William Optics UltraCat 76 f/4.8 WIFD

The William Optics UltraCat 76: A 4.98 kg Imaging Platform

The UltraCat 76 is a 76mm apochromatic refractor for astrophotographers who want clarity and true colour in a portable package. With a total weight of 4.98 kg including the ring and plate, it comes ready to image, with its own tilt adjuster, rotator and camera adapters.

At its core, the UltraCat 76 is for the wide-field imager. Its 365 mm focal length captures vast expanses of the night sky, framing large nebulae complexes and stellar nurseries in a single shot. The fast f/4.8 optical system reduces exposure times, enabling you to gather more data in a single night. The internal focuser, the Sensor Tilt Xterminator and the rotator are all part of the package.

5-Element Petzval Optics with Super ED and Lanthanum Glass

The optical heart of the UltraCat 76 is its advanced 5-element Petzval design, which eliminates field curvature, ensuring sharp, edge-to-edge stars without additional flatteners. The design includes super ED and lanthanum glass.

The elements are:

• Two Super ED Glass Elements: extra-low dispersion glass used to control false colour.
• One ED Glass Element: Working in concert with the Super ED elements, this lens further refines color correction across the visual spectrum.
• One Lanthanum Glass Element: a high-index glass type.
• One X-Element: the name given to the fifth element of the design.

f/4.8 Speed at 365mm for Wide-Field Imaging

The combination of a 365 mm focal length and a 76 mm aperture yields a fast f/4.8 focal ratio, a key specification that defines the UltraCat 76's primary purpose. A "fast" optical system like this gathers light more quickly than an f/7 or f/10 telescope, enabling deep-sky imaging with shorter exposure times.

The 365 mm focal length places the UltraCat 76 firmly in the wide-field imaging category. With a full-frame sensor the field is about 5.6 x 3.8 degrees, which frames the Andromeda Galaxy (M31), the Pleiades (M45) or the North America Nebula (NGC 7000) with the Pelican Nebula. Whole constellations such as Cygnus are far larger than this field and need a mosaic.

A 50mm Flat Image Circle for Full-Frame Sensors

The UltraCat 76 has a 50 mm image circle, the diameter of the usable field at the focal plane. A standard full-frame camera sensor has a diagonal of approximately 43 mm, so the 50 mm circle covers it with some margin.

The image circle fully supports full-frame cameras, APS-C and M4/3 sensors.

WIFD Focuser: 20mm of Internal Travel

William Optics equips the UltraCat 76 with its William's Internal Focuser Design (WIFD), which it describes as a patented internal rack-and-pinion focuser that ensures stable and precise focusing, with no vignetting or balance issues.

The focuser mechanism is a robust rack-and-pinion design with 0-20 mm of travel. For automated control, it is compatible with an EAF (Electronic Automatic Focuser) for precision focus control.

M54 & M48 Adapters with Integrated Sensor Tilt Control

The UltraCat 76 is designed for seamless camera integration. It comes with M54 and M48 camera adapters and a wrench. For DSLR and mirrorless cameras, T-mount support covers Canon EF and RF, Nikon F and Z, Sony E, Pentax K, MFT and FUJI X.

A standout feature of the imaging train is the included Sensor Tilt Xterminator (STX) and Rotator. It does two jobs:

• Camera Rotator: Turns the camera to frame your target.
• Tilt Adjuster: Tiny misalignments in the camera sensor can result in elongated stars in the corners of your image. The integrated tilt adjuster eliminates sensor misalignment, ensuring uniform image sharpness.

The 5-Element Design That Needs No Flattener

Many refracting telescopes, especially fast ones, suffer from field curvature, an optical aberration that causes stars at the edge of the field to appear out of focus even when the center is sharp. This typically requires the use of an additional lens called a field flattener. The William Optics UltraCat 76, however, is built on a 5-element Petzval design that has the field-flattening elements incorporated directly into the main optical tube.

The optical design naturally corrects field curvature, simplifying setup. There is no separate flattener to buy or space.

Visual Observing with the f/4.8 UltraCat 76

William Optics presents the UltraCat 76 as an astrograph and does not describe a visual configuration for it. Ask us before buying a diagonal and eyepiece for visual use.

As with any short telescope, the 365 mm focal length gives low magnification and wide fields rather than high power on planets or small objects.

A 3.81 kg OTA with Bahtinov Cap and Dual Dovetail

Every mechanical component of the UltraCat 76 reflects a focus on functionality and quality. The optical tube assembly (OTA) weighs 3.81 kg, and it has a lightweight yet robust build with an anodized finish.

Several key features enhance its usability in the field:

• Patented Bahtinov Mask Cap: The lens cap carries William Optics' patented Bahtinov mask, a built-in focusing aid. Point at a bright star and the diffraction pattern shows when you are at focus.
• Mounting Ring & Handle Bar: The mounting ring holds the OTA, and its patented handle bar design aids portability and ease of use.
• Dual Vixen/Arca-Swiss Plate: The dovetail plate is compatible with both the standard Vixen-style saddles found on most astronomy mounts and the Arca-Swiss clamps common on photographic tripods.

4.98 kg Total Weight and a 363mm Retracted Length

Portability is a central theme of the UltraCat 76's design. The total weight, including the mounting ring and dovetail plate, is just 4.98 kg (10.98 lbs). The tube length is 363 mm to 415 mm, and a soft carry case is included for transport and storage.

The box also holds riser blocks, the M48 and M54 adapters with their wrench, and an Allen key set.

Setup and Care for Your 76mm UltraCat

William Optics' Anti-Shrink Lens Cell Technology reduces thermal expansion effects to keep the optics aligned as the temperature changes.

For your first imaging session, use the Bahtinov mask cap to focus on a bright star. Once focused, you can move to your target and use the camera rotator to compose your shot. Check your corner stars and use the Sensor Tilt Xterminator if you notice any elongation. Finally, a critical safety warning: never aim the telescope at the Sun without a proper solar filter fitted over the front of the telescope; doing so for even a moment may permanently damage your vision.