William Optics Redcat 71 WIFD

A 4-Element Petzval Design with FPL-53 Glass

William Optics' manual describes a unique Petzval design featuring four optical elements, with high-quality FPL-53 glass for superior optical performance; the lens is marked "RedCat 71 Petzval APO f=350mm F4.9". The correction is part of the telescope's own optics, so there is no separate flattener to buy or space.

The manual includes the spot diagram and longitudinal aberration plots for the design, across wavelengths from 0.436 to 0.680 micrometres. William Optics' page calls the Cat71 WIFD an apochromatic refractor built on the Cat series with a larger aperture.

Imaging Performance: 350mm Focal Length at f/4.9

With a full-frame sensor, 350mm gives a field of about 5.9 x 3.9 degrees. That frames the Andromeda Galaxy (M31), the North America Nebula (NGC 7000) or most of the Veil Nebula complex in one shot.

The f/4.9 focal ratio shortens the exposure needed for each sub-frame compared with slower telescopes, which helps with faint emission nebulae through narrowband filters.

A 45mm Image Circle for Full-Frame Sensors

The Redcat 71 has a 45mm image circle, which William Optics' manual calls full frame: it covers a 36 x 24mm sensor (43.3mm diagonal). APS-C and smaller sensors sit well inside it.

Because the corrector is built in, a large sensor needs no extra flattener, which saves an accessory and a spacing step.

WIFD Focuser: 10:1 Fine Knob and EAF Steps

The William Optics Internal Focus Design (WIFD) is a rack-and-pinion focuser that the manual lists as compatible with EAF (an electronic automatic focuser). William Optics' page says it allows meticulous focus adjustments for visual and photographic use.

• 10:1 fine knob: the manual lists a 2 speed 10:1 ratio fine knob, and the focuser adjusting knob carries a thermometer.
• Tension and smoothness: tension screws (5mm Allen key) increase focuser tension; push and pull gear spacing screws (3mm and 2mm Allen keys) adjust smoothness, only when necessary. Do not over-tighten the focuser tension screw, and release it when focusing.
• EAF: remove the top screw (2mm Allen key), release the two inner knob screws, remove the knob, align the screw holes, lock the two set screws on the shaft and tighten both bracket screws. With a motor fitted, do not over-tighten the tension screw.

Camera Integration: 55mm Back Focus, Rotator and Tilt Adapter

The camera end has M54x0.75 and M48x0.75 threads, with the supplied M48 adapter for the camera mount and an M48 filter position shown in the manual. Back focus is 55mm to the camera sensor; set that spacing with your camera's own adapter.

A camera angle rotator with a locking screw lets you frame the target. Inside it is a tilt adapter: remove the rotator knob, twist the cover and remove it, then adjust the alternating push and pull screws to correct sensor tilt.

Build: Bahtinov Cap, Rings, Dovetail and Handle Bar

The Cat71 WIFD is made from CNC-machined aluminium. The dew shield is 105mm across and the front cover threads on at M105. A soft carry case is included.

• Bahtinov mask: the front dust cover carries a Bahtinov mask for focusing on a bright star.
• Mounting: mounting rings with knobs carry the 210mm dual-side plate, which the manual lists as a Vixen style dovetail plate. Set the tube so both focuser knobs are even and clear of the mount's saddle plate.
• Saddle handle bar: M4 locking screws, plus M4 and M6 screw holes for a finder bracket (remove the two holder screws with a 2.5mm Allen key first). The accessories map shows a red dot sight and the M-G32PB-RD guide scope (optional).

Redcat 71 vs. Redcat 51 WIFD

Both share the four-element Petzval layout, the f/4.9 focal ratio and the 45mm image circle.

• Redcat 51 WIFD: 51mm and 250mm, about 8.2 x 5.5 degrees on full frame, in a smaller and lighter package.
• Redcat 71 WIFD: 71mm gathers about 1.9 times the light of 51mm, and 350mm gives about 5.9 x 3.9 degrees with a larger image scale.

Visual Use

William Optics' page mentions visual as well as photographic use, but the manual does not show a visual configuration. Ask us before buying a diagonal or eyepiece for it. The 350mm focal length suits low magnification and wide star fields.

Getting Started and Care

Mount the telescope by its dovetail with both focuser knobs clear of the saddle, attach your camera to the M48 adapter at 55mm back focus, and focus on a bright star with the Bahtinov mask cover, releasing the focuser tension screw while you focus.

Before storing it, let it acclimatise to room temperature with the tube uncovered and open, and make sure it is dry and free of dew. Store it in a consistent-temperature, dry place. 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. Solar eyepiece filters are not considered safe.