William Optics Gran Turismo WIFD 81

81mm FPL-53 Triplet

The GT 81 WIFD features a triplet apochromatic lens with extra-low dispersion (FPL-53) glass from Ohara, Japan, for outstanding clarity and sharpness with virtually no chromatic aberration.

A new lens cell design with FPL-53 glass in the WO triplet series (GT and FLT) is designed for temperature compensation to better prevent pinched optics in cold weather. The precision-machined housing also maintains optimal alignment of the optical elements.

478mm at f/5.9

An 81mm objective has a Dawes limit of about 1.43 arcseconds. At 478mm a full-frame sensor covers about 4.3 x 2.9 degrees, and APS-C (about 23.5 x 15.6mm) about 2.8 x 1.9 degrees. That frames M31 or the North America Nebula (NGC 7000) on full frame.

At f/5.9 each sub-frame needs roughly half the exposure of an f/8 system for the same signal.

Image Circle

The image circle is >Full Frame (45mm). A 36 x 24mm sensor has a 43.3mm diagonal, so it is covered. For flat stars to the edge, pair the telescope with its 0.8x and 1x flatteners (below).

WIFD Focuser with 10:1 Fine Knob

The manual lists the WIFD focuser as compatible with EAF (an electronic automatic focuser), with a 2 speed 10:1 ratio fine knob and a focuser adjusting knob with a thermometer. It does not say which lens group moves, so we do not.

• Tension: tension screws (5mm Allen key) increase focuser tension. Do not over-tighten the focuser tension screw, and release it when focusing.
• Smoothness: push and pull gear spacing screws (3mm and 2mm Allen keys), adjusted only when necessary.
• 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.

M92 Rear Thread and Camera Angle Rotator

The rear threads at M92. The camera angle rotator, with a rotator knob and locking screw, turns the camera for framing. The included M92 to M63 adapter is for the FLAT6AIII.

Optional Flatteners: FLAT6AIII and FLAT68III

The price list says the flattener is optional and not included. The manual shows spot diagrams for two:

• P-FLAT6AIII (0.8x): threads onto the included M92 to M63 adapter. About 382mm and f/4.7; full frame then covers about 5.4 x 3.6 degrees.
• P-FLAT68III (1x): threads onto the rotator's M92 thread, keeping 478mm and f/5.9.

Visual Use

The WIFD focuser suits serious star-gazing as well as astrophotography. The visual setup is a 2-inch RotoLock (optional, not included) on the focuser, then a 2-inch diagonal and eyepiece, or a 2-inch to 1.25-inch adapter with a 1.25-inch diagonal or erecting prism. At 478mm, a 25mm eyepiece gives about 19x and a 7mm about 68x.

Rings, 120mm Handle Bar and 210mm Plate

• Mounting rings: with knobs.
• 120mm saddle handle bar: M4 locking screws and M4 and M6 screw holes for a finder bracket; remove the two finder bracket holder screws (2.5mm Allen key) before use.
• 210mm plate: William Optics' box list calls it a dual-side plate and the manual a 210mm Vixen style dovetail plate. It fits a Vixen-style saddle.
• Dew shield: retractable, 121mm across, with a lock screw.

Weight and Length

The optical tube weighs 3.58 kg (7.89 lbs), and 4.565 kg (10.06 lbs) with ring, dovetail and saddle. Length is 421mm retracted and 502mm extended. Add your camera and compare the total with your mount's rated payload.

Bahtinov Cover, Case and ZYGO Test Report

The front dust cover carries a Bahtinov mask and threads on at M121: aim at a bright star and focus until the central spike is centred. A ZYGO test report and a soft carry case are included.

Connecting a Camera with the FLAT68III

The steps: remove the M92 to M63 adapter from the rotator; screw the FLAT68III onto the M92 thread; optionally swap the flattener's shorter adjusting adapter for the M48 or M54 longer adapter (for the WIFD GT81 only); set 1.89mm for best performance with the flattener pointing at the zenith, to avoid tilt, and lock the retaining ring; then connect an M48 or M54 astronomy camera, or a T-mount and DSLR. Maximum camera loading is 18kg.

For an astronomy camera, the accessories map says the adapter must give 55mm back focus to the sensor.

Care and Storage

Before storing the telescope, leave it uncovered with the tube open long enough to adjust to room temperature, and make sure it is completely dry. Store it in a dry place with a consistent temperature.

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.