Exploring Plossl Eyepieces: Comprehensive Insights for Stargazers
Plossl eyepieces were once considered the gold standard for mid-tier eyepieces. Although their reputation has diminished, and they are often seen as 'token' eyepieces included with beginner telescopes, there are still reasons to utilize Plossls. These eyepieces have a longer history than many other designs and are generally less expensive.
Plossls can trace their origins to the 1860s but did not become popular until the 1980s when they began outperforming Orthoscopic and Kellner eyepieces on fast focal ratio telescopes like Dobsonians. Most Plossls use four lens elements, each comprised of a pair of cemented lenses with convex and flat surfaces. Many Plossls are of the "symmetrical" type, with two identical pairs of lenses. Some models incorporate an additional lens element. This symmetry makes manufacturing economical, making Plossls often bundled with beginner telescopes or sold in kits.
Key Factors to Consider in Plossl Eyepieces
- Field of View
A typical Plössl provides an apparent field of view of approximately 52 degrees. Longer focal-length Plossl eyepieces may have a narrower field due to mechanical limitations imposed by the barrel diameter.
- Optical Performance and Multi-Coating
Quality is essential in an eyepiece, and a good one should be fully multi-coated. Many cheap eyepieces claim to be multi-coated but lack coatings on some optical surfaces. Some may be labeled as "fully coated," but still exhibit more reflections and less efficient light transmission, leading to more glare and dimmer images.
Other important features include blackened lens edges, a darkened interior barrel, and good baffling. These features help minimize glare and maximize contrast in a well-made eyepiece.
In practice, Plossl eyepieces provide sharp views with reasonably controlled aberrations in telescopes with focal ratios down to f/4 or so. However, their limited field of view can feel narrow, especially compared to wide-angle eyepieces.
- Possible Magnification Range
No telescope can handle magnifications exceeding approximately 50× per inch of aperture. In practice, atmospheric conditions or telescope-related issues often limit magnification to quite a bit less, usually 300x or below. On the lower end, 4x magnification per inch of aperture is the lowest possible due to the limited diameter of our pupils. Under light-polluted skies, 5x or 6x per inch is a more appropriate limit.
- Reputable Brands
Over time, many inexpensive "Plossl" eyepieces have been found to be Kellner designs disguised as Plossls. It is essential to ensure the brand purchased from is reputable and to avoid shady, unknown brands. If a price seems too good to be true, it probably is. Manufacturers' specs can sometimes be intentionally vague or cover an entire series despite differences in individual eyepieces.
Focal Lengths to Avoid
- Too Short (< 11 mm)
At focal lengths below 11mm, eye relief becomes so short that viewing is impossible with eyeglasses. Even without glasses, short focal-length Plossl eyepieces can be uncomfortable.
I avoid Plossl eyepieces with focal lengths below 11mm and recommend other eyepieces that offer more comfortable eye lenses and longer eye relief. This is also why I never buy "eyepiece kits," as they invariably contain Plossls too short in focal length to use, and often, they are also substandard in quality or provide redundant focal lengths of eyepieces.
- Too Long
Longer Plossls are also best avoided because, at focal lengths longer than 32mm in the 1.25" format (or 56mm in the 2" format), the field of view is limited more by the eyepiece barrel than by optical design. They do not offer a wider true field but instead provide a more straw-like view at lower magnification. This can lead to an exit pupil too big for our eyes to handle or a background sky that is too bright. I recommend avoiding things like 40mm 1.25" Plossls or 60mm+ 2" Plossls.
Trustworthy Plossl Eyepieces
Extreme Performance Pick: TeleVue Plossls
Tele-Vue produces excellent Plossl eyepieces with minimal chromatic aberration, glare, and edge-of-field aberrations. They work well with Tele-Vue's Barlow lenses and other accessories.
The Next Best Plossls: Vixen NPL Series
The Vixen NPL eyepieces boast high-quality optics. I particularly appreciate the longer focal length units of the NPL series which feature twist-up plastic eyecups, making them easy to grab and comfortable to look through.
Best Value: SVBony SV207 Plossls
The SVBONY SV207 Plossls are high-value eyepieces. They feature comfortable, wide rubber eyecups and offer slightly more eye relief than competitors. I have been impressed by their sharpness and overall value.
Great Value: Apertura/GSO Plossls
Apertura's Plossl eyepieces and many authorized resale brands like Apertura are well-designed, with smooth, undercut-free barrels for easy insertion into focusers and star diagonals. However, the short (<15mm) focal length versions were uncomfortable for me to look through and featured a narrower field of view of below 50 degrees. The 15mm unit also has a fuzzy field stop, so the edge of the field of view does not cut off smoothly.
As always, eyepieces are easy to switch out, and all of these options are well-known and well-regarded. There is no reason why you cannot purchase eyepieces from different product lines depending on your desired price, focal length, and features.
- Despite their diminished reputation, Plossl eyepieces still have reasons for usage, such as their historical significance and lower cost compared to other designs.
- Plossls can trace their history back to the 1860s and became popular in the 1980s when they outperformed Orthoscopic and Kellner eyepieces on fast focal ratio telescopes.
- When considering Plossl eyepieces, it's crucial to ensure a good optical performance with multi-coating for better light transmission and minimal glare.
- Some disadvantages of Plossl eyepieces include their narrow field of view and the limited magnification range, especially under light-polluted skies.
- To avoid poor-quality Plossl eyepieces or those disguised as Kellners, it's essential to consider reputable brands and avoid cheaper, unknown brands when making purchases. Some examples of trustworthy brands for Plossl eyepieces include TeleVue, Vixen, SVBony, and Apertura.
