Eyepieces: A Quick Guide
A Difference Maker
“I placed a $450 eyepiece in a $200 telescope and it transformed it into an amazing space discovery instrument.”
This comment illustrates how better eyepieces will be the single most important upgrade to almost any telescope. All manufacturers mostly include a basic eyepiece or two in order to keep the cost of the telescope down24Panoptic and often a set of good eyepieces will exceed the original cost of the telescope. Photographers understand this about cameras and camera lenses.
It Won't Solve All Your Problems
“I’d like an eyepiece to see Saturn clearer and better.”
This comment expresses the hope that an eyepiece with more magnification and better optical qualities will produce a better view of planets. Unfortunately sky conditions and the position of the celestial body in the sky will determine the “sharpness” or “clarity” of the view. When viewing objects closer to the horizon, you are looking through two or three times as much atmosphere as higher in the sky. That atmosphere is often turbulent as the air warms and cools from the heat of the earth and may have more haze or low cloud. Amateur astronomers use the term, “steady skies” for the best viewing.
“I saw a YouTube video with the same scope as I purchased with much better views than what I’m getting.”
This comment from someone in northern Alberta is comparing views with someone in southern Arizona where the planet will be higher in the sky, experiencing less turbulence and less water vapor in the air. The solution is not a better eyepiece but better viewing location where the celestial object will be higher in the sky with less atmospheric interference. A moonless night, no light pollution and higher elevation with thinner air will contribute to better viewing. If you have the opportunity to visit Hawaii’s Mauna Kea or Haleakala you will quickly experience the advantage of higher elevation with less atmosphere for stargazing. In fact we’ve been confused by the abundance of stars that are not normally visible from home.
A Word About Magnification
It’s not all about magnification: The best way to understand magnification is to use your telescope regularly for several nights on several types of celestial objects - the moon, planets, nebulas, star clusters and galaxies. Manufacturers include an eyepiece or two that are usable - start with the eyepiece marked with the higher number - often 25 or 20. Unfortunately if you have a “department store trash scope” it may have come with one or two unusable, high power eyepieces in addition to a useless barlow lens (these are not sold by All-Star Telescope). While the conjunction of Saturn and Jupiter in December 2020 drew a lot of attention to viewing the planets which can benefit from higher magnification, 75% of your viewing likely will be at lower magnification. Why? Many celestial objects such as the Pleiades star cluster, the double cluster in Perseus, the Orion nebula, the twin galaxies M81 & M82 and Andromeda galaxy are large. The Andromeda galaxy is 4 or 5 times the diameter of the full moon. For best views you will want low magnification and large light gathering.
For most amateur astronomers, it is not about “how far” you can see. It’s about “how bright” those deep sky objects (nebulas, galaxies and star clusters) and how much detail you can see. While 75% of your viewing will be at the lowest power, 10% may be at high power for viewing the planets and lunar features with 15% of your viewing at medium power. Each telescope, eyepiece and nightly sky conditions will determine how much you can magnify. If you double the magnification you spread the same amount of light over a larger area and the object will be dimmer. This is not noticeable on brighter objects like the moon and planets but will be apparent on many deep sky objects. As you increase magnification you also magnify imperfections in our atmosphere and the celestial object can begin to look like a hockey puck at the bottom of a swimming pool. You will almost always have a sharper, clearer view of Saturn at a lower magnification and can increase the magnification until the planet begins to “swim” and offers a blurry view. To determine magnification, you divide the focal length of the telescope by the focal length of the eyepiece. Thus the 2000mm focal length of Celestron’s popular C-8 - (NexStar 8SE, etc.) comes with a 25mm eyepiece that results in 80X magnification. A 10mm eyepiece results in 200X magnification. Ignore the manufacturer’s “maximum magnification” which might apply if you are on the space station or moon where there is no atmosphere to contend with.
What Should I Buy?
While it is tempting to buy an eyepiece offering higher magnification, we recommend firstly buying one with lower magnification since 75% of your viewing will be done with that eyepiece. Terence Dickinson, author of NightWatch, co-author of Backyard Astronomer’s Guide and author of dozens of astronomy books once asked what he recommended. His answer was:
“The only way you are going to get sharp performance edge-to-edge across the field is by spending big bucks for top-of-the line glass.” He continues, “As for eyepieces, TeleVue Panoptics are superb, as are TeleVue Naglers and Ethos.” And, “Anything else is second rate... Expensive? Yes. But if you want to smile every time you look in the eyepiece, that's the answer. For more, see my book Backyard Astronomer’s Guide, 3rd Edition (2008), which has an entire chapter on eyepieces with comments about many brands.”
There’s a saying in astronomy, “Don’t look through an eyepiece more expensive than what you can afford.” Yes, quality eyepieces make a huge difference in your viewing.
On magnification, here’s what I recommend:
For a Celestron C11 or C925 (CPC, Evolution 11” and 9.25”) purchase
- The TeleVue Panoptic 41mm as the low magnification eyepiece you’ll use 75% of the time. Divide the 2800 mm focal length of the C11 by the 41mm focal length of the Panoptic 41 and you will have 69X magnification.
- You may need to purchase a 2” diagonal to use the 2” barrel.
- Purchase a 20mm to 24mm medium power eyepiece for your medium power. The TeleVue Ethos 21mm is the number one choice here.
- Then a 13mm eyepiece for your 10% higher magnification viewing. The TeleVue Ethos 13mm is the top eyepiece in this field. Note Dickinson’s comment above, “Expensive? Yes. But if you want to smile every time you look in the eyepiece, that's the answer.”
For the popular Celestron C8 on the NexStar 8SE with a focal length of 2000mm I recommend:
- a TeleVue Nagler 31mm or TeleVue Panoptic 35mm as the low power eyepiece that you will use 75% of the time. These will give you 65X or 57X magnification.
- A medium power 15mm to 18mm and high power 7mm to 10mm will complete your set. Again, purchase the best you can afford.
When we move to the Newtonian style telescope such as the Dobsonians or many short focal length refractors:
- You will find a 31mm to 20mm eyepiece to be a low magnification. Again the Nagler 31mm is a top choice. The TeleVue Panoptic 35mm would also be a good choice.
- Consider something in the 13mm range and down to the 5mm range for your medium and high power viewing. Note that if you buy a high power eyepiece, there may be evenings where the sky conditions don’t support good viewing at those higher magnifications. Thus an 8” or 10” Sky-Watcher Dobsonian with a 1200mm focal length will give you 39X magnification with a TeleVue Nagler 31mm eyepiece. If anything can be observed in the nighttime sky, you will always be able to use the TeleVue Nagler 31mm. You will get 120X magnification with a 10mm eyepiece. Just be aware that magnifications exceeding 200 to 250X may not give “better” or “clearer” views and may be the maximum magnification that you will want to use. Again - do the math of dividing the focal length of the telescope by the focal length of the eyepiece. (The lower the number on the eyepiece, the higher the magnification).
If you add TeleVue eyepieces to your eyepiece selection you not only will have a smile on your face every time you look in the eyepiece, you will have invested in eyepieces for a lifetime that will hold their value if you ever need to sell them or trade them in.
Other Things to Consider
But some brands offer the same magnification and “field of view” of the TeleVue eyepieces. Yes, but there are other qualities in an eyepiece to consider.
- Field of view - you will see an eyepiece characteristic called “Field of View” While this is not the same as magnification, it has to do with how much of the sky you can see - are you looking through a tunnel or through a picture window at the night sky? Wider fields of view are almost almost more enjoyable. Al Nagler developed the 82 degree field of view Nagler eyepiece which continues to be one of the best quality eyepieces. Now the TeleVue Ethos offer a 100 degree field of view so when you look in the eyepiece you almost only see the sky rather than the edge of the field of view. The basic eyepiece(s) that came with your telescope likely offer 50 to 60 degree field of view.
- Eye Relief - how far does your eye need to be from the eyepiece to see the entire field of view. You may find the basic 10mm eyepiece that came with your telescope has a “peep hole” and your eyeball is almost touching the eyepiece to see the entire field of view. You’ll want a minimum 12mm of eye relief and even more if you wear glasses when viewing. The TeleVue Delos line of eyepieces all feature 20mm of eye relief and are a favorite of those wearing eye glasses
- Glass quality - a cheap piece of glass in an eyepiece likely will not give pinpoint stars at the edge of the field of view. Typically they will look like comets and produce what is called “coma” If it is advertised as offering an 82 degree field of view but only 68% of the view is sharp, it is a low quality eyepiece. Additionally cheaper glass will allow a small amount of light from stars to bounce around in the imperfections of the glass resulting in the sky becoming gray and reducing the amount of starlight coming through to your eye. You’ll want the best glass with the best coatings for the best viewing.
Conclusion: Cheaper eyepieces may offer similar specifications to the better and best eyepieces. But they definitely will not offer similar viewing.
OK, I can’t afford the TeleVue eyepieces that Terence Dickinson recommends. What should I purchase? In the lower power eyepieces, from 30mm to 40mm you can consider the Baader Hyperion 36mm and 31mm as a good second choice to the top choice TeleVue eyepieces. In the 20mm to 30mm range you may be able to afford the TeleVue Panoptic eyepieces or consider the Baader Hyperion 24mm and 21mm eyepieces. From the 17mm and higher power, you can consider the Morpheus17, TeleVue Delos, or Baader Morpheus eyepieces. These are excellent. And in the higher power eyepieces, if you can’t afford the TeleVue 13mm, 9mm or 7mm or the TeleVue Delos 12mm, 10mm, 8mm or 6mm, take a look at the Baader Morpheus as a good second choice.
What Will I See?
Unfortunately this question is difficult or impossible to answer. However, getting out with the telescope under the nighttime sky can become a lifelong hobby and passion. In his book, Seeing in the Dark, Timothy Ferris says, “The universe is accessible to all, and can inform one’s existence with a sense of beauty, reason and awe as enriching as anything to be found in music, art of poetry.”