Kellner eyepieces have three glass pieces, a doublet and a singlet. Plossl eyepieces have four glass elements and two doublets. A Kellner’s field of view (FOV) is about 35 to 50 degrees, while a Plossl’s FOV is about 50 to 56 degrees. Kellners have less eye relief than Plossls. Kellner eyepieces are usually less expensive than Plossls.
Eyepieces have a big impact on your viewing experience. They come in many different shapes and sizes, each with its benefits.
If you’re looking for a quality eyepiece for your telescope, you may have come across the terms “Kellner” and “Plossl.” But what do these terms mean, and which one is right for you?
In this blog post, I will explore the differences between Kellner and Plossl eyepieces and help you decide which one is best for your needs.
How Does the Kellner Eyepiece Work?
The Kellner eyepiece was invented in 1849 by German optician Karl Kellner. He intended to solve some of the issues with earlier, simpler eyepieces.
The first achromatic eyepiece was the Kellner lens, which did not break light into its colors. Thus, the Kellner eyepiece displayed colors more accurately compared to earlier eyepieces.
It is a three-element design consisting of a doublet and a singlet. Kellner eyepieces are designed for use with Newtonian reflectors and refractors.
They offer a wide field of view (FOV) and good color correction but have less eye relief than other designs.
Kellner eyepieces are a good choice for amateur astronomers on a budget. They are usually less expensive than Plossl eyepieces and provide good image quality.
Currently, it is believed that the Kellner eyepiece design should be used as the minimum standard for serious observing; however, this does not necessarily mean that it should not be used in more expensive setups. It simply means that older and less sophisticated designs, such as Huygens and Ramsden, are best avoided and are actually not all that common.
Even though the Kellner design made everything seem perfect, the adjustments it made were simply enhancements; additional corrections were needed to address internal reflection issues, which can still leave color fringing and distortions.
Pros of the Kellner Eyepieces
The Kellner eyepiece has several advantages:
Perfect Color
The Kellner eyepiece is designed to provide perfect color correction. The three glass elements in the eyepiece help to break light into its individual colors, which results in more accurate color reproduction.
Good Field of View
Kellner eyepieces offer a wide field of view (FOV), which is perfect for amateur astronomers who want to take in as much of the night sky as possible. The FOV for a Kellner eyepiece ranges from 35 to 50 degrees.
Simple Design
The simple design of the Kellner eyepiece makes it easy to use and maintain. The three glass elements in the eyepiece are held in place by a metal or plastic housing, which can be easily disassembled for cleaning.
Less Expensive
Price is a very important consideration for many amateur astronomers. Kellner eyepieces are usually less expensive than Plossl eyepieces, which makes them a good choice for budget-conscious stargazers.
Cons of the Kellner Eyepiece
In addition to its advantages, the Kellner eyepiece also has some drawbacks:
Less Eye Relief
Kellner eyepieces have less eye relief than other designs, which can be uncomfortable for some users. If you wear glasses or contact lenses, you may find it difficult to use a Kellner eyepiece.
Not Ideal for SCTs or Maks
Kellner eyepieces are not well suited for use with Schmidt-Cassegrain telescopes (SCTs) or Maksutov-Cassegrain telescopes (Maks). These instruments have a very short focal length, which can result in vignetting (darkening of the edges of the field of view).
Image Quality Issues
When you upgrade your telescope, you may experience image quality issues with your Kellner eyepieces. This is because the Kellner eyepiece’s simple design does not correct all optical aberrations.
You may notice distortions such as color fringing and chromatic aberration when using a Kellner eyepiece.
Not Well Corrected for Spherical Aberration
Spherical aberration is an optical error that occurs when light waves are not focused on a perfect point. This can result in blurred or distorted images.
Kellner eyepieces are not well corrected for spherical aberration, which means that you may see some blurring and distortion around the edges of the field of view.
How Does the Plossl Eyepiece Work?
The Plossl eyepiece was invented in 1860 by Georg Simon Plossl, an Austrian inventor. Unlike the Kellner eyepiece, designed for Newtonian telescopes, the Plossl eyepiece was designed specifically for use with Schmidt-Cassegrain telescopes (SCTs) and Maksutov-Cassegrains (MAKs).
The Plossl eyepiece is a four-element design consisting of two doublets. It offers good color correction and high-contrast images.
Plossl eyepieces have a wide FOV and are known for their clarity and sharpness. They also have good eye relief, making them comfortable to use for extended periods.
Plossl eyepieces are a good choice for amateur astronomers who want high-quality images. They are usually more expensive than Kellner eyepieces but provide better image quality.
Pros of Using the Plossl Eyepiece
The Plossl eyepiece has several advantages:
Good Color Correction
The Plossl eyepiece offers good color correction thanks to its four-element design. This makes it a good choice for amateur astronomers who want high-quality images.
High Contrast Images
Plossl eyepieces are known for their high-contrast images. This is because they have good color correction and a wide FOV.
You can view a wider range of stars and galaxies with a Plossl eyepiece than with a Kellner eyepiece.
Comfortable to Use
The Plossl eyepiece has good eye relief, making it comfortable for extended periods. This is especially important for amateur astronomers who want to spend long hours observing the night sky.
Wider Fields of View
Plossl eyepieces have a wider field of view than Kellner eyepieces. This makes them ideal for use with Schmidt-Cassegrain telescopes (SCTs) and Maksutov-Cassegrains (MAKs).
You can see more of the night sky with a Plossl eyepiece than a Kellner eyepiece.
No Need to Change Your Eyepieces
There are many amazing things to see in the night sky. With a Plossl eyepiece, you won’t need to change your eyepiece as often as you would with a Kellner eyepiece.
This means you can spend more time observing and less time fiddling with your equipment.
Cons of the Plossl Eyepiece
The Plossl eyepiece also has some drawbacks:
More Expensive
Plossl eyepieces are usually more expensive than Kellner eyepieces, which can disadvantage budget-conscious amateur astronomers.
More Complex Design
The Plossl eyepiece is a more complex design than the Kellner eyepiece. This means it is more difficult to manufacture and may require more maintenance.
May Require Collimation
The Plossl eyepiece may require collimation, which involves aligning your telescope’s optics. This can be a daunting task for amateur astronomers.
Kellner vs. Plossl | What’s the Difference?
Now that you know how each of these eyepieces works, let’s look at the key differences between a Kellner and Plossl eyepiece:
Number of Lenses
The first difference is the number of lenses. The Kellner eyepiece has three lenses, while the Plossl eyepiece has four.
Focal Length
The Kellner eyepiece has a shorter focal length than the Plossl eyepiece. This means it provides a wider field of view but at the expense of image quality.
Eye Relief
Eye relief refers to the distance between your eyes and the eyepiece at which you may easily view the entire image. Compared to the more powerful eyepieces, neither the Plossl nor the Kellner eyepiece has very good eye relief, but the Plossl wins out in the end.
Short eye relief of the Kellner might make it uncomfortable to use, especially for those wearing glasses. The Plossl eyepiece is easier to use because of its slightly longer eye relief.
However, even this more expensive lens will experience issues at higher power ranges.
Cost
The Kellner eyepiece is usually cheaper than the Plossl eyepiece. This makes it a good choice for amateur astronomers on a budget.
Image Quality
The Plossl eyepiece provides better image quality than the Kellner eyepiece. This is because it has a longer focal length and four lenses instead of three.
Field of View
The Plossl eyepiece provides a wider field of view than the Kellner because of its four-lens construction. The field of view refers to the largest viewable region that your gadget allows you to observe.
The field of view of a Kellner eyepiece ranges from 30 to 50 degrees, with an average of 40 degrees.
The three lenses on the Kellner are to blame for the limited field of view. The Plossl has a greater field of view of 50 degrees on average, thanks to its four symmetrical lenses.
Similarities Between the Kellner and Plossl
Now that we’ve looked at the key differences between these two types of eyepieces, let’s take a moment to consider their similarities:
There aren’t nearly as many similarities between the Kellner and Plossl eyepieces as there are differences. The mid-1800s saw the development of both eyepieces, which addressed issues with color and spherical aberrations present in earlier eyepieces.
They then addressed these issues in significantly different ways.
The biggest similarity between Kellner and Plossl eyepieces is that they are both constructed with achromatic lenses that lessen chromatic aberrations. Simply put, achromatic lenses produce more realistic colors when viewed through them.
The Kellner and Plossl lenses were revolutionary when they were developed, even if achromatic lenses aren’t perfect, and more recent eyepieces will have an even better color correction.
The Bottom Line: Which Eyepiece Should You Use?
So, which eyepiece should you use? The answer depends on your needs and preferences.
In general, Plossl eyepieces tend to be superior to Kellner eyepieces but are also more expensive. The only catch here is that Plossls have a larger quality spectrum because of their greater complexity.
Plossl eyepieces can be better or worse than Kellner. There are good and bad Plossl eyepieces.
You must be more cautious when selecting a cheap, generic eyepiece because its quality will rely on the brand and model.
The quality of Kellners will generally be very consistent across the board; therefore, there is a minimal probability that the definition of a picture will vary significantly between brands.
Kellner eyepieces are perfect if you’re just starting with astronomy. When contrasting a comparable Kellner and a Plossl with similar specs, you won’t be able to notice the difference, but as you acquire skill, you’ll start noticing the small differences and likely outgrow the Kellner.
At that time, you’ll probably want to upgrade to Plossl eyepieces or even better ones, so you’ll end up paying money twice.
Choose the telescope with the Plossl even if it is a little more expensive if you are purchasing your first telescope and must choose between models that have one or the other and all other similar specifications.
Kellner vs. Plossl FAQs
What is the difference between a Kellner and a Plossl eyepiece?
The biggest difference between these two types of eyepieces is that the Plossl provides a wider field of view than the Kellner because of its four-lens construction. Additionally, Plossl eyepieces tend to be more expensive than Kellner eyepieces.
What is a long eye relief eyepiece?
An eyepiece with long eye relief means that you can hold it further away from your eye while still being able to see the entire field of view. This is especially important for people who wear glasses, as they may not be able to get close enough to the eyepiece otherwise.
What is an angled eyepiece?
An angled eyepiece is simply an eyepiece that is not perpendicular to the telescope. This can help you view objects high in the sky, as you won’t have to lie on your back to see them.
Final Thoughts
The main difference between Kellner and Plossl eyepieces is their number of lenses and the resulting field of view. Plossl eyepieces usually provide better image quality and a wider field of view, but they also tend to be more expensive.
I hope this article has helped you understand the difference between these two types of eyepieces and which one might be right for you.
Please be careful and use at your own risk
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