The Ultra-Cat 131 30th Anniversary Edition is a special commemorative refractor marking 30 years of William Optics craftsmanship and innovation. Based on the renowned five-element advanced Petzval optical design, this limited release renders remarkable astrophotography performance wrapped in a distinctive anniversary finish exterior.

Showcasing a finely laser-engraved emblem of a cat embracing “30,” a lens cover personally hand-signed by founder William Yang, and an included 30th Anniversary Certificate, the Ultra-Cat 131 stands as both a high-performance imaging instrument and a collectible tribute to the enduring legacy of the Cat series.

Corrects field curvature at the optical design level, delivering consistently sharp, edge-to-edge flat-field images across the entire sensor. This integrated correction removes the need for external field flatteners or additional optical accessories, simplifying the imaging setup while preserving maximum light transmission and optical precision.

Engineered to fully illuminate and resolve the demands of both 35mm full frame and medium format sensors, this system enables expansive wide-field compositions while preserving exceptional fine detail across the entire image circle. Its generous coverage and superior optical correction ensure pinpoint stars from center to corner, even on larger sensors.

Compared to standard 106mm-class telescopes, it delivers greater light-gathering capability, improved resolution, and a wider corrected field, resulting in deeper exposures, sharper structures, and more immersive astrophotography overall.

With its fast f/4.9 focal ratio, the system captures a substantial amount of light in a shorter period of time, making it exceptionally well-suited for deep-sky imaging. The accelerated light-gathering performance allows for shorter exposure times while still revealing faint nebulae, delicate dust lanes, and subtle galactic structures with impressive clarity and contrast.

This speed enhances imaging efficiency while producing richer colors, stronger signal-to-noise ratios, and more detailed results—ideal for vivid, high-impact deep-sky compositions.

A major advantage of the Ultra-Cat 131 is WIFD Internal Focusing System. Designed specifically for imaging applications, this patented system eliminates the external rear focuser obstruction common in traditional designs. All focusing takes place internally within the optical tube, maintaining a cleaner optical path while reducing vignetting risk and preventing unwanted mechanical shifts that could compromise alignment.

Reduces the effects of thermal expansion and contraction, preserving precise optical alignment and consistent performance across a wide range of environmental conditions.

A patented, fully integrated sensor tilt correction system is built directly into the optical assembly, effectively eliminating image distortion caused by minute sensor misalignment. By allowing precise micro adjustments, it ensures the imaging plane remains perfectly orthogonal to the optical axis.

The result is uniform star shapes, corner to corner sharpness, and maximized detail across the entire frame, meeting the precision standards of professional astrophotographers.

Featuring a built-in, patented Bahtinov mask, this design streamlines the focusing process and enables exceptionally precise, repeatable accuracy. By generating a clear diffraction pattern directly within the optical path, users can fine-tune focus quickly and confidently for perfectly sharp, pinpoint stars.

Its seamless integration removes the need for separate accessories, reduces setup time, and ensures faster workflow with consistently reliable results, clearly distinguishing it from competing instruments.