeters, this number is the magnification an eyepiece provides when used in combination with a telescope. Magnification values are determined simply by dividing the focal length of the telescope by the focal length of the eyepiece, which means that an eyepiece with a smaller number provides a higher magnification.  For example, a 10mm eyepiece would provide twice as much magnification as a 20mm eyepiece.  It is worth noting that the same eyepiece will provide different magnifications for different telescopes. It is normal for astronomers to have 3 types of eyepieces as a minimum: low power, medium power, and higher power eyepieces. Although magnification range varies per telescope, the normal range for most scopes will be from 50x to 250x.

Field of View

Field of view is very important for getting the best view and one can change the field of view in an eyepiece by changing magnification. by changing tDetermined by the magnification and the apparent field of view of the eyepiece, the true field of view is the amount of sky seen through an eyepiece. Apparent field of view is a design characteristic taken into account during eyepiece design as they can vary from narrow to wide apparent fields of view. To view very large objects such as the Andromeda Galaxy or Pleiades star cluster, one will need an eyepiece with very large field of view and hence a very low magnification. 

Different Options

  • 0.965 in. (24.5 mm) – These are no longer manufactured for high-end telescopes, but the smallest diameter eyepieces can still usually be found in telescopes at the toy store and the other retail telescopes. Many of these eyepieces that come with such telescopes are plastic, and some even have plastic lenses.
  • 1.25 in. (31.75 mm) – This is the most popular telescope eyepiece, which have a practical upper focal length limit of 32 mm. The issues with longer focal lengths is that the edges of the barrel intrude into the view. This is especially seen when the focal length of the eyepiece is greater than 32 mm, the available field of view will fall below 50°, which is the minimum acceptable width that most would consider using.
  • 2 in. (50.8 mm) – With an upper focal length limit of 55 mm, these eyepieces are the most popular after the 1.25 inch variants. The popularity of these eyepieces is because they are able to provide greater focal lengths than other options. However, the sacrifice is that these are usually more expensive and may be too heavy for some telescopes.
  • 2.7 in. (68.58 mm) – Allowing for slightly larger fields of view, many high end telescope optics manufacturers are now producing and incorporating these eyepieces.
  • 3 in. (76.2 mm) – With a 120 degree field of view and even larger focal lengths, the large barrel size in 3″ eyepieces does come with many disadvantages. Expensive and rare to find, these eyepieces are almost too heavy. Smaller telescopes will lack the eyepiece size to support these sizes, plus there are balancing issues. If one is interested in purchasing these eyepieces, check out manufacturers such as Explore Scientific. Explore Scientific also makes telescopes that can support their weight. Issues that plague telescopes too small for these optics include balance issues and severe vignetting.
  • 4 in. (102 mm) – Because of low demand as their only common use is with observatories, very few manufacturers make eyepieces of this size. .

What To Choose

It is recommended that for viewing planets, the rule that is often mentioned is the “sixty power per inch” rule, which says, “Never use an eyepiece that will magnify more than 60 times for every inch of aperture.” Often times, it isn’t the most high powered optics that work the best for viewing planets.

For viewing galaxies, it is recommended to determine how large the galaxy is and the power of your telescope. This is the case because there are very wide galaxies, like the Andromeda, which will require a very different lens than a small planetary nebula.

References And Further Reading

Great article that is a guide for eyepieces

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