Eyepiece GSO Plossl 40mm 1.25" – astronomical eyepiece, 40 mm focal length, 1.25-inch barrel

The GSO Plossl 40mm 1.25" eyepiece is an astronomical eyepiece with a 40 mm focal length and a standard 1.25-inch barrel, intended for visual observation with telescopes that accept 1.25" oculars. Engineered to deliver low magnification and a wide true field of view, this eyepiece helps with comfortable, low-power observing sessions, wide-field views of star fields, open clusters and large deep-sky objects, as well as initial planetary framing and star hopping.

Advantages

This 40 mm Plossl provides a large apparent and true field at low magnification while maintaining good edge performance typical for Plossl designs. The 1.25-inch barrel ensures compatibility with a wide range of amateur telescopes and the longer focal length reduces eye strain during extended observing. The eyepiece construction balances field size and contrast to make wide-field observation practical without complex optical accessories.

Suitable for

• Wide-field visual observation of star fields and large deep-sky objects
• Low-power surveys and star hopping to locate targets
• Comfortable, extended observing sessions with minimal eye fatigue
• Initial framing of planets and large lunar features

Key specifications

• Focal length: 40 mm
• Barrel diameter: 1.25 inches
• Optical design: Plossl-style eyepiece
• Intended use: visual astronomy, low-power wide-field viewing

How to use

Insert the eyepiece into the 1.25" focuser or diagonal of a compatible telescope and secure it with the focuser thumbscrew. Begin observing at low magnification to scan large areas of sky; adjust focus slowly until the image is sharp. For improved comfort, set your eye position to match the eyepiece's eye relief and use a comfortable observing posture. When changing magnification, remove or replace this eyepiece with shorter focal length oculars as needed.

Usage recommendations

Use the eyepiece in telescopes with focal ratios of approximately f/5 and slower for best full-field performance; in faster systems, vignetting or edge distortion can increase. Combine with a stable mount and appropriate finder scope for efficient star hopping. Clean lenses only with appropriate optical cleaning tools and store the eyepiece in a protective case when not in use.