William Optics RedCat 51 II "U" 51mm f/4.9 Petzval Apo Imaging Refractor Telescope
Availability: Item has been discontinued by the manufacturer
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- Ultra-compact 51mm Petzval apochromatic refractor with razor sharp optics and a full frame flat field
- Ideal for wide-field astronomical or terrestrial imaging with APS-C or full-frame DSLR, mirrorless, or dedicated astronomy cameras
- Precision helical focuser, field rotator, and Bahtinov mask for fast and precise set-up
- Includes slot for 48mm astronomy filters and M48 threads to accommodate a wide range of cameras
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, 2nd generation 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.
The RedCat 51 focal length is 250mm, which many find to be a sweet spot for wide field astrophotography. The Orion Nebula, Andromeda Galaxy, Heart and Soul Nebulae, North America Nebula, and many more popular regions of the sky easily fit into the field of view. With premium apochromatic optics in a Petzval design, the RedCat 51 is sharper and better corrected than many brand-name camera lenses of similar focal length. The superb optical design and a fast focal ratio of f/4.9 make this little scope a powerful imaging tool.
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 telescope offers an expansive field of view nearly 10 degrees across the sensor diagonal.
Please note that unlike previous versions of the Redcat telescope, this latest II-U version CANNOT be used visually. The William Optics D-EP90-125-RC51-SL diagonal will not fit on this scope.
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. Astrophotographers looking for an autofocus solution for the RedCat 51 may want to browse the ProAstroGear Black-CAT Mount, which allows attachment of a ZWO EAF among other accessories.
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 telescope also includes M48x0.75 threads to let you attach a Micro-4/3, Nikon, Canon, Sony, or Pentax camera with a 48mm wide T-ring (sold separately). 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.
4. 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.
5. 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 228mm x 80mm (9" x 3.1") with the retracted dew shield, and it weighs just 1.3kg (2.9 lbs).
With a tube made of attractive anodized aluminum alloy, the RedCat 51 is available in a premium red/black finish.
6. 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.
7. Specifications
- Focal Length: 250mm
- Diameter: 51mm
- Aperture: f/4.9
- Focuser: Helical focuser
- Image Circle: >43mm
- Tube length (fully retracted): 228mm
- Tube length (fully extended): 291mm
- Dew Shield Circumference: 255mm
- Dew Shield Diameter: 80mm
- Weight:
- 2.9 lbs / 1.3kg (OTA only)
- 3.9 lbs / 1.76 kg (OTA with ring and dovetail)
- Interchangeable T-mounts (sold separately): Canon EF, Nikon F, Sony E, Pentax, Micro Four Thirds, Fuji
| Product Banner Type | discontinued |
|---|---|
| Free Shipping Graphic | Free Contiguous US Shipping |
| Telescope Focal Length | Up to 300mm |
| Hide Found a better Shipped Price on Frontend? | No |
- Soft carry case
- RedCat 51 II 51mm f/4.9 OTA - Red OTA with Black Accents
- Tube ring and dovetail rail
- M48 Camera Threaded Mount (Agena note: We don't know why WO lists this as a separate item. It isn't. This is just the male M48 male thread at the rear end of the telescope on the attached tilt adapter, and it is not a separately included item. Required camera-specific 48mm Wide T-Ring sold separately and is reqired to connect your DSLR to this thread. See the "Related Accessories" section above for compatible adapters.)
- Patented Bahtinov Mask
- Ultra-compact 51mm Petzval apochromatic refractor with razor sharp optics and a full frame flat field
- Ideal for wide-field astronomical or terrestrial imaging with APS-C or full-frame DSLR, mirrorless, or dedicated astronomy cameras
- Precision helical focuser, field rotator, and Bahtinov mask for fast and precise set-up
- Includes slot for 48mm astronomy filters and M48 threads to accommodate a wide range of cameras
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, 2nd generation 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.
The RedCat 51 focal length is 250mm, which many find to be a sweet spot for wide field astrophotography. The Orion Nebula, Andromeda Galaxy, Heart and Soul Nebulae, North America Nebula, and many more popular regions of the sky easily fit into the field of view. With premium apochromatic optics in a Petzval design, the RedCat 51 is sharper and better corrected than many brand-name camera lenses of similar focal length. The superb optical design and a fast focal ratio of f/4.9 make this little scope a powerful imaging tool.
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 telescope offers an expansive field of view nearly 10 degrees across the sensor diagonal.
Please note that unlike previous versions of the Redcat telescope, this latest II-U version CANNOT be used visually. The William Optics D-EP90-125-RC51-SL diagonal will not fit on this scope.
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. Astrophotographers looking for an autofocus solution for the RedCat 51 may want to browse the ProAstroGear Black-CAT Mount, which allows attachment of a ZWO EAF among other accessories.
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 telescope also includes M48x0.75 threads to let you attach a Micro-4/3, Nikon, Canon, Sony, or Pentax camera with a 48mm wide T-ring (sold separately). 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.
4. 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.
5. 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 228mm x 80mm (9" x 3.1") with the retracted dew shield, and it weighs just 1.3kg (2.9 lbs).
With a tube made of attractive anodized aluminum alloy, the RedCat 51 is available in a premium red/black finish.
6. 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.
7. Specifications
- Focal Length: 250mm
- Diameter: 51mm
- Aperture: f/4.9
- Focuser: Helical focuser
- Image Circle: >43mm
- Tube length (fully retracted): 228mm
- Tube length (fully extended): 291mm
- Dew Shield Circumference: 255mm
- Dew Shield Diameter: 80mm
- Weight:
- 2.9 lbs / 1.3kg (OTA only)
- 3.9 lbs / 1.76 kg (OTA with ring and dovetail)
- Interchangeable T-mounts (sold separately): Canon EF, Nikon F, Sony E, Pentax, Micro Four Thirds, Fuji
| Product Banner Type | discontinued |
|---|---|
| Free Shipping Graphic | Free Contiguous US Shipping |
| Telescope Focal Length | Up to 300mm |
| Hide Found a better Shipped Price on Frontend? | No |
- Soft carry case
- RedCat 51 II 51mm f/4.9 OTA - Red OTA with Black Accents
- Tube ring and dovetail rail
- M48 Camera Threaded Mount (Agena note: We don't know why WO lists this as a separate item. It isn't. This is just the male M48 male thread at the rear end of the telescope on the attached tilt adapter, and it is not a separately included item. Required camera-specific 48mm Wide T-Ring sold separately and is reqired to connect your DSLR to this thread. See the "Related Accessories" section above for compatible adapters.)
- Patented Bahtinov Mask