William Optics RedCat 61 WIFD

The 4-Element, f/4.9 Petzval Optical System

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 61 Petzval APO f=300mm F4.9". No separate field flattener is needed. The f/4.9 focal ratio shortens the exposure needed for each sub-frame compared with slower telescopes.

Because the correction is part of the telescope's own optics, there is no separate flattener to buy or space. The manual includes the spot diagram and longitudinal aberration plots for the design.

A 45mm Flat Field for Full-Frame Sensors

The RedCat 61 has a 45mm image circle, which William Optics' manual calls full frame: it covers a 36 x 24mm sensor (43.3mm diagonal). With a full-frame sensor the field is about 6.9 x 4.6 degrees, enough for the Andromeda Galaxy (M31) or the North America Nebula (NGC 7000) in one frame.

The camera connects to the M48 adapter supplied with the telescope; the manual's drawing gives 55mm from the rear reference face to the camera sensor.

WIFD: The William Optics Internal Focus Design

This RedCat uses the William Optics Internal Focus Design (WIFD), a rack-and-pinion focuser that the manual lists as compatible with EAF. William Optics' page says the WIFD focuser allows fine control for both visual and photographic use.

• Tension and smoothness: tension screws (5mm Allen key) increase focuser tension, and push and pull gear spacing screws (3mm and 2mm Allen keys) adjust smoothness, which should be done only when necessary.
• 10:1 fine knob: the manual lists a 2 speed 10:1 ratio fine knob. A focuser tension screw should not be over-tightened and should be released when focusing.
• EAF: the manual's six steps are to 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.
• Thermometer: the focuser adjusting knob carries a thermometer, and a scale window on the focuser helps you return to a position.

The Imaging Train: 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.

The rear also carries a camera angle rotator with a locking screw, for framing, and a tilt adapter inside it. To reach the tilt adapter, remove the rotator knob, twist the cover and remove it, then adjust the alternating push and pull screws.

Build: Ring, Handle Bar, Dovetail and Bahtinov Cap

The optical tube weighs 2.465 kg (5.43 lbs), or 3.390 kg (7.47 lbs) with ring, dovetail and saddle, and is 365mm long with a 92mm dew shield. The body is CNC-machined aluminium. A soft carry case is included.

• Mounting: mounting rings with knobs carry the 210mm dual-side plate, which the manual calls a Vixen style dovetail plate. The manual also advises setting the tube so both focuser knobs are even and clear of the mount's saddle plate.
• Saddle handle bar: it has M4 locking screws and M4 and M6 screw holes for a finder bracket. The accessories map shows a red dot sight and the M-G32PB-RD guide scope (optional).
• Bahtinov mask: the front dust cover carries a Bahtinov mask for focusing on a bright star.
• Dew shield: the 92mm dew shield sits around the objective; the front cover threads on at M92.

Visual Astronomy with the RedCat 61

William Optics' page mentions visual as well as photographic use for the WIFD focuser, but the manual does not show a visual configuration. Ask us before buying a diagonal or eyepiece for it.

The short 300mm focal length suits wide star fields rather than high magnification on planets.

RedCat 61 vs. RedCat 51: Aperture and Image Scale

The RedCat 51 WIFD (51mm, 250mm, f/4.9) and the RedCat 61 WIFD share the four-element Petzval layout and the 45mm image circle.

• RedCat 51 Advantage: Its 250mm focal length gives about 8.2 x 5.5 degrees on full frame, a wider field for very large regions.
• RedCat 61 Advantage: Its 61mm aperture gathers about 43% more light, and its 300mm focal length gives about 6.9 x 4.6 degrees on full frame with a larger image scale for targets such as M31 or the Pleiades (M45).

Getting Started with Your RedCat 61 WIFD

On the first night, 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. Release the focuser tension screw while focusing, then use the camera angle rotator to frame your target.

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.