Telescope eyepieces are a huge topic. After you buy your telescope, you should consider upgrading or replacing eyepieces included in the box with something better because it will vastly improve the image you see.
Only you have to choose the right eyepiece depending on the type of your scope (refractor or reflector) and your preferences of what you want to observe- either planetary or deep sky objects. You also have to decide what magnification you want, how big the field of view and the eye relief you are aiming for. I will explain here all the options and recommendations for different situations.
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Eyepiece Barel Diameter
Eyepieces for telescope are made in two barrel sizes, 1.25″ and 2″. So, the first thing you have to know is what size is your eyepiece holder on your focuser. The majority of beginner level telescopes have 1.25″ eyepiece holder. 2″ holders are usually on bigger and more advanced scopes. However, if you have 2″ focuser, you can buy 1.25″ reducer so, you can use both sizes there. It’s only up to your preference. 2″ eyepieces tend to be more expensive but have a wider field of view.
Focal Length Of The Eyepiece
The main specification of the eyepiece is the focal length. It is a number written on the eyepiece, and it is measured in millimeters. Choosing the focal length of the eyepiece is very important because it will determine the magnification, the field of the view and the eye relief. Later, I will explain what every term means. Everyone should have a variety of focal lengths in eyepiece collection for the telescope. The numbers are usually starting at 4mm and go up to maybe 30-40mm. Certain focal lengths are better for a certain type of objects. Low number focal lengths are high power and are perfect for the planets and the moon craters. The higher numbers which are low power are suitable for observing deep sky objects.
It is perfectly fine to have just 3 eyepieces in your collection. One high power with short focal length, one low power with long focal length and perhaps one in the middle range for special situations.
Magnification of The Eyepiece
I have mentioned above terms “high power” and “low power”. Both these terms mean the magnification and obviously, the high power is high magnification, and the low power is low magnification. The focal length of the eyepiece will set the magnification for each telescope differently. The focal length of the eyepiece alone will not tell you the magnification. You need to know the focal length of your telescope as well to be able to calculate the magnification of the eyepiece. That being said, one eyepiece with a focal length of 10mm will have different magnification with different telescope focal lengths. It is very easy to calculate the magnification of your setup. Let’s say we have 10mm eyepiece and we want to use it with the telescope of 650mm focal length. We have to divide the focal length of the telescope with the focal length of the eyepiece. In our example, it’s 650÷10. The magnification is 65x.
Telescope Highest Useful Magnification
You have to be careful about choosing very high power eyepieces. Every telescope has its own “highest useful magnification” which means that if you exceed this number you will get a blurry image and there is no way to focus beyond this point. The highest useful magnification is related to the aperture(diameter) of the telescope. To calculate it, just multiply the aperture by 2x. So, for an aperture of 130mm, the maximum useful magnification is 260x.
Nevertheless, I would not recommend using such high magnification. What you will be using really depends on sky conditions and 99% of the time you will not exceed 200x magnification, no matter how big aperture you have. So, keep this in mind when choosing the focal length for the magnification. Don’t overpower it! Magnification is only important for the moon and planets. Even if they are much closer to us than nebulas, they are very small on the sky. For the deep sky objects, you have to focus more on the field of view.
Eyepiece Field Of View
Field of view depends on the barrel size, type of eyepiece and focal length of the eyepiece. 2″ eyepieces are dedicated for wide field observing, and they are the best choice for nebulas or star clusters because these objects are quite wide on the sky. Many times bigger than the Moon. As for the focal length, you perhaps figured out that the low power eyepiece will have a wider field of view. When you use 4mm eyepiece, it has very narrow field and high power. It is mostly being used for observing details of the planets and craters on the Moon. It won’t be very useful for the deep sky object. For those, you want to use low power, wide field eyepiece.
The field of view is measured in degrees.
Apparent Field Of View(AFOV): is the angular size of the amount of the sky you will see, and it ranges from 30 to 110 degrees. The AFOV is always written on the side of the eyepiece next to the focal length.
True Field Of View(TFOV): is a measure of the angular size of the image seen through the eyepiece. In other words, it’s how large the image appears in the eyepiece, and it ranges from 0.1 to 2 degrees. The TFOV is different with different types of eyepieces and has to be calculated because two different types of eyepieces with the same focal length will have different TFOV. To calculate TFOV, we have to divide the AFOV with the actual magnification in the telescope with a given eyepiece. So, for example, if you have an eyepiece with 52 degrees AFOV in a telescope, it is giving you 20x magnification. Now, divide AFOV with the magnification, and you get the TFOV figure of 2.6 degrees.
What Is Eye Relief?
The eye relief is a kind of practical aspect of the eyepiece. It is a distance between the lens of the eyepiece and your eye. This distance will tell you how far away your eye can be from the eyepiece to see the 100% field of view. It is very important for people who wear glasses because they can’t go too close to the eyepiece. And if you have short eye relief eyepiece and you wear glasses, you won’t see much through it. The rule of thumb is that short focal length eyepiece has short eye relief. But we already have specially designed long eye relief eyepieces with short focal length. They use more elements, usually 6-7, than standard eyepieces. But, of course, you will pay a bit extra for that kind of comfort.
Barlow Lens – The Handy Friend
I have to include the Barlow lens in the article although it is not an eyepiece. But, it is an important part of your set, and it is upgrading your eyepieces. Barlow lens is essentially multiplying the magnification of the eyepiece. The amazing thing about Barlow is that it will not affect the field of view of your eyepiece so, you can enjoy low power wide field eyepiece with more magnification. Common Barlows are 2x and 3x (2x will double the magnification).
In many cases, Barlow lens comes with the telescope. However, it might be a low-quality one with only one lens element. Good Barlow has at least 2 elements. I recommend you upgrading this handy tool and if you don’t have it yet, definitely get one.
The exciting category is parfocal eyepieces. If you have a telescope, you already know this. When you have a specific focal length eyepiece in the focuser with the image already focused, and you want to change it to a different focal length eyepiece, you have to focus the image again. It seems like no big deal, but sometimes the vibrations and struggle of focusing again are major. Parafocal eyepieces are solving this problem because you can use one, focus and then change for another parfocal eyepiece with no additional focusing of the object. The image will stay focused on, and you can keep changing focal lengths according to your needs. Cool right?
Another interesting eyepiece is “zoom eyepiece,” and it’s doing exactly what is in its name. It’s only one eyepiece with focal length range, and by twisting it, you can change the magnification without changing the eyepiece. The one thing I enjoy about the zoom eyepiece is when you look at some star cluster like M13 with low zoom and then start zooming in it feels like you are flying and getting closer to it. Exciting feeling. However, zoom eyepiece comes with one trade of which is a smaller field of view compared to fixed focal length eyepiece. But, it’s really fun to have one in the collection.
Illuminated Reticle Eyepiece
Illuminated reticle eyepieces are very practical if you have motorized GoTo mount. During the star alignment, you have to point the telescope on few stars and have them exactly in the middle of the eyepiece to get the perfect alignment and tracking afterward. The eyepiece will help you do that because it has illuminated reticle inside so, you can get the stars in the dead center of the eyepiece when performing the alignment.
Kellner or Plossl Eyepiece?
Now, when you start searching for your eyepieces, you can come across terms “Plossl” and “Kellner”. These are types of eyepieces, and the difference between them is inside with the elements. Kellners have simple but effective design with 3 elements, and they are usually cheaper and mostly included in the package of the telescope. Plossls have 4 elements (2 achromatic lens pairs), and they have a sharper image for the higher price tag. The sharpness in both is almost identical in the center of the view, but farther from the center, the Plossls are winning. The image is sharper around the edges compared to Kellner. However, the Kellner is cheaper, and if you are using it with a slow telescope (like f/8 and higher) mainly for planets, you are fine. The difference compared to Plossl would probably be negligible.
Celestron Eyepiece and Filter Kit
This eyepiece kit with filters is very popular among beginner astronomers. Most people will buy it with their first telescope. The eyepieces are plossl design and come in 5 focal lengths, 6,8,13,17 and 32mm. One 2x Barlow is also part of the set with additional 7 filters. But, if you ask me if I recommend this kit for a beginner, I would probably say no. This set has more cons than pros, and I think there are better options for beginners.
- 17 and 32mm eyepieces are quite good
- good Barlow lens
- solid case
- short eye relief
- short focal lengths(6,8,13) are almost impossible to use because of eye relief
- useless planetary filters, only the moon filter is useful
- high price for what you will actually use
I don’t want to discourage you, it’s a nice kit, and I love the carry case, but you will do better when you try to buy everything separately or cheaper kit with fewer components. As I said at the beginning, you will be more than fine with 3 eyepieces. One high power, second low power and one in the middle. Many people are using only two. One for planets and one for deep sky objects.
Recommended Cheap Eyepiece Kit
I found out that sometimes, cheap eyepieces are good quality and better than stock eyepieces that come with your telescope. The example is SVBONY brand. It’s not Celestron, but you can have a nice kit of 3 eyepieces with Barlow lens for half the price of Celestron kit. I bought them for myself because the reviews on the forums and the price are irresistible. Also, I think this is a useful method, and you can benefit from it in the future. Don’t spend too much money on eyepieces or eyepiece kits first, try cheaper ones and find out what you like and don’t like to use. Then you can invest more money in specific focal length eyepieces that you are sure you will love to use.