When you’re looking at the stars through a telescope, the eyepiece is what brings the view to life. It’s the last stop for the light from the stars before it reaches your eye, so choosing the right one is important for having the best possible experience. There are so many eyepieces out there, it can be tough to pick the right one. But don’t worry, just keep reading this post and you’ll be on your way to finding the perfect match for a clear and enjoyable viewing experience.
The exit pupil is a small circular hole in a telescope that allows light to pass through and is visible as a glowing disk when the telescope is pointed toward a bright surface. When you look through the eyepiece, you see the exit pupil.
Eye relief is the distance between your eye and the eyepiece when you can see the entire field of view and is crucial for comfortable viewing, especially for those who wear glasses.
Field Of View
The field of view is the circle of the sky that can be seen through the eyepiece and is measured in degrees or fractions of a degree. Wide-field eyepieces are popular as they allow you to see a larger portion of the sky at any given magnification.
The power, or magnification, of a telescope, is determined by dividing the focal length of the telescope by the focal length of the eyepiece. The recommended maximum usable power is around 50-60 times the aperture of the telescope to get the best results with bright and clear images with a wider field of view. A 2x Barlow lens can be used to increase magnification while preserving eye relief.
Different Types of Eyepieces
The Plössl eyepiece is a commonly used lens in telescopes that offers a clear and vibrant image through its four pieces of glass arranged in two back-to-back lens systems. With a wide field of view of around 52°, it is suitable for both planetary and deep-sky viewing. Although the Plössl is an economical choice, there is a range of prices based on the quality of the lens.
One downside to this type of eyepiece is the short eye relief, particularly with focal lengths less than 12mm, requiring the observer to position their eye close to the lens to see the full field of view. Despite this limitation, the Plössl remains a popular choice and is often included in new telescope packages.
Radian eyepieces are a must-have for those who appreciate quality optics in their telescope. With either six or seven lens elements, these eyepieces are perfect for planetary and general observation and are designed with the comfort of glasses-wearers in mind, featuring a 20mm eye relief.
One unique feature of the Radian is the Instadjust system, which helps in proper eye positioning for non-glasses wearers and maximizes contrast at the same time. This is important because if you are not wearing glasses and get too close to the eyepiece, you might experience vignetting where the edge field rays will miss your pupil. The push-pull, click-stop adjustment helps position your eye quickly and eliminates stray light.
Al Nagler created Nagler eyepieces in the early 1980s, and they are highly regarded by astronomers for their wide field of view. Unlike typical eyepieces that offer a 50-degree field of view, Nagler eyepieces offer an impressive 110-degree field of view, which gives you a “spacewalk” feeling. The secret to this wide view lies in the design, which features a Barlow-like lens group within a long focal-length eyepiece. Astronomers appreciate Nagler eyepieces for their clarity and sharpness, as well as the ability to see more sky without constantly moving the telescope. However, these eyepieces can be heavy, which may not be suitable for all telescopes or drive mechanisms. They also tend to be pricier and may only be compatible with 2-inch focusers.
An orthoscopic eyepiece is a type of lens used for viewing objects through a microscope or telescope. It was invented by German physicist Ernst Abbe in 1880 and is sometimes referred to as the “Abbe” lens. It’s relatively inexpensive and has a good level of clarity, especially for close-up views of inner planets like the moon. However, it has a narrow field of view, and the short eye relief can be a problem for people who wear glasses. Despite these drawbacks, the orthoscopic eyepiece offers reduced geometric distortion and comfortable eye relief. In recent years, it has gained popularity among amateur sky gazers and scientists due to its good performance and affordability.
A Barlow lens is a tool used by astronomers to increase the magnification of their eyepieces. It was invented in the 19th century by Peter Barlow and contains a concave lens placed between the objective lens or mirror and eyepiece. This results in a 2 or 3 times increase in magnification, doubling the effective focal length of the telescope. Barlow lenses are a cost-effective and simple option compared to zoom lenses and provide a fixed magnification level with a smaller field of view.
Things To Consider
The focal length of an eyepiece is a crucial factor in determining the type of objects you can observe through your telescope and the quality of the view. It refers to the distance between the eyepiece lens and the point where light comes into focus. Different focal lengths result in different magnifications. Magnification = focal length of the telescope / focal length of the eyepiece, so the lesser the focal length of the eye piece, the higher the magnification.
Apparent Field Of View
True Field of View (FOV) refers to the actual width of the area that is visible through a binocular or telescope. It is expressed in degrees and represents the angular width of the view. On the other hand, Apparent Field of View (AFOV) refers to how large the image appears when viewed through the binocular or telescope. It takes into account the magnification of the lens and is calculated by multiplying the True Field of View by the magnification. In other words, AFOV gives an idea of the perceived width of the visible area, which is larger than the actual width of the area in the case of high magnification.
To prevent light from scattering and reflecting unnecessarily, optical surfaces are covered with a thin film of a special material called anti-reflection coating. This helps to reduce the amount of light lost. The technique makes use of the wave-like nature of light and takes into account the way light phases oppose each other and how reflectivity is affected by the index of refraction. By using a single coating that is 1/4 of a wavelength thick, it is possible to eliminate reflections at a specific wavelength because the two light beams cancel each other out. Typically, a single coating on a lens is designed to work best with a green light in the middle of the visible spectrum. However, by using multiple layers of coatings, it is possible to reduce light loss across the whole visible spectrum.
Care and Maintenance of Telescope Eyepiece
To take care of a telescope’s eyepieces, follow the right steps and guidance to maintain the health of the telescope and its eyepieces for a long time:
- Avoid touching the surface of the lens to prevent damage from skin oils to the optical coating.
- Remove dirt and dust to keep the lenses safe.
- Clean the eyepieces using a lens-cleaning solution, pure isopropyl alcohol, or a soft, solvent-impregnated cleaning pad.
- Clean the mirrors regularly for better image formation, especially for reflector telescopes.
The eyepiece is an important component in the viewing experience of a telescope. Understanding factors such as exit pupil, eye relief, field of view, and magnification, along with the different types of eyepieces like Plossl, Radian, and Nagler, can greatly enhance the quality of your stargazing experience. Whether you’re a beginner or an experienced astronomer, having the right eyepiece and reading a proper guide on choosing the best telescope will definitely help you see the stars as vividly as possible.