William Optics MiniCat 51mm Apochromatic Refractor

The William Optics MiniCat 51 WIFD is a compact apochromatic astrograph designed primarily for wide-field astrophotography. With a 51mm aperture and a focal length of just 178mm, it is designed to capture a broad area of the night sky rather than provide the higher magnification associated with longer focal-length telescopes.

Its f/3.5 focal ratio is particularly useful for astrophotography because it allows the telescope to collect light quickly. William Optics specifically describes the MiniCat as being designed for wide-angle views and states that its fast f/3.5 optics allow faint nebulae and comets to be captured in relatively short exposures.

The optical system uses a six-element Petzval design incorporating two Super ED glass elements and lanthanum lenses. The ED elements are intended to reduce chromatic aberration, helping to maintain accurate colour and sharp images. The Petzval design also provides field correction within the telescope itself, so William Optics states that no separate field flattener is required.

The MiniCat produces a 43.2mm image circle and is compatible with full-frame cameras, although William Optics specifically recommends APS-C cameras. The wide field of view makes the telescope particularly suited to large deep-sky subjects and extensive areas of the Milky Way where a longer focal length would capture a smaller portion of the sky.

The MiniCat uses William Optics’ WIFD (William Optics Internal Focus Design) focuser. Unlike a conventional rear-mounted focuser, the WIFD focusing mechanism is positioned within the telescope. William Optics states that this design helps eliminate tube tilt caused by the weight of cameras and accessories while maintaining an unobstructed optical path. The focuser uses a rack-and-pinion mechanism, has 28mm of travel and includes a 1:10 fine-adjustment control. It is also compatible with electronic automatic micro focusers.

At the rear of the telescope, a camera angle rotator allows the camera orientation to be adjusted, while an integrated sensor tilt adjuster provides a means of correcting sensor alignment when required. A M54-to-M48 adapter is supplied for connecting imaging equipment, and William Optics specifies a recommended astrophotography back spacing of 55 ± 3mm.

The MiniCat is particularly compact for an astrophotography telescope. The optical tube is 210mm long and weighs 1.67kg, while the complete package weighs 2.29kg. A dual-sided mounting plate is supplied and is compatible with both Vixen-style and Arca-Swiss mounting systems.

A Bahtinov mask is incorporated into the front lens cover. A Bahtinov mask is a focusing aid that produces a pattern of diffraction spikes around a bright star, allowing the photographer to make precise adjustments to focus.

The MiniCat is supplied with a mounting ring with integrated handle bar, dual-sided mounting plate, M54-to-M48 adapter, Bahtinov mask, soft carrying case and Allen key set.