GSO Plössl 4mm 50° 1.25″ eyepiece
Looking for an eyepiece that lets you get the most out of your telescope when observing planets, the Moon or double stars? The GSO Plössl eyepiece with a 4mm focal length is a classic choice for anyone who wants to achieve high magnification while maintaining solid image quality. The four-element Plössl design has been proven over decades and remains one of the most popular eyepiece types in the amateur astronomy community.
Optical design and image quality
The Plössl 4mm eyepiece uses a symmetrical four-element arrangement that provides good correction of aberrations and a pleasant, high-contrast image. The 50° apparent field of view follows the classic standard for this type of eyepiece - it may not be the widest on the market, but for planetary observing it is perfectly adequate and well-organised. The FC (Fully Coated) anti-reflection treatment applied to all optical surfaces reduces unwanted reflections, increases light transmission and contributes to better contrast of the observed object. The resulting image is sharp, bright and free from ghost reflections.
Magnification and compatibility
With a focal length of 4mm, this eyepiece delivers high magnification in combination with common amateur telescopes. For example, paired with a refractor of 600mm focal length you will achieve a magnification of 150x. This is ideal for detailed study of the lunar surface, the cloud belts of Jupiter, Saturn's rings or surface details on Mars.
The 31.75mm (1.25″) barrel diameter ensures full compatibility with the vast majority of modern telescopes and astronomical instruments. The adjustable rubber eyecup allows comfortable viewing even with glasses and shields the eye from surrounding stray light.
What to keep in mind
It is worth bearing in mind that very short focal lengths typically result in a shorter eye relief - in the case of this eyepiece, that is a natural characteristic of the Plössl design and should be taken into account, especially if you wear prescription glasses. For observers without glasses, viewing comfort is excellent.
Key specifications
| Parameter |
Value |
| Focal length |
4mm |
| Apparent field of view |
50° |
| Barrel diameter |
31.75mm (1.25″) |
| Optical design |
Plössl (4 elements) |
| Coating |
FC - Fully Coated |
| Eyecup |
Adjustable rubber |
Who is this eyepiece for?
The GSO Plössl 4mm is a reliable and affordable tool that deserves a place in every eyepiece collection. Whether you are a beginner just discovering the wonders of the night sky or an experienced observer looking for a solid high-magnification eyepiece for everyday use, the GSO Plössl 4mm 1.25″ will meet your expectations without unnecessary compromises.
This eyepiece is especially suitable for:
- Detailed observation of the Moon's surface
- Planetary observing - Jupiter, Saturn, Mars and more
- Splitting close double stars
- Observers looking for a cost-effective high-magnification option
- Anyone who already owns a 1.25″ focuser and wants to expand their eyepiece range
