William Optics Ultra-Cat 131 f/4.9 Petzval Astrograph

The Ultra-Cat 131: A 131mm Astrograph for Demanding Imagers

The Ultra-Cat 131 is the flagship of the Ultra-Cat series, designed for serious astrophotographers and observatories. It pairs a large 131mm objective with a fast f/4.9 optical system, creating an ideal platform for capturing faint deep-sky structures with shorter exposure times. This instrument is built for those using large-format cameras, from full-frame to medium-format sensors, who want a flat field without an external flattener.

The combination of a 648mm focal length and a 60mm image circle provides the versatility to frame expansive targets like the Andromeda Galaxy (M31) or zoom in on detailed nebular regions. Its total weight of 13.97 kg necessitates a stable, high-capacity equatorial mount,

5-Element Petzval Optics: Two Super ED Elements for Apochromatic Correction

At the heart of the Ultra-Cat 131 is a sophisticated 5-element Petzval optical system. It uses two Super ED glass elements, one ED element, one lanthanum element and one X-Element, and the design eliminates field curvature, providing sharp, flat-field images without additional flatteners.

Colour correction is exceptional, with sharp stars across the flat image circle. With no separate flattener, there is one less part to space in the imaging train.

131mm Aperture at f/4.9: Capturing Faint Detail Faster

The 131mm objective gathers more light than smaller refractors. This aperture advantage allows you to capture fainter details in your target with the same exposure time or, alternatively, achieve the same signal-to-noise ratio with shorter sub-exposures. This is particularly beneficial for narrowband imaging, where long exposures are often required to capture faint emission lines.

The fast f/4.9 focal ratio complements the large aperture by concentrating that collected light onto your camera sensor more quickly. A faster system lets you collect the same signal in less time. The native 648mm focal length strikes a balance between wide-field context and sufficient image scale for resolving details in galaxies and nebulae.

A Massive 60mm Image Circle for Medium-Format Sensors

The Ultra-Cat 131 has a 60mm image circle, larger than the 43.3mm diagonal of a full-frame sensor. It fully supports full-frame and medium-format cameras.

Its spec lists medium format, full frame, APS-C and M4/3 sensors.

Patented WIFD Focuser: 20mm of Internal Travel

William Optics’ patented WIFD (William’s Internal Focuser Design) is a patented internal focuser that, in William Optics' words, ensures precise, stable and hassle-free imaging, avoiding balance issues and eliminating vignetting. It gives 0-20mm of focus travel.

• Balance: The design avoids balance issues.
• Vignetting: It eliminates vignetting.
• EAF compatible: the focuser accepts an electronic automatic focuser (EAF).

Full Imaging Train Control: Adapters, Tilt, and 360° Rotation

The Ultra-Cat 131 comes with the Sensor Tilt Xterminator (STX-R) Rotator, which adjusts sensor tilt and rotates the camera for framing.

The Sensor Tilt Xterminator is an integrated and patented design that optimises sharpness and detail. To connect your camera, the package includes M68, M54 and M48 adapters with a wrench.

Inherent Field Correction: A 5-Element Design That Needs No Flattener

A significant advantage of the Ultra-Cat 131's 5-element Petzval design is that it requires no external field flattener. Many refractors need a separate flattener to correct the curvature of the focal plane.

The Ultra-Cat 131's design eliminates field curvature without additional flatteners, so there is no flattener spacing to set up.

Visual Use

William Optics presents the Ultra-Cat 131 as an astrograph and does not describe a visual configuration for it. Ask us before buying a diagonal and eyepieces for visual use. At 648mm, magnifications stay low to medium with typical eyepieces.

Build: Thermal Stability and Integrated Tools

The Ultra-Cat 131 is built to provide a stable and reliable imaging platform. The optical tube features Anti-Shrink Lens Cell Technology to minimize the effects of thermal expansion and contraction, which maintains optical alignment as the temperature changes.

The telescope includes several practical, patented design features. The objective cap is a Bahtinov mask, which simplifies focusing. The patented handle bar combines ergonomic use with portability. The tube sits in the mounting ring and handle bar on a Losmandy plate.

13.97 kg Total Weight: What's Included in the Soft Carry Case

The Ultra-Cat 131 has a total assembled weight of 13.97 kg (30.8 lbs), with the main optical tube assembly (OTA) alone weighing 11.3 kg (24.91 lbs). Choose an equatorial mount rated comfortably above that weight plus your camera and guiding gear.

The astrograph comes with a soft carry case for protection during transport and storage. The package includes the main tube, the Bahtinov mask cap, the mounting ring and handle bar, a Losmandy plate and riser blocks. It also contains the full set of M48, M54, and M68 camera adapters with a wrench, the Sensor Tilt Xterminator (STX-R) rotator, and an Allen key set for adjustments.

First Light Guide: Focusing and Care for the Ultra-Cat 131

Setting up for your first imaging session is straightforward. After mounting the 13.97 kg telescope securely on your equatorial mount, attach your camera using the appropriate included adapter (M48, M54, or M68). To focus, use the Bahtinov mask in the lens cap: point at a bright star and adjust the focuser until the central diffraction spike sits centred between the other two, then take the cap off to image.

Like any large-aperture instrument, let the telescope reach the outdoor temperature before you focus. When not in use, store the telescope in its soft carry case with the lens cap on to protect the optics from dust and moisture. Never aim the telescope at the Sun without a proper solar filter fitted over the front of the telescope.

Ultra-Cat 131 vs. 106mm Refractors: The Aperture Advantage

William Optics itself compares the Ultra-Cat 131's 60mm image circle with standard 106mm telescopes. The figures below follow from the apertures alone.

• Light Gathering Power: The 131mm objective has over 50% more light-collecting area than a 106mm objective. (131 squared over 106 squared is about 1.53).
• Image Circle: The 60mm circle fully supports full-frame and medium-format cameras.
• Integrated Design: While many refractors require a separate, carefully spaced flattener, the Ultra-Cat 131 needs no additional flattener.