William Optics Apochromatic refractor AP 51/250 SpaceCat WIFD OTA (83498)

William Optics Apochromatic Refractor AP 51/250 SpaceCat WIFD OTA (83498)

The AP 51/250 SpaceCat is a compact and highly refined apochromatic refractor designed primarily for astrophotographers who demand portability without compromising image quality. Despite its small size, it delivers outstanding optical performance and is also suitable for creative nature photography. Its advanced Petzval optical design ensures a flat, well-corrected image field, making it an excellent choice for modern full-frame cameras. With its fast focal ratio of f/4.9, it enables short exposure times and efficient light capture.

At the core of the telescope is a quadruplet lens system with 4 elements in 3 groups, incorporating high-quality FPL53 and FPL51 glass. This configuration produces color-accurate, high-contrast images with a corrected image circle exceeding 44 mm in diameter. A built-in field corrector is integrated into the optical system, eliminating the need for additional flatteners.

Camera connection is achieved via an M48 thread, and the helical focuser allows smooth focusing by turning a textured ring on the tube. This system easily supports heavier cameras without loss of precision. A rotating mechanism and tilt adapter provide additional flexibility for accurate image alignment.

The 110 mm prism rail offers three mounting options: a standard 1/4" photo thread, Arca-Swiss compatibility, and Vixen-style compatibility. This ensures easy attachment to common photo tripods, camera mounts, or astronomical mounts. The telescope is supplied with a protective nylon carrying case and a specially designed Bahtinov focusing mask, which is permanently integrated into the lens cap for precise focusing.

Optical Specifications

• Design: Refractor, Apochromat

• Lens system: Quadruplet (4 elements in 3 groups)

• Aperture: 51 mm

• Focal length: 250 mm

• Focal ratio: f/4.9

• Resolving power: 2.71

• Limiting magnitude: 10.3

• Light gathering capacity: 50

• Maximum useful magnification: 102x

• Coating: Multi-coated

• Corrector: Integrated

Tube and Construction

• Tube material: Aluminium

• Tube length: 346 mm

• Tube weight: 2.4 kg

• Tube design: Full tube

• Dew shield diameter: 80 mm

• Color: Silver

• Series: Cat

Focuser and Connectivity

• Focuser type: Helical

• Tilt adapter: Yes

• Camera connection: M48 thread

• Backfocus: 59.7 mm

• Free aperture: 48 mm

• Adjustment range: 0–23 mm

• Rotatable: Yes

Mounting

• Type: OTA (Optical Tube Assembly only)

• Mount included: No

• Prism rail: Arca-Swiss and Vixen-style compatible

• Tripod thread: 1/4"

Included Accessories

• Dew shield: Yes

• Carry case: Yes

• Integrated Bahtinov focusing mask

Recommended Applications

• Astrophotography: Very good

• Nebulae and galaxies: Yes

• Moon and planets: Yes

• Solar observation: Yes (only with appropriate solar filter)

• Nature observation: Not recommended

Recommended For

• Advanced users: Yes

• Beginners: No

• Observatory use: No