The telescope is relatively simple to use, and you just have to make minimal adjustments in order to find the correct magnification settings. However, there are certain aspects in several parts of the telescope that is too complex for beginners and even some veterans to understand, and one of these parts is the eyepiece.
Most of us probably don’t need to think about eyepiece sizes and types, as almost every telescope that we purchase online and in hobby stores have eyepieces. As the eyepieces come in each package of a telescope, some of us may say that we don’t need to replace these eyepieces since they work fine. However, there are eyepiece types that are supposed to enhance the experience for telescopes with different focal lengths and apertures, and these specific eyepieces may not even come with their compatible telescopes at all.
If you feel that the eyepieces that came with your telescope lessen its magnifying quality, then you might need to replace them. Before purchasing an eyepiece, there are different aspects of the telescope eyepiece that you should know about. Here are several things to consider when buying a replacement eyepiece for your telescope.
Telescope and Eyepiece Focal Lengths
In order for a telescope to magnify the object that it is viewing, it must have sufficient focal length. The focal length is a term used for the measurement of the distance between the lens and the end of the telescope where your eye is supposed to peek through.
The longer the focal length is on the telescope, the higher the magnification it can provide. However, your eye will not be able to see images directly on the telescope, as your eyes usually cannot focus on a “real image” that the telescope is producing. But your eyes will be able to focus on a “virtual image,” which is the one that eyepieces can produce, as long as it is at the end or at the focal point of the telescope.
Without the eyepiece, your telescope is basically useless, and that is why choosing the correct eyepiece for your telescope and your eyes are essential for the best viewing experience.
The telescope is not the only one that has focal length; eyepieces typically have one too. When two focal lengths found in both the eyepiece and the telescope are combined, they can have a stronger magnifying power.
Before we get into the numbers concerning eyepiece and telescope focal lengths, let us take a look at the formula below:
Magnification = (Focal Length of the Telescope) / (Focal Length of the Eyepiece)
In this formula, you can deduce that there is a higher magnifying power for telescopes with longer focal length, but eyepieces with longer focal length produce less magnifying power. All in all, the focal length of the eyepiece must be shorter, while the telescope’s focal length must be higher to provide better magnification.
For example, if you have a telescope at home that has a 400mm focal length, using a 30mm eyepiece on it produces lower magnification settings while utilizing a 10mm eyepiece can help it magnify objects much clearer.
Eye Relief
The focal length is not the only aspect of the eyepiece that matters, but eye relief as well. Eye relief is a term for the distance between your eye and the eyepiece, and the shorter the distance, the more blurry the image will appear. You shouldn’t really put your eye directly on the eyepiece, as you should place a little bit of allowance until the image becomes clearer to your sight.
Eyepieces that have longer focal lengths tend to have longer eye relief, but eyepieces with shorter focal length usually have varying lengths of eye relief. If you have 20/20 vision, short focal length eyepieces that you own should have shorter eye relief, but if you are wearing glasses, then you must get an eyepiece with longer eye relief.
Field of View
There are two fields of view that you need to know to use the telescope and the eyepiece correctly. One is the true field of view, while the other is the apparent field of view. The size of how much space you see on the eyepiece is called the true field of view, and the bigger true field of view is achieved if a telescope has higher magnifying power and if the eyepiece has a larger apparent field of view, which is the size that your eyepiece can offer.
Most eyepieces with longer focal length usually have a large apparent field of view, as they are specifically designed for viewing larger objects like galaxies or star clusters. On the other hand, shorter focal length eyepieces typically have a smaller apparent field of view since they are made to magnify a single object like a planet or the moon.
Eyepiece Lengths
As mentioned previously, eyepieces have various lengths that provide different magnifying settings. A telescope made for beginners often has three eyepieces that are set to low, medium, and high settings. If you want to replace these eyepieces, then you must know about the different focal lengths that eyepieces have so you can determine which one would be the best replacements.
2mm to 9.9 mm – These have excellent focal lengths for viewing the surface of the moon up-close. These eyepieces are recommended for telescopes with longer focal lengths.
10mm to 24.9mm – Eyepieces with these focal lengths are well-balanced and are suitable for telescopes of all sizes. Telescope users will be able to see the surface of the moon, although not as clear as the previous focal lengths, and star clusters with ease.
25mm to 40mm – These lengths are perfect for telescopes with shorter focal lengths, and it is recommended for viewing star fields and nebulae.
If you are left with long focal length eyepieces at home, then you can use a Barlow lens, which is commonly included in beginner telescopes, to increase the magnifying power of the eyepieces.
Choosing the correct eyepiece to put on your telescope is crucial to have a better view of the object that you are magnifying. In addition, knowing the focal length of your telescope is also important, as you will be able to understand which eyepiece size you need to enhance the telescope’s magnifying capabilities.