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RedCat 51 mm f4.9 APO Astrograph Refractor Telescope

(2 customer reviews)


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 refractor 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.

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SKU: ASC261TEL Categories: , ,
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William Optics New RedCat 51 f4.9 refractor telescope

From the beginning, the goal was to create the best possible design for an affordable, small, lightweight, and fast Apochromatic refactor for astrophotography. The unique optical design includes the highest quality glass for the best possible color correction and flat-field.  The result is a 51mm lens in a Petzval design with is matched for the majority of available astronomy camera’s and DSLR’s Full frame sensors are fully illuminated. This telescope is a perfect match with the SkyHunter Portable mount and the CEM26 mount.

The RedCat 51 Refractor Telescope comes with 48 mm threads, so it compatible with a variety of wide Canon to Nikon, Sony E to Pentax, or Micro 4/3 T-mounts. This Cat Series refractor also has a camera sensor tilt adjuster to help you get those great images.

You can also put a diagonal and eyepiece on the back for spectacularly sharp, high contrast images of both terrestrial and celestial targets. The RedCat mounts on a photographic tripod or an equatorial mount with the reversible base that has Arco-Swiss mounting on one side and a standard Vixen dovetail on the other. That’s just the beginning, though! Check out all the features of this flexible APO refractor that can do triple duty as a camera lens, a spotting scope, or a telescope:

Amazing Refractor Optics on the RedCat 51

The William Optics RedCat 51 sports optics made of Ohara FPL53 & FPL51 synthetic fluorite glass for beautiful high contrast, spectacularly sharp results when used in both visual and photographic applications. APO performance means you’ll have virtually zero chromatic aberration. The Petzval design, which consists of four elements in three groups, also known as a Quad, produces a full frame flat field, with a image circle of 44mm.

Innovative Design

The RedCat body is made of a strong aluminum alloy and is anodized a beautiful red for strength and durability. A matching dew/sun shield can be attached when needed and then inverted to store the lens without the additional length the dew shield provides. An aluminum threaded cap protects the optics whenever the RedCat 51 refractor is not in use.

In the middle of the RedCat 51 you will find a nicely designed ring mount that attaches to the reversible base and wraps securely around the body of the lens. A red anodized knob allows you to loosen the ring enough to fully rotate the RedCat 51, and of course, it is also used to lock the ring in place. The “ring” has a flat section that has two 1/4-20 holes spaced far enough to allow the addition of a red dot finder, microphone, or other compatible accessories.

Calibrated Helical Focuser with 360-Degree Rotator

The helical focuser with a focusing index on the RedCat 51 has a focus tension ring that allows you to lock focus or just increase or decrease the drag to your liking. A field rotator is included with this 250 mm lens as well, and every degree is plainly marked…all 360 of them! William Optics says they were inspired by luxury watches when they designed this rotator, but what it means to you is mosaics of large celestial objects or sections of the sky will be much easier!

A filter slot is built into the OTA just after the rotator and accepts 48 mm filters, and the back of the RedCat is fitted with M48 threads. Attach a full-frame (M48) M 4/3, Nikon, Canon, Sony, or Pentax T-ring to the threads and use your DSLR to produce beautiful images. Or, attach a diagonal and eyepiece and turn this 250mm f/4.9 beauty into a telescope or spotting scope.

Includes These Accessories

You will also receive with your RedCat 51 a Bahtinov Mask for focusing assistance during astrophotography (that might give you a hint as to why the Cat’s whiskers look like they do…just look at the threaded cap), the M48 camera mount (T-ring mentioned above not included) and a soft carry case. Aluminum covers are also included for the front and back of the telescope.

Weight2.95 kg
Dimensions31 × 34 × 17.5 cm

2 reviews for RedCat 51 mm f4.9 APO Astrograph Refractor Telescope

  1. English

    Jerald Jacob (verified owner)

    Best product in its category !

  2. English

    SHIRISH M. NARSAPUR (verified owner)

Only logged in customers who have purchased this product may leave a review.

  • Model Number
    Telescope Series
    William Optics Cat

    Focal Ratio

    Optical Design
    Telescope Aperture
    51mm (2″)
    Telescope Mount Type
    No Mount – OTA Only
    Type of Electronics
    OTA Only – No Electronics
    Number of Refractor Elements
    4 (Quad)
    Focal Length (mm)
    250 mm
    Image Circle
    Focuser Style
    Refractor OTA Length – Extended (in.)
    12″ (305 mm)
    Refractor OTA Length – Retracted (in.)
    8.86″ (225 mm)
    Telescope OTA Weight (lb.)
    3.24 lb. (1.47 kg)


    William Optics 2 Years
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