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The compact full-frame astrograph that made wide-field imaging easy
The William Optics RedCat 51 is one of the most popular small astrographs ever made, and the third-generation RedCat 51 III WIFD is the best version yet. Its 51mm f/4.9 Petzval design uses four lens elements with FPL-53 and FPL-51 ED glass to deliver a flat, color-free 48mm image circle, so it covers full-frame, APS-C and smaller sensors with no separate flattener. At 250mm focal length it frames the largest nebulae, Milky Way star fields and the Andromeda Galaxy, and the whole scope weighs just 5.25 lbs with its ring, handle and dovetail.
Why imagers choose the RedCat 51
- Full-frame flat field, no flattener: a 4-element Petzval design with FPL-53 and FPL-51 glass for pinpoint stars across a 48mm image circle.
- Fast f/4.9 optics: collect signal quickly, great for short summer nights and quick sessions between clouds.
- WIFD internal focuser: a dual-speed rack-and-pinion focuser that moves the optics inside the sealed tube, with a focus position window and a removable knob for an electronic autofocuser.
- Imaging train built in: an included camera angle rotator and tilt adjuster let you frame targets and correct sensor tilt without extra adapters.
- Travel friendly: small enough for a carry-on bag and light enough for a star tracker like the SkyGuider Pro or Star Adventurer GTi.
Field of view with popular cameras
| Camera sensor | Field of view | Image scale |
|---|---|---|
| Full frame (36 x 24mm) | 8.2° x 5.5° | 3.1"/pixel (3.76µm pixels) |
| APS-C (23.5 x 15.7mm), e.g. ZWO ASI2600MC Pro | 5.4° x 3.6° | 3.1"/pixel (3.76µm pixels) |
| 1" square (11.3 x 11.3mm), e.g. ZWO ASI533MC Pro | 2.6° x 2.6° | 3.1"/pixel (3.76µm pixels) |
With an APS-C camera, the whole Andromeda Galaxy, the North America and Pelican Nebulae, the Veil Nebula complex or Orion's Belt with the Horsehead and Flame Nebulae fit in one frame. Full frame takes in huge regions like the Rho Ophiuchi cloud complex or the Heart and Soul Nebulae together.
IC 4603 Reflection Nebula in Ophiuchus by Jaroslav Salva
What you'll need to complete your setup
- A mount: at 5.25 lbs, the RedCat 51 rides on a star tracker such as the iOptron SkyGuider Pro or the GoTo Sky-Watcher Star Adventurer GTi, or on a strain-wave mount like the Sky-Watcher Wave 100i or ZWO AM5N for a full imaging rig.
- A camera: a DSLR or mirrorless camera with a wide M48 T-ring, or a cooled astronomy camera such as the ZWO ASI2600MC Pro or ASI533MC Pro. William Optics recommends 55mm (±3mm) of back focus.
- Autofocus (optional): the WIFD focuser accepts an electronic focuser such as the ZWO EAFN.
- Guiding (optional): the handle's mini Vixen base takes a UniGuide 32 guide scope.
Compare the William Optics Cat astrographs
| MiniCat 51 | RedCat 51 III | UltraCat 56 | RedCat 61 | |
|---|---|---|---|---|
| Aperture | 51mm | 51mm | 56mm | 61mm |
| Focal length | 178mm (f/3.5) | 250mm (f/4.9) | 269mm (f/4.8) | 300mm (f/4.9) |
| Lens elements | 6 | 4 | 5 | 4 |
| Image circle | 43.2mm | 48mm | 50mm | 46mm |
| Total weight | 5.05 lbs | 5.25 lbs | 7.08 lbs | 7.47 lbs |
| Best for | The widest, fastest views | The classic lightweight full-frame astrograph | Sharper corners on high-megapixel sensors | A little more reach and aperture |
Prefer space gray? The SpaceCat 51 III is the same telescope in a gray finish.
Why buy from Mile High Astronomy
- No surprise import duties: our price includes all import duties. Telescopes ordered directly from overseas are often listed without duties, which are then billed separately before delivery.
- Ships from the US: your RedCat 51 ships from our Denver store, not from overseas.
- Expert help from astrophotographers: we can help you choose a mount, camera and focuser for your setup, and we're here to help after the sale.
What's in the box
- William Optics RedCat 51 III WIFD f/4.9 astrograph with rotator and tilt adjuster
- Bahtinov mask cap for precise focusing
- M48 camera adapter and cap
- Mounting ring and handle bar with mini Vixen base
- 210mm dual Vixen / Arca-Swiss dovetail plate
- Soft carry case
- Allen key set
RedCat 51 FAQ
Do I need a field flattener?
No. The RedCat 51's Petzval design has field flattening built in. Use the recommended 55mm (±3mm) of back focus with the right T-ring or spacers.
What's new in the RedCat 51 III?
The RedCat 51 III uses William Optics' WIFD internal rack-and-pinion focuser in place of the earlier helical focuser, and its focus knob removes to fit an electronic autofocuser. It also includes a camera angle rotator and a sensor tilt adjuster.
Can I use the RedCat 51 for visual observing?
No, the Redcat is an astrograph designed specifically for imaging. standard diagonals and eyepieces are not compatible.
Can I use the RedCat 51 with a star tracker?
Yes. At 5.25 lbs with its ring and dovetail, plus a camera, it fits within the payload of popular trackers like the iOptron SkyGuider Pro and Star Adventurer GTi. The 250mm focal length is forgiving of small tracking errors.
RedCat 51 or UltraCat 56?
The RedCat 51 is lighter and less expensive, and its 4-element design is excellent on APS-C and most full-frame cameras. The UltraCat 56 uses a 5-element design with tighter stars in the far corners of high-megapixel full-frame sensors, and a slightly longer focal length.
Will it fit my mount?
It works great with all Harmonic mounts including the ZWO AM3N, AM5N, and AM7, as well as the Sky-Watcher Wave 100i and Wave 150i. For some older equatorial mounts, you may need a riser block for the focuser, or you can rotate the RedCat in the ring 180 degrees around to the focuser is on top.
Optical performance
Specifications
| Optical design | 4-element Petzval apochromat |
| Glass | FPL-53 and FPL-51 ED |
| Aperture | 51mm (2") |
| Focal length | 250mm |
| Focal ratio | f/4.9 |
| Image circle | 48mm |
| Camera formats | Full frame, APS-C and Micro Four Thirds |
| Field flattener | None required |
| Back focus | 55mm (±3mm) |
| Camera connection | M48 x 0.75 male; accepts 48mm filters |
| Focuser | WIFD internal dual-speed rack and pinion, 1:10 fine focus, EAF compatible |
| Focuser travel | 23mm |
| Included adapters | Tilt adjuster, camera angle rotator, M54 to M48 adapter |
| Dovetail | 210mm dual Vixen / Arca-Swiss |
| Tube length | 298mm |
| Tube diameter / dew shield outer diameter (for solar filter sizing) | 84mm |
| OTA weight | 3.9 lbs (1.77 kg) |
| Total weight with ring, handle and dovetail | 5.25 lbs (2.38 kg) |
| Dawes limit | 2.3 arcseconds |
| Tube color | Red |
| Warranty | 2 years from William Optics |
| Manufacturer part number | L-RC51RDIII |
Online manual: back focus, tilt adjustment, electronic focuser installation and mounting.