William Optics ULTRA-CAT 56 mm APO Refractor Imaging Telescope
The William Optics UltraCat 56 is a premium ultra-wide-field apochromatic refractor designed from the ground up for serious astrophotography in a compact, travel-ready form. Featuring a 56 mm aperture and fast f/4.8 focal ratio (269 mm focal length), the UltraCat 56 is built to capture expansive nebulae, star fields, and Milky Way structures with exceptional sharpness, contrast, and colour correction across the entire frame.
At its core is an advanced 5-element Petzval optical design, incorporating high-grade Super ED, ED, Lanthanum, and specialised glass elements to deliver a naturally flat field with pinpoint stars from centre to edge — no external field flattener required. This makes the UltraCat 56 exceptionally well suited to modern imaging sensors, including Full Frame, APS-C, Micro Four Thirds, and even larger formats, simplifying the imaging train while maintaining professional-level image quality.
The telescope utilises William Optics’ patented WIFD (William Optics Internal Focuser Design), an internal rack-and-pinion focuser system that keeps the optical path rigid and stable during focusing. This design minimises image shift, flexure, and tilt — critical for long-exposure astrophotography — and is paired with an integrated Sensor Tilt Xterminator and camera rotator, allowing precise alignment of the imaging sensor for perfectly shaped stars across the field.
Despite its performance, the UltraCat 56 remains impressively portable. Its short tube length and lightweight CNC-machined aluminium construction make it ideal for grab-and-go imaging setups, airline travel, and lightweight equatorial mounts. It is equally at home in a permanent observatory or a remote dark-sky location, offering professional astrograph capability without the bulk of larger systems.
Supplied as a complete, imaging-ready package, the UltraCat 56 includes essential accessories such as a Bahtinov focusing mask, multiple threaded camera adapters, mounting rings with handle, and a protective carry case — everything needed to integrate seamlessly into an astrophotography workflow straight out of the box.
For astrophotographers seeking a compact APO refractor with uncompromising optical performance, minimal setup complexity, and premium mechanical execution, the William Optics UltraCat 56 stands out as one of the most refined ultra-wide-field astrographs available today.
Specifications
Lens Type: 5-Element Advanced Petzval
Lens Elements: Super ED Glass x2 ED Glass x1 Lanthanum Glass x1 X-Element x1
Focal Length: 269 mm
Diameter: 56 mm
FocalRatio: f/4.8
Image Circle: 50 mm
Focuser: WIFD R&P Focuser Compatible with EAF
Focuser Travel:0-30 mm
Adapter Included:Sensor Tilt Xterminator Rotator M54, M48 Camera Adapters
Optional Flattener:No Flattener Required
T-Mount Support:Canon EF, RF Nikon F, Z Sony E Pentax K MFT (M4/3) FUJI XCamera Format:Full Frame APS-C M4/3
Dovetail Type:Vixen ↔︎ Arca-SwissTube
Length:296.3 mm - 348.3 mm
OTA Weight:2.56 kg / 5.64 lbs
Total Weight:3.21 kg / 7.08 lbs