William Optics UltraCat 108 f/4.8 WIFD

The UltraCat 108: A 108mm Astrograph for Demanding Imagers

The UltraCat 108 is a 108mm apochromatic refractor for astrophotographers. With its 108mm aperture and fast f/4.8 focal ratio, it is designed to capture faint, extended deep-sky objects such as the Rosette Nebula or parts of the Veil Nebula with shorter exposure times. Its 60mm image circle fully supports full-frame and medium-format cameras.

The optics need no separate flattener, and the box includes the Sensor Tilt Xterminator rotator and M48, M54 and M68 camera adapters, so the telescope is ready to connect to a camera.

5-Element Petzval Optics: Super ED and Lanthanum Glass

At the heart of the UltraCat 108 is a sophisticated 5-element Petzval optical design. It eliminates field curvature, providing sharp, flat-field images without requiring additional flatteners.

The lens formula includes five distinct elements, each chosen for its specific refractive properties. Two elements are Super ED (extra-low dispersion) glass, one is ED glass, one is lanthanum glass and one is the X-Element. Low-dispersion glass is what controls colour fringing around bright stars in an apochromat.

Fast f/4.8 Optics: Capturing Faint Signal at 518mm

The UltraCat 108's f/4.8 focal ratio places it in the category of fast astrographs, designed for efficient light collection. A faster f-ratio means that for a given exposure time, you will record more signal from faint targets compared to a slower telescope (e.g., f/7 or f/10).

With a focal length of 518mm, the UltraCat 108 provides a wide field of view that is ideal for framing large nebulae, open clusters, and entire galactic regions. With a full-frame camera the field is roughly 4.0 x 2.7 degrees, enough to frame the Andromeda Galaxy (M31) or the Rosette Nebula in a single frame.

A Massive 60mm Image Circle for Medium Format Sensors

The UltraCat 108 has a 60mm image circle, larger than the 43.3mm diagonal of a full-frame sensor.

The 60mm circle fully supports full-frame and medium-format cameras; supported formats are medium format, full frame, APS-C and M4/3. Medium format cameras need a suitable adapter on the camera side.

Patented WIFD Focuser: 30mm of Internal Travel

The William Optics Internal Focuser Design (WIFD) is, in William Optics' words, a patented internal focuser that ensures precise, stable and hassle-free imaging, avoiding balance issues and eliminating vignetting.

• Focus Travel: 0-30mm.
• EAF Compatible: The focuser is EAF compatible (electronic automatic focuser).

Camera Integration: Rotator, Tilt Adjuster, and M48/M54/M68 Adapters

The box includes William Optics' Sensor Tilt Xterminator rotator. The rotator turns the camera to frame a target, and the integrated, patented tilt corrector adjusts the camera's sensor angle so stars stay sharp across the field.

To ensure broad compatibility, the telescope ships with a set of three standard camera adapters: M48, M54, and M68. Use the one that matches your camera, filter wheel or off-axis guider.

• M48: the thread of 48 mm T-mounts and many filter wheels.
• M54: a wider thread found on many larger filter wheels and cameras.
• M68: the widest of the three, used by some large-format camera accessories.

Flat-Field by Design: No Flattener Required

The advanced 5-element Petzval optical system of the UltraCat 108 has field flattening built directly into its design. Many doublet and triplet refractors need a separate field flattener at a set back focus. With the UltraCat 108, no additional flattener is required: connect the camera with the included adapters.

Visual Use

William Optics presents the UltraCat 108 as an astrograph and does not describe a visual configuration for it. Ask us before buying a diagonal and eyepieces for visual use. As with any short-focus refractor, 518mm gives low magnification and wide fields rather than high power.

Mechanical Design: 7.96 kg OTA, Vixen Dovetail, and Bahtinov Cap

The UltraCat 108 is built with the practical needs of the imager in mind. The optical tube assembly (OTA) weighs 7.96 kg. William Optics' Anti-Shrink Lens Cell Technology minimises thermal expansion and contraction to keep the optics aligned.

Several thoughtful features are integrated into the design:

• Patented Bahtinov Mask Cap: The dust cap has an integrated Bahtinov mask, a built-in focusing aid that simplifies focusing.
• Mounting Rings and Handle Bar: The mounting ring holds the OTA, and the patented handle bar design combines ergonomic use with portability.
• Vixen-style Dovetail: The telescope comes with a standard Vixen-style plate (as listed in William Optics' box contents).

UltraCat 108 Dimensions: A 9.58 kg System in a Portable Case

The total weight of the UltraCat 108 system, including the OTA, rings, handle, and dovetail, is 9.58 kg (21.12 lbs). The tube length is 512.9mm to 617.9mm. A soft carry case is included for transport and storage.

First Light with the UltraCat 108: Setup and Care

Getting started with the UltraCat 108 is straightforward. After mounting the telescope on your equatorial mount, use the included adapters to attach your camera. Find rough focus on a distant object in daylight, then use the Bahtinov mask cap on a bright star at night for fine focus. Use the rotator to frame and the tilt corrector if corner stars are elongated.

Like any high-end optical instrument, the UltraCat 108 requires proper care. Let the telescope reach the outdoor temperature before you focus. Keep the dust cap on when not in use to protect the optics. Most importantly, never point 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.