William Optics Apochromatic refractor AP 91/448 RedCat 91 WIFD OTA (85222)

The AP 91/448 is a remarkable optical instrument, blending compact size, competitive price, and exceptional image quality—ideal for both astrophotographers and nature photographers. At its core is a Petzval optical design with four elements in three groups, utilizing premium FPL53 and FPL51 glass. This setup provides a color-accurate, well-corrected image circle over 55mm in diameter, perfect for full-frame cameras. The fast f/4.9 aperture enables short exposure times for sharp, detailed images.

The telescope is equipped with both M48 and M54 camera threads, and two appropriate adapters are included. Focusing is helical—just rotate the textured focus ring on the tube for smooth, internal focusing. This robust design easily accommodates even heavier camera systems. The “WIFD” (With Internal Focus Design) ensures that all focusing movement takes place inside the tube.

The included prism rail offers multiple mounting options: you can screw it on with a standard photo thread, mount via Arca-Swiss, or insert it into a Losmandy-style prism clamp. These versatile options make it easy to attach the RedCat 91 to nearly any photo tripod, camera mount, or astronomical mount.

The scope comes complete with a durable nylon carrying case and a specially designed Bahtinov mask, permanently integrated into the lens assembly, to assist with precise focusing.

The Cat 91 is an outstanding choice for photographers dedicated to capturing the night sky, as well as wildlife enthusiasts. Not only does it deliver superb optical performance, but you’ll also appreciate the rugged and stylish finish that William Optics is known for.

Specifications

Optics

• Optical type: Refractor (Apochromat)

• 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

• Max. useful magnification: 182x

• Tube weight: 6.3 kg

• Tube length: 556 mm

• Tube material: Aluminum

• Coating: Multi-coated

• Tube construction: Full tube

• Lens design: Quadruplet (4 elements)

Focuser

• Tilting adapter: Yes

• Focuser type: Helical, internal focus

• Camera thread: M48 / M54

• Clear aperture: 55 mm

• Rotatable: Yes

Mount

• Type: OTA (Optical Tube Assembly only—no mount included)

Included Accessories

• Dew shield: Yes

• Carry case: Yes

• Integrated Bahtinov focusing mask: Yes

• Prism rail: Yes (Arca-Swiss & Losmandy compatible)

• Thread for camera tripod: Yes

• Integrated field corrector: Yes

• Eyepiece adapter: 1.25" & 2"

General

• Series: Cat

• Color: Red

Applications

• Astrophotography: Yes (excellent)

• Moon & planets: Yes

• Nature observation: Yes

• Nebulae & galaxies: Yes

• Solar: No (unless using a proper solar filter)

Recommended for

• Beginners: No

• Advanced users: Yes

• Observatories: No