Plossl eyepieces used to be the gold standard for mid-tier eyepieces. Today, they are often relegated to the status of “token” eyepieces thrown in with beginner telescopes. However, there are some good reasons to use Plossls, and they are a lot less expensive than many other eyepiece designs.
Advantages & Disadvantages of Plossl Eyepieces
Plossl eyepieces have been around since the 1860s, but only became common in the 1980s when they were finally economical to manufacture and began to provide advantages in performance over Orthoscopic and Kellner eyepieces with then-new fast focal ratio telescopes like Dobsonians. They usually use four elements, each a pair of cemented lenses with convex and flat surfaces; the two convex surfaces face each other. Most Plossls are of the “symmetrical” type, with two identical pairs of lenses. Some add an additional lens element. The symmetrical nature of the optics in most Plossls makes manufacturing cheap, and as such, they are often bundled with many beginner telescopes or sold in kits.
Plossl eyepieces can achieve an apparent field of view no greater than about 52 degrees. However, some long focal-length Plossl eyepieces may have a narrow field due to mechanical limitations imposed by the barrel diameter.
Plossl eyepieces provide sharp views with reasonably controlled aberrations down to f/4 or so. However, their limited field of view can feel narrow, especially if you’ve used a wide-angle eyepiece or two. They also are short on eye relief at focal lengths below about 11mm, to the point that viewing is impossible with eyeglasses on and extremely uncomfortable to use even without glasses with short focal-length Plossl eyepieces.
Things to Look For in Plossl Eyepieces
Any good eyepiece should be fully multi-coated, which means that every lens surface has multi-coatings. Cheap “coated” or “multi-coated” eyepieces may lack coatings on some optical surfaces, and “fully coated” optics have more reflections and less efficient light transmission, leading to more glare and dimmer images. Blackened lens edges and interior barrels as well as good baffling are also extremely important features to minimize glare and maximize contrast in a well-made eyepiece.
As previously mentioned, we would recommend you avoid Plossl eyepieces with focal lengths below 11mm and stick to eyepieces with other optical designs that offer more comfortably sized eye lenses and longer eye relief. Most people find it difficult to use plossls with focal lengths less than 9 mm without experiencing severe discomfort. Do not buy “eyepiece kits,” as they inevitably contain Plossls too short in focal length to actually use, and often they are also substandard in quality or provide you with redundant focal lengths of eyepieces.
Plossls with focal lengths longer than 32mm in 1.25” format or 56mm in 2” format will have their field of view limited more by the eyepiece barrel itself than the optical design, and thus won’t provide a wider true field but instead a more straw-like view at lower magnification. This can even lead to an exit pupil too big for your eyes to handle or simply a too-bright background sky. As such, don’t bother with things like 40mm 1.25” Plossls or 60+mm 2” Plossls.
As always, no telescope can handle magnifications over about 50x/inch, and atmospheric conditions or issues with the telescope’s optics, cooling, or collimation might limit you to quite a bit less than that, usually 300x or below. Similarly, 4x magnification per inch of aperture is the lowest you can go due to your pupil’s limited diameter, and under light-polluted skies, 5x or 6x per inch is often a more appropriate limit. Your magnification is calculated by dividing your telescope’s focal length by your eyepiece’s focal length; for example, a 1200mm telescope and 10mm eyepiece will produce 120x.
Many cheap “Plossl” eyepieces are actually Kellner or even worse designs masquerading as Plossls. Always check to make sure the brand you’re buying is reputable, and stay away from shady, unknown brands on sites like Amazon. If it seems too good to be true, it probably is. Many manufacturers’ specs are purposely vague or talk about an entire series, even though each eyepiece might be very different.
Our Plossl Eyepiece Recommendations
1. Tele-Vue Plossls: The Tele-Vue eyepieces provide excellent sharpness and contrast, even with fairly fast instruments, with minimal chromatic aberration, glare, and edge-of-field aberrations. They also work great with Tele-Vue’s Barlow lenses and other accessories.
2. Vixen NPL: The Vixen NPL eyepieces have high-quality optics, and the longer focal length units of the NPL series use twist-up plastic eyecups, making them easy to grab and extremely comfortable to look through.
Best Value:
3. SVBONY SV207 Plossls: The SVBONY SV207 Plossls are high-quality and use comfortable, wide rubber eyecups for easy viewing with or without glasses. They have slightly more eye relief than competing designs and provide excellent sharpness as well as value for the money.
4. Apertura/GSO Plossls: The GSO Plossl eyepieces and many authorized resale brands such as Apertura are well-designed eyepieces with smooth, undercut-free barrels for easy insertion into focusers and star diagonals. However, the short (<15mm) focal length versions are especially uncomfortable to look through and seem to have a narrower field of view 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.
5. Celestron Omni Plossls: The Celestron Omni Plossls are optically and mechanically similar, if not identical, to the GSO designs, with the same pros and cons. These are not to be confused with the generic Plossl eyepieces thrown in Celestron’s kits or with beginner telescopes, which are quite a bit different. The “E-Lux” 25mm and 40mm Plossls are simply rebadged Omni Plossls, however.
6. Orion Sirius Plossls: The Orion Sirius Plossls are slightly sharper than GSO and Celestron equivalents but have slightly inferior coatings, which theoretically makes them worse performers for deep-sky viewing. However, in practice, the difference is minimal. These eyepieces are great, and they have a little bit more variety of focal lengths than some of their rivals.
As always, eyepieces are easy to switch out, and all of the ones on this list are well-known and come highly recommended. There’s nothing stopping you from buying eyepieces from different product lines depending on the price, focal length, and features you want.
