William Optics Ultra-Cat 56 f/4.8 WIFD Petzval Refractor Telescope

William Optics’ Ultra-Cat: The Ultra-Compact 56mm Petzval Refractor!

5 element Petzval optics: Sharp, flat-field images across the full frame without extra flattener.
Internal Focuser (WIFD): Smooth focus with zero image shift, even with heavy cameras.
Sensor Tilt Xterminator (STX): Quickly corrects sensor tilt for perfect alignment.
Fast f/4.8 Wide Optics: Bright, versatile imaging for full-frame and APS-C cameras.

The Ultra-Cat 56 is the latest evolution in the William Optics acclaimed Ultra-Cat telescope lineup, bringing advanced optical performance to a more compact and accessible package. Built around a 5 element advanced Petzval optical design, this 56 mm refractor delivers sharp, flat-field images across the 50 mm image circle without the need for an external field flattener, making it ideal for wide-field astrophotography straight out of the box! Gathering light quickly to produce stunning, detailed views of expansive celestial targets like nebulae and star clusters, and is compatible with a wide variety of camera sensor sizes for flexible composition.

Beyond its excellent optics, the Ultra-Cat 56 is engineered for ease of use. It includes William Optics’ patented Internal Focuser Design (WIFD) for smooth, stable focusing under heavy imaging loads, and a Sensor Tilt Xterminator (STX) with integrated rotator to help fine-tune your imaging train for perfect alignment. A thoughtful suite of accessories and adapters, along with a sleek and durable build, ensures the Ultra-Cat 56 is ready to elevate your astrophotography experience from the very first night!

William Optics Ultra-Cat 56mm f/4.8 Petzval Refractor Telescope 3 different side views

Notable Features

Compact & Lightweight: 56 mm travel-friendly refractor.
Advanced Petzval Optics: 5 lens element design for sharp, flat-field images, no external flattener needed.
Fast & Bright: f/4.8 with 269 mm focal length.
Ready for Imaging: Built-in sensor tilt with camera rotator gives your compositional flexibilty.
Stable Focusing: Internal rack-and-pinion focuser design.

See Why the Ultra-Cat 56mm is Crafted for Astrophotography!

Built-in Advantages

William Optics Ultra-Cat 56mm f/4.8 Petzval Refractor Telescope STX and rotator

Featuring a camera rotator and tilt adjustment plate - this refractor is thoughtfully constructed for ease of use. A tilt plate is crucial at ensuring your camera sensor is parallel to the telescope's focal plane, correcting oblong stars to pinpoint perfection! The camera rotator gives you compositional freedom, and easy movement to get the perfect shot.

EAF Ready

Enabling EAF compatibility out of the box is another attentive feature tailored to stress-free astrophotography. Without the need for tracking down and attaching bulky brackets, you’re able to spend less time setting up and more time under the stars. (Electronic auto focuser sold separately)

Priority on Portability

William Optics Ultra-Cat 56mm f/4.8 Petzval Refractor Telescope

William Optics packed a punch in this tiny but mighty imaging refractor. Weighing in at 5.64 lbs (2.56 kg) it still holds a 5 lens design, and a sturdy internal Rack & Pinion style focuser! The included handle bar with vixen mounting base for a guide scope, and dual side dovetail plate makes this compact and adventure-ready!

William Optics Ultra-Cat 56mm f/4.8 Petzval Refractor Telescope retractable dew sheild

Optical Excellence

Starting off strong with a 5 element Petzval lens design, this refractor ensures sharp, edge-to-edge stars across the large 50 mm image circle. The Ultra-Cat 56 mm also packs a punch with fast f/4.8 and a wide 269 mm focal length, making it possible to get the majority of larger targets in one frame! Compatible with a full frame, APS-C, M 4/3, and medium format sensors, you are able to use a wide variety of cameras! Due to its Petzval design that naturally corrects field curvature, no field flatteners are required.

William Optics Ultra-Cat 56mm f/4.8 Petzval Refractor Telescope internal design and rack and pinion focuser

Internal Focusing Control

William Optics included their highly regarded WIFD (William’s Internal Focuser Design) to the Ultra-Cat 56 mm refractor. Internal focusing prevents image shift, and ensures stable alignment, especially with heavy cameras or accessories like Off-Axis Guiders (OAGs). While the rack-and-pinion focuser offers superior stability, preventing slipping focus with their gear-driven mechanism. Topping off the focuser features, it is compatible with EFAs (Electronic auto focusers) right out of the box, making it straightforward and a joy to use.

Engineered with Astrophotography in Mind

The Ultra‑Cat 56 mm is packed with thoughtfully designed features to make imaging effortless. The built-in Sensor Tilt Xterminator with camera rotator provides compositional freedom while ensuring pinpoint stars across the full 50 mm imaging circle. The carrying handle doubles as a Vixen‑style guide scope mount, combining two essential functions into one smart accessory. The M68 to M48 adapter adds versatility, bridging common astrophotography thread standards so your gear fits together cleanly. A built-in temperature gauge helps track environmental changes during imaging for optimal performance. Finally, the dual 210 mm dovetail plate (Vixen‑style & Arca‑Swiss) underscores the attention to detail in every component, making this telescope a strong choice for both beginners and advanced imagers.

Diagram Reports for the Ultra-Cat Refractor

The spot diagram demonstrates how tightly the telescope can focus starlight, small, compact point clusters across the field indicate highly corrected optics with minimal aberrations. The longitudinal aberration chart shows how well different wavelengths of light stay in the same focus; the closer those colored lines are, the better the chromatic correction, which means cleaner, sharper images with little to no color fringing. The MTF chart highlights how well the telescope preserves contrast and fine detail from the center to the edge of the field. When all three charts show tight groupings, low aberration, and strong contrast retention, it confirms that the Ultra-Cat 56 mm delivers high-precision, high-quality optical performance across the board. To dive deeper into these charts and their purpose we reccomend checking out: Interpretation of Telescope Diagram Reports