William Optics Apochromatic refractor AP 91/448 GreyCat 91 WIFD OTA (88166)

William Optics Apochromatic Refractor AP 91/448 GreyCat 91 WIFD OTA (88166)

The AP 91/448 GreyCat 91 combines compact dimensions, an attractive price, and excellent optical performance. It is designed for both astrophotographers and nature photographers who require high image quality and reliable construction.

The optical system is based on a Petzval design with four elements arranged in three groups, using high-quality FPL53 and FPL51 glass. This configuration produces a color-accurate, fully corrected image field with a diameter of over 55 mm, making it suitable for full-frame cameras. The fast focal ratio of f/4.9 enables shorter exposure times and efficient light gathering.

Camera connection is provided via M48 and M54 threads, and both matching adapters are included. Focusing is achieved through a helical mechanism by turning a wide, easy-grip ring on the tube. Because the system features a WIFD (Internal Focus Design), focusing occurs internally, allowing the telescope to handle even heavy cameras without mechanical strain.

The included prism rail offers versatile mounting options. It can be attached using a standard photo thread, mounted via an Arca-Swiss system, or inserted into a Losmandy-style prism clamp. This flexibility ensures compatibility with common photo tripods, camera mounts, and astronomical mounts.

The telescope comes with a durable nylon carrying bag and a specially designed Bahtinov mask integrated into the lens cover. The mask assists with precise focusing.

The GreyCat 91 is well suited for night sky photography as well as wildlife imaging. In addition to its strong optical performance, it features the distinctive, high-quality finish typical of William Optics products.

Optical Specifications

• Optical design: Refractor, apochromat

• Lens design: Quadruplet (Petzval system)

• Aperture: 91 mm

• Focal length: 448 mm

• Focal ratio: f/4.9

• Resolving power: 1.52 arcseconds

• Limiting magnitude: 12.3

• Light gathering power: 170x

• Maximum useful magnification: 182x

• Fully corrected field: Over 55 mm

• Multi-coated optics

Tube and Construction

• Tube material: Aluminium

• Tube construction: Full tube

• Tube weight: 6.3 kg

• Tube length: 556 mm

• Color: Grey

• Series: Cat

Focuser and Connections

• Helical internal focuser (WIFD design)

• Rotatable

• Integrated tilt adapter

• Camera-side threads: M48 and M54

• Free aperture: 55 mm

• Eyepiece adapter: 1.25 inch and 2 inch

Included Accessories

• Integrated corrector

• Bahtinov focusing mask

• Dew shield

• Prism rail

• Thread for camera tripod

• Nylon carrying case

• Optical tube assembly only (no mount included)

Application

• Astrophotography: Very good suitability

• Nebulae and galaxies: Yes

• Moon and planets: Yes

• Nature observation: Yes

• Solar observation: Only with an appropriate solar filter

• Recommended for advanced users

• Not specifically designed for permanent observatory installation