A Guide on Choosing the Best Telescope Eyepieces
The eyepieces allow you to focus the light collected by the telescope, so you can observe a sharp image of the object or area where the telescope is pointing. It may seem like a small link in the chain, but it has a big effect on your telescope’s optical system, and finding the right eyepieces will greatly increase its potential.
With so many options to choose from, getting the right set of eyepieces for you and your telescope can seem a bit tricky. This guide offers some insights and explanations about the different types of eyepieces, specifications and how it all connects to optimize your astronomy and astrophotography sessions!
Focal Length and Magnification
Focal length is an important specification that must be considered when determining the magnification, also known as power, of the eyepiece and telescope with which it is used. The following formula will help you determine the magnification based on the specifications of your eyepiece and telescope:
Magnification = Focal length of the telescope (mm) / Focal length of the eyepiece (mm)
The 20 mm eyepiece of the 2000 mm telescope (2000/20) gives you 100 power (100x), which makes objects appear 100 times closer to you through the telescope than they look to your naked eye.
Field of view: Apparent and true
The apparent field of view of the eyepiece (AFOV) is expressed in degrees (°). This is how much of the sky can be seen from end to end only through the eyepiece. AFOV range from narrow (25 ° – 30 °) to extremely wide angle (80 ° or more).
The true field of view of the eyepiece is the angle of the sky that is visible through the eyepiece when attached to the telescope. The true field can be calculated using the following formula:
True field = visible field / magnification
Suppose, for example, that you have an 8-inch Schmidt-Cassegrain telescope with a focal length of 2000 mm and a 20 mm eyepiece with a field of view of 50 °. The magnification will be 2000 mm / 20 mm = 100x. The true field would be 50 \ 100 or 0.5 ° – approximately the same visible diameter as the full moon.
Long Eye Relief
Eye Relief refers to the distance between your eye and the eyepiece lens when the image is in focus. The relief of the eyes is traditionally proportional to the focal length: The shorter the focal length, the shorter the relief of the eyes. However, some of the more modern eyepiece designs provide long-eye relief, regardless of the focal length, which is especially useful for those who wear glasses. If you want to keep your glasses while using a telescope, the relief of the eyepiece is an important feature that must be taken into account (we recommend looking at the eyepieces for long-eyed relief).
How many eyepieces do I really need?
Although there is no specific number of eyepieces to own, with a few different telescope eyepieces you have a better chance of reaching the optimal power for the particular object you are observing, given the sky conditions at the time. You will usually want to start with low power (i.e., a long focal length of the eyepiece, for example 25 mm or 30 mm) to get the object in the field of view of the telescope. Then you can try an eyepiece with a little more power (shorter focal length, maybe 18 mm or 15 mm) and see if the view looks better. If this happens, swap the eyepiece for even more power, etc., until you reach this “sweet spot” where the brightness of the image, the scale of the image, and the amount of visible detail combine to form the most pleasing view.
What about Barlow lenses?
You can also choose a long focal length eyepiece with comfortable eye relief and use image amplifiers to increase power, such as a Barlow lens. Barlow increases the effective focal length of the lens, increasing the magnification. The idea is that two eyepieces and a Barlow will give you the flexibility to zoom in on four eyepieces and will give higher magnifications with less powerful eyepieces.
The use of different eyepieces can significantly increase the flexibility and functionality of any telescope.
What is a Zoom Eyepiece?
The zoom eyepieces are a single eyepiece that you can adjust to change its focal length, which changes the magnification. You just insert the eyepiece into your telescope and simply rotate the zoom eyepiece barrel to zoom in or out of whatever you are looking at.
They tend to occupy the middle level of focal length / magnification, so you may need very high and low magnification eyepieces (short and long focal lengths, respectively), but a zoom one can handle everything between you.
Telescope Eyepiece Sets
Telescope eyepiece sets are a great idea for anyone new to backyard astronomy. For one price, they provide a selection of eyepieces and filters to improve the versatility of your telescope setup.
The Eyepiece Set offers package of various eyepieces, conveniently providing a full array of different magnifications and powers in one place. This lets you find numerous eyepieces that are all compatible with your telescope.. The big three astronomy manufacturers, Celestron, Orion and Meade, produce their own eyepiece kits and we have selected the best one here in this article.
Basic Tips to Follow When Shopping for Eyepieces
- Consider the focal length of your telescope, or telescopes, to make sure the eyepiece will provide an appropriate magnification to suit your needs.
- If you wear eyeglasses while using a telescope, pay attention to the eye relief specification of different eyepieces, as ample eye relief can improve comfort and ease-of-use while wearing corrective lenses.
- Depending on your observing goals, consider the apparent field of view of your eyepiece choices.
- If versatility is paramount, consider a zoom eyepiece or Barlow lens to increase the number of possible magnifications to use.