William Optics RedCat 51 III WIFD f/4.9 Petzval Refractor Telescope

The RedCat 51 Gen III is an apochromatic refractor that delivers stunning, crystal-clear images. With anti-reflective lens coatings and high-quality Petzval optics, it offers pinpoint stars across the entire field and reduces chromatic aberration. Chromatic aberration is when colors appear distorted in your images. It happens because different colors of light bend differently through the lenses of a refractor. This causes colors to focus at different points, making bright objects like stars appear with purple or blue halos around them.

The Gen 3 improves on what made the original RedCat great! It has the new William Optics Internal Focus Design (WIFD). This design works perfectly with electronic focusers like the ZWO EAF. By integrating the focuser inside the optical tube, the WIFD eliminates any need for an external drawtube. This reduces the chance of tilt, sag, and mechanical flexure, which means greater stability and sharper images.

The Gen III RedCat 51 has an updated tilt adapter with M54 threads to reduce vignetting on full-frame cameras. It also comes with an M48 x 0.75 adapter for standard setups. With a fast f/4.9 focal ratio and 250mm focal length, the RedCat 51 Gen III is perfect for wide-field shots of deep-space objects. If you're upgrading from a camera lens, new to astrophotography, or looking for a portable telescope, the RedCat 51 WIFD is a fantastic choice!

• 4-Element Petzval Optics: The RedCat 51 III features a 4-element Petzval design. This provides ultra-sharp, color-free images with minimal distortion. Stars stay sharp across the entire field, including the edges, so you get stunning images every time!
• Short Focal Length: With a focal length of 250mm and a large image, the RedCat 51 III excels in wide-field astrophotography.
• Fast Focal Ratio: A fast focal ratio of f/4.9 means the RedCat 51 III gathers light quickly. So you can capture bright, detailed images in less time! 
• Internal Focus System (WIFD): The internal focus system provides smooth, precise adjustments without the telescope body extending. 
• Premium Glass: The RedCat 51 III uses FPL-53 glass, a top-tier material that provides excellent light transmission. It also minimizes chromatic aberration (color fringing). 
• Easy Framing: The built-in field rotator with markings makes framing a breeze!
• Built-In Filter Thread: The RedCat’s built-in 48mm filter thread gives you the flexibility to use standard 2" filters without a filter wheel or drawer. 
• Full-Frame & APS-C Compatible: Whether you're using a full-frame or APS-C camera, the RedCat 51 III works flawlessly with both thanks to its 45mm image circle. 

Earlier versions of the RedCat 51 used a helical-style focuser, similar to a camera lens. It had a rubber grip that wasn’t ideal for autofocus systems. With the RedCat 51 Gen III, William Optics took user feedback into account and introduced the William Optics Internal Focus Design (WIFD). One of the most advanced focusing systems available in consumer telescopes today!

Unlike traditional focusers that extend out the back of the telescope, WIFD moves the optical elements inside the tube. This eliminates vignetting, prevents sensor tilt from the telescope itself, and provides rock-solid stability. With no external drawtube, there’s no risk of sag, even when using heavy accessories like a large filter wheel or DSLR camera. To ensure even greater precision, William Optics includes a built-in tilt adapter to correct any misalignment caused by other components in the imaging train.

The RedCat 51 III features four lens elements arranged in a Petzval optical design with high-quality FPL-53 glass, ensuring excellent color correction and a flat field across a 45mm image circle. This design ensures exceptional image sharpness across the entire field of view by addressing issues like field curvature and chromatic aberration. This is a huge benefit for astrophotography, where sharp, clear images help reveal more detail in nebulae and galaxies.

The Petzval design eliminates the need for a separate field flattener, which is required in other refractor telescopes to ensure round stars across the entire image. Removing this component saves on cost, complexity, and total system weight. 

With this design, you also remove the need for specific backfocus calculations. With a traditional refractor and field flattener, there is often a fixed back-focus distance, typically 55 millimeters. But with the RedCat 51 WIFD, you can achieve focus using any imaging system that is 68.2 millimeters or less from the M54 threads on the back of the scope. This means you can easily add accessories like cameras, rotators, or filter wheels, and seamlessly adjust focus without worrying about rigid back-focus standards.

The William Optics RedCat 51 Gen III WIFD includes everything needed for high-quality results, making it both simpler to use and more cost-effective! 

The 250mm focal length and f/4.9 focal ratio of the RedCat 51 make it an ideal choice for wide-field astrophotography, allowing you to capture expansive views of nebulae, galaxy clusters, and large open star fields. 

The RedCat is great for beginners who may be using portable tracking mounts like the Sky-Watcher Star Adventurer GTi, Star Adventurer 2i Pro, or iOptron SkyGuider. Because of its wide field of view, minor imperfections and distortions in star shape are less noticeable. This means you don’t need to track the night sky as precisely as you would if you were using longer focal lengths. 

The combination of a fast f/4.9 aperture and short focal length allows the telescope to gather light quickly! For instance, an image that would take 4 hours at f/8 can be captured in just 2 hours at f/4.9. This is a significant time-saver when photographing faint objects.  It’s especially valuable when you're working with limited clear skies or light pollution, allowing you to capture detailed images faster.

The RedCat 51 Gen III WIFD accommodates high-resolution cameras and heavier, cooled setups with ease, offering a versatile imaging experience with a design that ensures minimal vignetting on full-frame sensors.

Specifications

• Optical Design: Petzval (4-element)
• Glass Type: FPL-53
• Focal Length: 250mm
• Aperture: 51mm
• Focal Ratio: f/4.9
• Image Circle: 48mm (full frame)
• Focuser: William Optics Internal Focus Design (WIFD)
• Focuser Compatibility: ZWO EAF
• Focus Travel: 0 - 23mm (Ruler to 30mm)
• Adapter Included: Tilt adjuster, Rotator, M54 to M48 adapter
• Optional Flattener: No flattener required
• Camera Format: Full frame, APS-C, M4/3
• Tube Length: 346mm - 298mm
• Dew Shield Circumference: 265mm
• Dew Shield Diameter: 84mm
• Weight of OTA only: 3.90 lbs (1.77 kg)
• Weight of OTA with tube rings, dovetail, & saddle: 5.25 lbs (2.38 kg)
• Soft Carrying Case External Dimensions: 39cm x 25cm x 27cm

Download the instruction manual