William Optics RedCat 51 II “U” 51mm f/4.9 Petzval Apo Telescope M48 Mount Lens

The William Optics RedCat 51 II “U” is a compact apochromatic (APO) astrograph telescope designed primarily for astrophotography, using a Petzval optical design to deliver flat, well-corrected images across the field for wide-field deep-sky imaging. It’s built for astrophotographers who want a lightweight, high-performance imaging telescope that can also double as a telephoto lens for terrestrial photography via its M48 mount.

Optically, the RedCat 51 II offers a 51mm aperture with a fast f/4.9 focal ratio, giving it strong light-gathering capability well suited to capturing faint deep-sky objects such as nebulae, galaxies, and star clusters with shorter exposure times than slower optical systems require. The Petzval design, using multiple lens groups rather than a traditional mirror-based telescope layout, is specifically engineered to produce flat fields with minimal coma and chromatic aberration across the entire imaging area, a critical consideration for astrophotographers using large sensor cameras where edge-to-edge sharpness and star shape matter significantly.

As an apochromatic (APO) design, the lens uses specialized glass elements to bring red, green, and blue wavelengths of light into much closer focus alignment than a standard achromatic lens, substantially reducing chromatic aberration and color fringing, particularly around bright stars, a common issue with lower-tier astrophotography optics. The “U” designation in this version typically refers to an updated iteration in William Optics’ RedCat lineup, generally incorporating minor design refinements over previous versions.

The M48 mount system allows the RedCat 51 II to connect to a wide range of astronomy imaging cameras, filter wheels, and other accessories using the M48 thread standard common in astrophotography setups, while adapters are also generally available to allow the telescope to mount on standard camera bodies for use as an ultra-telephoto prime lens for terrestrial or wide-field photography. Focus is achieved manually via a dedicated focuser mechanism rather than a traditional camera lens focus ring, consistent with astrophotography telescope design rather than conventional photographic lenses.

Build quality includes a compact, lightweight construction relative to its optical performance, making it a popular choice among astrophotographers who want a portable, travel-friendly imaging telescope, along with a distinctive red-colored barrel that has become a recognizable design element of William Optics’ RedCat series.

Key Features
– 51mm aperture Petzval apochromatic (APO) astrograph telescope
– Fast f/4.9 focal ratio
– Flat-field optical design optimized for astrophotography
– Reduced chromatic aberration (APO design)
– M48 mount for astronomy imaging cameras and accessories
– Manual focuser mechanism
– Compact, lightweight, portable construction
– Adaptable to standard camera bodies via appropriate adapters (confirm specific adapter compatibility)