William Optics UltraCat 56 f/4.8 WIFD

5-Element Petzval Optics: A Flat 50mm Image Circle

The UltraCat 56's 5-element Petzval optics correct field curvature across its 50mm image circle, so no separate field flattener is required. The 5-element design uses two Super ED glass elements, one ED element, one lanthanum element and one X-Element. The design eliminates field curvature for sharp, edge-to-edge stars without additional flatteners.

By correcting for field curvature internally, the UltraCat 56 simplifies your imaging train. There is no separate flattener to space, which keeps the imaging train short. William Optics optimises the UltraCat 56 for full-frame, APS-C and M4/3 sensors and also describes it as compatible with medium format cameras.

f/4.8 Speed and 269mm Wide-Field Capture

With a fast f/4.8 focal ratio, the UltraCat 56 gathers light significantly faster than typical f/6 or f/7 refractors. This speed shortens the exposure time needed, allowing you to collect more signal on faint, expansive targets in a single imaging session. For imagers working under light-polluted skies or with limited time, this is a critical advantage for maximizing data quality.

The 269mm focal length gives a wide field of view for large targets. With a full-frame sensor (36 x 24 mm) the field is roughly 7.7 x 5.1 degrees, enough to frame the Andromeda Galaxy (M31) or the North America Nebula (NGC 7000) together with the Pelican Nebula (IC 5070) in one shot.

WIFD Focuser: 30mm of Stable, Internal Travel

The UltraCat 56 uses the William Optics Internal Focuser Design (WIFD), a patented internal rack-and-pinion focuser for stable and precise focusing, with 0-30 mm of focus travel.

• No balance issues: The WIFD design has no vignetting or balance issues, which makes it ideal for astrophotography.
• EAF Compatibility: The focuser is compatible with an EAF (Electronic Automatic Focuser) for precision focus control.

STX Rotator and M54/M48 Camera Adapters

The UltraCat 56 is designed for seamless camera integration. It ships with M54 and M48 camera adapters (with an adapter wrench), two common camera-side threads for astronomy cameras, filter wheels and T-mounts.

William Optics includes its Sensor Tilt Xterminator (STX) rotator in the box:

• 360° Camera Rotator: A built-in rotator allows for effortless framing of your target. You can turn the camera to frame a target without unscrewing the imaging train.
• Sensor Tilt Xterminator (STX): This integrated tilt adjuster allows you to make micro-adjustments to your camera's sensor plane. If you notice elongated stars in one corner of your frame, the STX lets you dial them out, ensuring uniform, pinpoint stars across the entire image.

Field-Ready Build: 2.56 kg OTA with Vixen/Arca-Swiss Plate

Engineered for imagers on the go, the UltraCat 56 optical tube assembly weighs only 2.56 kg (5.64 lbs). That low weight helps with transport and setup on portable mounts. The total weight is 3.21 kg (7.08 lbs).

The telescope includes several practical design features for field use. The lens cap incorporates a patented Bahtinov mask, providing a simple and highly accurate tool for achieving critical focus. The mounting ring is attached to a dual-side dovetail plate compatible with both traditional Vixen-style saddles and modern Arca-Swiss tripod heads. Its patented handle bar design aids portability and ease of use.

Adapting the UltraCat 56 for Visual Observing

The UltraCat 56 is a photographic instrument first. William Optics' material for the UltraCat 56 does not describe a visual configuration, so check with us before buying a diagonal and eyepiece for it.

As with any short telescope, the 269mm focal length gives low magnifications and wide fields rather than high power on planets or close double stars.

UltraCat 56 vs. RedCat 51: Aperture and Illumination

Both are compact William Optics astrographs; these figures come from William Optics' own specifications for each.

• RedCat 51 WIFD: 51mm aperture, 250mm focal length at f/4.9, 45mm image circle and 1.77 kg for the optical tube.
• UltraCat 56's Advantage: The UltraCat 56 features a larger 56mm aperture compared to the RedCat's 51mm, about 20% more light-gathering area, a 50mm image circle and a slightly faster f/4.8 ratio, with an optical tube weight of 2.56 kg.

Getting Started and Care

William Optics builds the UltraCat 56 with what it calls Anti-Shrink Lens Cell Technology, which reduces thermal expansion effects to keep the optics aligned as the temperature changes during the night. Even so, give the telescope time to reach outdoor temperature before you take your focus frames.

• Mount the dual-side plate in a Vixen-style saddle or an Arca-Swiss clamp and balance the telescope with your camera attached.
• Thread the M54 or M48 camera adapter onto the rotator, then add your camera or T-mount.
• Point at a bright star, fit the Bahtinov mask cap and focus with the WIFD focuser until the central diffraction spike is centred, then remove the cap.
• Take a test frame; if stars are elongated in one corner, use the Sensor Tilt Xterminator to correct sensor tilt.
• Rotate the camera to frame your target, lock it, and start imaging.

Never aim the telescope at the Sun without a proper solar filter fitted over the front of the telescope.