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William Optics RedCat 51 APO 250mm f/4.9

1 unit/s in stock

Regular Price: US$ 1,080.71

US$ 966.40 Special Price


Short Description

From the beginning, the goal was to create the best possible design for an affordable, small, lightweight and fast Apochromatic refractor for astrophotography needs. The unique optical design includes the highest quality glass for the best possible color correction and flat-field. The result is a 51mm objective lens in a sleek Petzval design, for a full frame flat field image. The optical quality provides for exceptional performance at a fast focal ratio of f/4.9


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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, 2nd generation William Optics RedCat 51 / SpaceCat 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 / SpaceCat 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 / SpaceCat 51 is a work of art with precision mechanical design and manufacturing and a rock-solid build that’s ideal for serious imagers.

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

The RedCat 51 / SpaceCat 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 / SpaceCat 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 telescope offers an expansive field of view nearly 10 degrees across the chip diagonal. 

Usable as a Portable Visual Telescope too

The RedCat 51 / SpaceCat 51 is optimized for use as an imaging instrument, but it also makes a superb little scope for visual use (with certain constraints). Just add William Optics' optional 1.25" erecting prism diagonal made specifically for this scope (sold separately) and your favorite eyepiece, and you’re ready for a rewarding night of sweeping the Milky Way or for a pleasant day of birdwatching.

The RedCat / SpaceCat does not have a conventional focuser and has limited available backfocus for visual use. Hence, the use of the special 1.25" diagonal requires the removal of the RedCat's tilt adapter so while you can use this scope visually or for imaging, switching between one confiuration and the other requires a bit of effort. The RedCat is unable to accept standard astronomy 1.25" or 2" diagonals, nor can you attach the RedCat diagonal to the M48 thread or the tilt adapter and still reach focus.

Precision Focuser and Field Rotator

As you’ve come to expect from William Optics, the RedCat 51 / SpaceCat 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 / SpaceCat 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 telescope also includes M48x0.75 threads to let you attach a Micro-4/3, Nikon, Canon, Sony, or Pentax camera with a T-ring (sold separately). This is truly a telescope built for imagers. 

The RedCat 51 / SpaceCat 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.

Generous Back-Focus to Accommodate DSLRs and Many Astro-Imaging Cameras

The lens assembly can be moved within the telescope body over 33mm by rotating the focusing collar in the front. The distance from the base of the M48 male thread on the Redcat to the chip plane should be 59.7mm with the lens cell completely retracted (at the 0mm position; see image below). With the lens cell fully extended to 33mm, the chip-to-thread distance must at least be 59.7 - 33 = 26.7mm. In other words, the thread-to-chip distance can be anywhere between 26.7mm and 59.7mm to reach focus. This is easily achieved with DSLRs with standard M48 T-rings. Many astroimaging cameras will also reach focus with a standard 2" M48 threaded nosepiece or with accessories such as filter wheels.

A Carefully-Selected Range of Included Accessories

The William Optics RedCat 51 / SpaceCat 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 / SpaceCat 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 / SpaceCat 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 (3.24 lbs).

With a tube made of attractive anodized aluminum alloy, the RedCat 51 / SpaceCat 51 is available in either a premium red/black (this item - RedCat 51) or space gray/black (SpaceCat 51 - sold separately).

Ideal for Bird, Wildlife, and Nature Photography

With focal ratio of f/4.9, the RedCat 51 / SpaceCat 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 / SpaceCat 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/ SpaceCat 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.

What’s in the Box

  • Soft carry case
  • RedCat 51mm f/4.0 OTA - Red OTA with Black Accents
  • M48 Camera Threaded Mount
  • Patented Bahtinov Mask


  • 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

Additional Information

Additional Information

ManufacturerWilliam Optics
NameWilliam Optics RedCat 51 APO 250mm f/4.9
Warranty2 Years
Optical SchemeFlatfield APO refractor
Diameter51 mm
Focal Length250 mm