The Ultra-Cat 56 is a high-performance 56mm apochromatic refractor, engineered for astrophotographers who demand exceptional portability without sacrificing optical precision, color accuracy, and wide-field performance. Featuring a 5-element advanced Petzval optical design with super ED and lanthanum glass, paired with a fast f/4.8 focal ratio, the Ultra-Cat 56 delivers razor-sharp, perfectly flat images across a 50mm image circle. With a 269mm focal length, it is optimized for Full Frame, APS-C, and M4/3 sensors, and remains fully compatible with Medium Format cameras, making it an ideal ultra-wide-field imaging platform for deep-sky and large-scale nebula photography. [b]KEY FEATURES:[/b] [b]Advanced 5-Element Petzval Optics[/b] Eliminates field curvature, ensuring sharp, edge-to-edge stars without additional flatteners. [b]50mm Large Image Circle[/b] Fully supports Full Frame cameras, APS-C and M4/3 sensors for optimal performance. Compatible with Medium Format. [b]Fast f/4.8 Optics[/b] Superior light-gathering capability, enabling deep-sky imaging with shorter exposure times. [b]WIFD (William Optics Internal Focuser Design)[/b] Patented internal rack-and-pinion focuser ensures stable and precise focusing. No vignetting or balance issues — ideal for astrophotography. Compatible with EAF (Electronic Automatic Focuser) for precision focus control. [b]Sensor Tilt Xterminator (STX) & Rotator[/b] Integrated tilt adjuster eliminates sensor misalignment, ensuring uniform image sharpness. [b]No Flattener Required[/b] The optical design naturally corrects field curvature, simplifying setup. [b]Anti-Shrink Lens Cell Technology[/b] Reduces thermal expansion effects, maintaining perfect optical alignment in any climate. [b]Patented Bahtinov Mask for Focusing[/b] Built-in focusing aid ensures pinpoint star sharpness with ease. [b]Premium Build & Ergonomic Design[/b] Lightweight yet robust (4.31 kg / 9.5 lbs) construction with a sleek anodized finish. Patented handle bar design for enhanced portability and ease of use. [b]Versatile Camera Compatibility[/b] Supports Canon EF, RF, Nikon F,Z, Sony E, Pentax K, MFT, and FUJI X mounts.