Digital Photography Review

William Optics RedCat 51mm f/4.9 Petzval Apo Refractor Telescope – Red

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1. Ultra-Portable Petzval Apo Refractor for Astronomical and Terrestrial Imaging

Is it a telescope? Is it a camera lens? Is it both? Call it what you will, but the new William Optics RedCat 51 is an affordable, compact, light-weight Petzval refractor with razor sharp optics in a compact and rugged package. It’s ideal for ultra-wide-field imaging of the deep sky and for terrestrial imaging of birds, landscapes, and wildlife.

With an aperture of 51mm and focal length of 250mm, the RedCat 51 is sharper and better corrected than many brand-name camera lenses of similar focal length. Superb optical design and a fast focal ratio of f/4.9 makes this little scope a powerful imaging tool.

And, like other refractors by William Optics, the RedCat 51 is a work of art with precision mechanical design and manufacturing and a rock-solid build that’s ideal for serious imagers.

2. Fast, Razor-Sharp Optics and Flat Field with Big Image Circle

The RedCat 51 incorporates high-quality synthetic fluorite FPL53 and FPL51 glass in a 4-element, 3-group classic Petzval design for an superb color correction and an exceptionally flat field. No field flattener is needed with this scope. The RedCat is engineered to have a big 43mm image circle compatible with APS-C and full-frame DSLRs, mirrorless, and dedicated astronomy cameras. With a camera with a full-frame (36mm x 24mm) sensor, the RedCat 51 offers an expansive field of view nearly 10 degrees across the chip diagonal.

The RedCat 51 also makes a superb little scope for visual use. Just add William Optics’ optional 1.25″ erecting prism diagonal made specifically for this scope (sold separately; no ETA or specifications available at this time) and your favorite eyepiece, and you’re ready for a rewarding night of sweeping the Milky Way or for a pleasant day of birdwatching. Note that the RedCat does not have a conventional focuser and is therefore unable to accept standard astronomy 1.25″ or 2″ diagonals.

William Optics RedCat 51mm f/4.9 Petzval Apo Refractor Telescope - Red

3. Precision Focuser and Field Rotator

As you’ve come to expect from William Optics, the RedCat 51 includes solid and precision-designed mechanics. The helical focuser has an attractive finish and engraved calibration markings, and it includes a tension adjustment to increase focusing drag or to lock the focuser in place.

The RedCat 51 also includes an amazingly well-engineered field rotator to make it easy to frame your images. The field rotator is laser etched and fully rotatable by 360 degrees. And, in a touch inspired by the design of luxury watches, the field rotator is marked to a precision of one degree.

Just after the field rotator, you’ll find a slot for a 48mm threaded astronomy filter. The back of the RedCat also includes M48 x 0.75 threads to let you attach a Micro-4/3, Nikon, Canon, Sony, or Pentax camera with a T-ring (not included). This is truly a telescope built for imagers.

The RedCat 51 optical tube is held with a single ring that’s sufficiently rigid to hold the tube and a large DSLR without flexure. You can easily rotate the tube with the help of a large locking knob. The tube ring also includes female threads that let you add a red-dot finder, guide camera, microphone rig, or other accessories.

William Optics RedCat 51mm f/4.9 Petzval Apo Refractor Telescope - Red

4. A Carefully-Selected Range of Included Accessories

The William Optics RedCat 51 also comes with a range of useful accessories with astronomy imaging in mind. There’s a super-bright Bahtinov mask that makes it a snap to focus the telescope in just a few seconds. Cap the mask with the ‘whiskered cat’ cover, and it serves as a protective cap for the objective lens.

The RedCat also comes with a dual-spec reversible mounting base for Vixen-style astronomy mounts or an Arca-Swiss mount used in photography tripod heads.

A sun-shade/dew-shield keeps stray light out of the telescope, and you can invert the shield and slide it over the objective cell to minimize the length of the tube for transport and storage. Not that the RedCat 51 is all that big: the tube is just 225mm x 80mm (8.9″ x 3.1″) with the retracted dew shield, and it weighs just 1.47kg (3.2 lbs).

With a tube made of attractive anodized aluminum alloy, the RedCat 51 is available in either a premium red/black (this item) or classic William Optics white/gold finish (sold separately).

5. Ideal for Bird, Wildlife, and Nature Photography

With focal ratio of f/4.9, the RedCat 51 also makes a superb camera lens for birders and wildlife and nature photographers. The apochromatic optics are razor sharp and free of false color, much better than most camera lenses. With a weight of just 1.47kg, the RedCat is easy to use hand held. And with a soft rubber grip and adjustable tension, the precision helical focuser makes it easy to achieve focus in a hurry. Owners of Sony mirrorless cameras can even enjoy autofocus with the addition of the optional TechArt Pro adapter than includes a built-in focusing motor.

You can get close with the RedCat 51- it has a close focusing distance of just 3.7m. Use the 2″ nosepiece from the diagonal as an extenstion tube and the focusing distance drops to 3.0m. Add more extension tubes to reduce the focusing distance further, close enough to do respectable macro imaging.

6. What’s in the Box

  • Soft carry case
  • RedCat 51mm f/4.0 OTA – Red OTA with Black Accents
  • M48 Camera Threaded Mount (Required 48mm Wide T-Ring sold separately)
  • Patented Bahtinov Mask

William Optics RedCat 51mm f/4.9 Petzval Apo Refractor Telescope - Red

7. Specifications

  • Lens Type: Prime Lens
  • Optical Path: Four-element Petzval
  • Focal Length: 250mm
  • Objective Diameter: 51mm
  • Focal Ratio: f/4.9
  • Image Circle: Full Frame
  • Focus Mode: Manual Focus
  • Minimum Focus Distance: 3.7m
  • Dimensions: 225mm x 80mm x 80mm
  • Weight: 1.47 kgs
  • Mounting Plate: Vixen-style/Arca-Swiss reversible
  • Focuser: Calibrated Helical
  • Interchangeable T-Mounts (Canon EF, Nikon F, Sony E, Pentax, Micro 4/3

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