William Optics MiniCat 51 WIFD f/3.5

The MiniCat 51 Astrograph: A 178mm f/3.5 Wide-Field Specialist

The MiniCat 51 is engineered specifically for astrophotographers who need to capture immense areas of the night sky in a single frame. William Optics designs it for photographers who love wide-angle views, and says the f/3.5 ratio lets you capture faint nebulae or comets in a very short time. With a full-frame sensor the 178mm focal length gives a field of about 11.6 x 7.7 degrees.

Its 1.67 kg optical tube and 2.29 kg total weight make it easy to carry to a dark site.

Optical Design: A 6-Element Petzval with 2 Super ED Elements

At the heart of the MiniCat 51 is a sophisticated 6-element Petzval optical design. It is the latest Petzval design, with 6 lens elements, and no flattener is required. Many doublet and triplet refractors need a separate flattener; the MiniCat 51 does not.

The six elements include two Super ED (extra-low dispersion) lenses and lanthanum lenses, which control chromatic aberration.

Speed & Field of View: The Power of a 51mm f/3.5 System

The MiniCat 51's defining feature is its extremely fast f/3.5 focal ratio. An f/3.5 system gathers light roughly twice as fast as a comparable f/5 telescope, allowing you to achieve the same signal-to-noise ratio in half the time. Put another way, the f/3.5 aperture lets in twice as much light as an f/5 aperture.

Combined with its 51mm aperture and 178mm focal length, the 178mm focal length gives a wide field. With a full-frame sensor you can frame the North America Nebula (NGC 7000) or the Andromeda Galaxy (M31) with room around them.

Illumination: A 43.2mm Flat Image Circle for Full-Frame Sensors

The MiniCat 51 has a 43.2mm image circle, about the diagonal of a full-frame sensor (36mm x 24mm, 43.3mm diagonal). Supported camera formats are full frame, APS-C and M4/3.

• Full frame: listed by William Optics.
• APS-C and M4/3: listed by William Optics.
• No flattener required: the design needs no separate flattener.

Focusing System: The Patented WIFD with 28mm of Internal Travel

William Optics' patented WIFD (William Optics Internal Focus Design) is a rack-and-pinion focuser compatible with EAF. The MiniCat holds three unique patents for easy focusing and coaxial image quality, to prevent drawtube tilt issues caused by optical misalignment.

The focuser gives 0-28mm of travel (ruler to 40). Precise focus matters at f/3.5, where the zone of sharp focus is short. For automated imaging, the focuser is compatible with an EAF (electronic automatic focuser).

Camera Connection: 58-30mm Back Spacing with Rotator and Tilt

The MiniCat 51 provides a flexible and robust connection for your camera. The package includes a tilt adjuster rotator and an M54 to M48 adapter, so a 48 mm T-mount or an M48 astronomy camera connects directly.

• Back Spacing: It is 58-30mm, so plan filter drawers, filter wheels or off-axis guiders within that range.
• Rotator: turns the camera to frame your target.
• Tilt Adjuster: Adjusts the sensor angle to correct corner star elongation caused by tilt.

Field Correction: Natively Flat Design, No Flattener Required

William Optics' specification for the MiniCat 51 reads "No Flattener Required", so the camera connects without an extra corrector.

With many refractors you buy a separate flattener and set its back focus. With the MiniCat 51 you connect the camera within the 58-30mm back spacing.

Visual Use

William Optics presents the MiniCat 51 as an astrograph for wide-angle photography and does not describe a visual configuration for it. Ask us before buying a diagonal and eyepiece for visual use. At 178mm any eyepiece gives a low magnification.

Mechanical Build: Rings, Handle, and a Dual Vixen/Arca-Swiss Plate

The optical tube sits in a mounting ring with a handle bar on top.

For maximum mounting flexibility, the telescope includes a dual-format dovetail plate. It fits Vixen-style saddles and Arca-Swiss clamps. The front lens cap is a Bahtinov mask for focusing.

Portability & Inclusions: A 1.67 kg OTA with a Full Accessory Kit

The optical tube assembly (OTA) weighs 1.67 kg (3.68 lbs), the total weight is 2.29 kg (5.05 lbs) and the tube length is 210mm.

The box holds the main tube, the Bahtinov mask cap, the mounting ring and handle bar, the dual Vixen/Arca-Swiss plate, the M48 camera adapter, the tilt adjuster and rotator (M54 to M48), an Allen key set and a soft carry case.

First Night Out: Setup, Focusing, and Care for the MiniCat 51

Getting started with the MiniCat 51 is straightforward. First, attach the dual dovetail plate to your mount's saddle. Connect your camera to the M48 adapter, ensuring any filter drawers or other accessories are installed within the 58-30mm back spacing range. Before imaging, it's crucial to achieve precise focus.

Fit the Bahtinov mask cap and point at a bright star; adjust the WIFD focuser until the central diffraction spike is centred between the other two. Once focused, you can remove the mask and begin imaging. When not in use, store it dry in the provided soft padded carry case with the lens cap on. Never aim the telescope or finder at the Sun without proper solar filters installed on the front of the telescope; doing so for even a moment may permanently damage your vision.

MiniCat 51 vs. RedCat 51: Choosing Your 51mm William Optics Astrograph

Both are 51mm William Optics astrographs with WIFD focusers, so the choice comes down to focal length and focal ratio. The RedCat 51 WIFD figures are below.

• RedCat 51 (f/4.9, 250mm): The longer 250mm focal length gives more image scale, and its image circle is 45mm.
• MiniCat 51 (f/3.5, 178mm): The MiniCat 51 is significantly faster at f/3.5 and has a much wider field of view due to its shorter 178mm focal length. That suits the largest nebulae and star fields.

Your choice depends on your main targets: the widest field and fastest ratio, or a little more image scale.