William Optics Mini Spacecat 51 WIFD - 51 mm f/3.5 Flatfield Refractor for Astrophotography

William Optics Mini Spacecat 51 WIFD Travel Apochromat for Astrophotography and Nature Photography

This apochromatic refractor is intended for all astrophotographers and nature photographers who are searching for a transportable and tack-sharp telephoto lens. Due to the 6-element Petzval design, a fully corrected image field of 43.2 mm diameter without chromatic aberration is achieved. Thus even full-frame cameras can be used.

Focusing works internally, as with many telephoto lenses, but via knobs with an additional fine focus knob (dual speed), as with telescopes. However, with 210 mm length, the apo ist not larger than a normal telephoto lens. But it can do more.

The application areas of the Mini Spacecat 51 apo:

Astrophotography: With 178 mm focal length and fast f/3.5, short exposure times are possible. The working distance of 55 mm allows using all popular astro cameras. mirrorless system cameras and DSLR/SLR cameras. The fine focusing allows a precise focus, the rotation the selection of the suitable image section. The supplied Bahtinov mask helps with focusing and is integrated in the objective cap.

Countryside photography and animal photography: The Mini Spacecat 51 is also perfect for, for example photographing animals. Internal focusing allows you to focus without shifting the weight, and therefore also without changing the position. The image field is thus not altered. The image quality will impress you.

Important Property: Gathers Light Twice as Fast as the RedCat 51
The RedCat 51 series has kept one thing constant: the f/4.9 focal ratio. This all changes with the MiniCat and Mini Spacecat 51. It now features a blazing fast f/3.5 focal ratio, meaning it can gather the same amount of light in less than half the time as the RedCat 51!

This faster focal ratio lets you focus more on capturing your target in staggering detail. Of course with the focal ratio change also comes a change in focal length: the focal length is reduced from 250 mm down to 178 mm. This focal length is ideal for capturing large widefield views of the night sky.

Important Property: Same Full Frame Image Circle
Despite a faster focal ratio, the Mini Spacecat 51 retains its 43 mm image circle from the RedCat 51. William Optics was able to achieve this by using 6 elements for this scope instead of 4, with two of them being super ED elements in the familiar Petzval design. As usual, William Optics has spared no quality components even in the smallest scope they´ve ever produced.


In Detail: Petzval Optical Design
Typically made with 4 optical elements, the MiniCat adds two Super ED additional elements to create a 6 element Petzval design. Favored by astroimagers for crystal clear images with little to no aberration, the Petzval design is known for its ability to produce a flat field, which minimizes field curvature. With a flat field, the fine details of astronomical objects, such as galaxies, nebulae, and star clusters, are captured more accurately across the entire frame. The result is higher-quality images with better resolution and contrast.

William Optics has mastered refined Petzval designs and has managed to take it to the next level by downsizing an already compact refractor design. The MiniCat body is a full 88 mm (nearly 3.5") shorter than the travel friendly f/4.9 Redcat 51.

In Detail: William Optics WIFD Focuser
William Optics´ MiniCat 51 telescope features their patented WIFD focuser system, which offers a significant improvement over traditional refractor focusers. Unlike conventional designs that extend and collapse a tube outside the main telescope, potentially causing tilt and vignetting, the WIFD system adjusts focus by moving the optics inside the telescope. This design ensures a stable, tilt-free connection and allows the full light cone to pass through, preserving image quality.

The MiniCat 51´s WIFD focuser is compatible with electronic autofocus systems like the ZWO EAF, simplifying the process and eliminating the need for third-party autofocus kits. For manual focus enthusiasts, it also includes a dual-speed focus knob and an analog thermometer-equipped coarse focus knob, making it a versatile and user-friendly option for both automated and manual focusing.


Thoughtful Features, Made to Travel
Features like the clear index markings on the rotator, easy to reach and turn knobs, and a handle bar make using the Minicat a comfortable and intuitive experience. William Optics also includes everything you need to start using the telescope, right out of the box: a V series dovetail and tube ring, a handle that doubles as a top mount bracket for accessories like a guidescope or remote control hub, and an included Bahtinov mask to fine tune your focus tucked into the front dust cap. The only thing you will not find included is the built in filter drawer. Ready to accept the same range of accessories as its larger counterpart, the Minicat is easy to pack, grab and go.

While slightly heavier than the f/4.9 version due to the additional lens elements, this compact refractor is just 2.3 kg (5.05 lbs) and only 210 mm long. Paired with a travel-friendly tracking mount, you could easily bring this system along with you in a carry on bag on a flight or have a condensed and lightweight system to take out to your favorite dark sky locations.

Image by a customer

Our customer Sascha Ebeler from Emsdetten (Germany) captured several nebulae together with this telescope and an APS-C format camera:

Objects: Lobster Claw Nebula (SH2-157), Bubble Nebula (NGC 7635), Cave Nebula (SH2-155)

Photographer: Sascha Ebeler from Emsdetten (Germany)
Bortle: 5
Telescope: William Optics MiniCat 51 WIFD (MINICAT51)
Imaging and guiding camera: ZWO ASI2600MC Duo (ASI2600MC-DUO)
Mount: ZWO AM3 (ZWO-AM3-Tripod)

Congratulations on this great result!

Safety informations: PDF Download