William Optics SpaceCat 51 III WIFD f/4.9 Astrograph

The RedCat 51 full-frame astrograph, in space gray

The William Optics SpaceCat 51 III WIFD is the space gray edition of the RedCat 51 III, one of the most popular small astrographs ever made, with the same optics, focuser and accessories. 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 SpaceCat 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

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, imaged with the RedCat 51 (same optics), by Jaroslav Salva

What you'll need to complete your setup

• A mount: at 5.25 lbs, the SpaceCat 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

Prefer red? The RedCat 51 III is the same telescope in the classic red 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 SpaceCat 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 SpaceCat 51 III WIFD f/4.9 astrograph (space gray) 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

SpaceCat 51 FAQ

Is the SpaceCat 51 the same as the RedCat 51?

Yes. The SpaceCat 51 III and RedCat 51 III share the same optics, WIFD focuser, adapters and accessories. Only the color is different: the SpaceCat is space gray.

Do I need a field flattener?

No. The SpaceCat 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 version III?

The SpaceCat 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 SpaceCat 51 for visual observing?

Yes, as a wide-field grab-and-go scope with a 1.25" diagonal or erecting prism (sold separately). With a 250mm focal length, a 10mm eyepiece gives 25x, so it's best for star fields, large clusters and daytime nature viewing rather than planets.

Can I use the SpaceCat 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.

SpaceCat 51 or UltraCat 56?

The SpaceCat 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 dual-saddle mount?

You may need a riser block on some dual-saddle mounts. The focuser contacts the Celestron Advanced VX saddle that accepts both CG-5 and CGE plates. It clears the Celestron CGX saddle, the iOptron CEM70 and GEM45 / CEM40 dual-saddle plates, and the Sky-Watcher EQ6 and HEQ5.

Optical performance

Specifications

Online manual: back focus, tilt adjustment, electronic focuser installation and mounting.