William Optics Apochromatic refractor AP 51/250 RedCat WIFD OTA (82930)

The AP 51/250 is a remarkable optic that combines compact size, affordability, and outstanding optical quality—perfect for astrophotographers and nature photographers alike. At its core is a Petzval optical design, featuring four elements in three groups, made from high-grade FPL53 and FPL51 glass. This results in a color-true, well-corrected image circle over 44mm in diameter—making it ideal for full-frame cameras! Its fast f/4.9 aperture enables short exposure times.

The telescope provides an M48 thread for camera connection. Compatible T2 adapters are listed under "Recommended Accessories." Focusing is accomplished internally via a rack-and-pinion system—simply rotate the grippy focusing ring on the tube. This internal design easily supports heavy camera setups.

The 110mm prism rail offers three mounting options: you can screw it in using a photo thread, or slide it in using either Arca-Swiss or Vixen-style standards. Thanks to these versatile options, attaching the telescope to virtually any standard photo tripod, camera mount, or astronomical mount is straightforward.

Included with the RedCat 51 are a protective nylon carrying case and a specially designed Bahtinov mask for precision focusing, which is permanently integrated into the lens assembly.

The RedCat 51 is a fantastic choice for photographers who want to capture both the night sky and wildlife. Along with impressive optical performance, you'll appreciate the robust and attractive craftsmanship typical of William Optics.

Innovative Focus Design

The William Optics Internal Focus Design (WIFD) is a groundbreaking system, blending a patented, ultra-precise internal focusing mechanism with top-quality optics. It delivers a high-end apochromatic refractor designed specifically for astrophotography. Built to support even the heaviest cooled cameras, this design is uncompromising in performance. The focuser's motion is contained entirely within the main tube—no external components move.

Users enjoy the dual fine-focus controls of a traditional focuser, but without the limitations (such as vignetting) that rear-mounted focusers can introduce. An integrated window on the focuser allows you to instantly check the numerical position indicator.

The focuser is also compatible with ZWO’s Electronic Automatic Focuser (EAF).

Technical Specifications

Optics

• Optical type: Refractor (Apochromat)

• Aperture: 51mm

• Focal length: 250mm

• Focal ratio: f/4.9

• Resolving power: 2.71 arcseconds

• Limiting magnitude: 10.3

• Light gathering power: 50x

• Max. useful magnification: 102x

• Tube weight: 2.4kg

• Tube length: 346mm

• Tube material: Aluminum

• Lens coating: Multi-coated

• Tube design: Full tube

• Dew shield diameter: 80mm

• Lens design: Quadruplet (4 elements)

Focuser

• Tilting adapter: Yes

• Focus mechanism: Internal

• Camera thread: M48

• Backfocus: 59.7mm

• Clear aperture: 48mm

• Focuser travel: 0–23mm

• Rotatable: Yes

Mount

• OTA (Optical Tube Assembly) only (no mount included)

Included Accessories

• Dew shield: Yes

• Carry case: Yes

• Bahtinov focusing mask: Yes (integrated)

• Prism rail: Arca-Swiss & Vixen-Style compatible

• Integrated field corrector: Yes

General Information

• Series: Cat

• Color: Red

Application

• Astrophotography: Excellent

• Moon & planets: Yes

• Nature observation: No

• Nebulae & galaxies: Yes

• Solar (with proper filter): Yes

Recommended For

• Beginners: No

• Advanced users: Yes

• Observatories: No