William Optics Gran Turismo 71 APO Triplet

71mm f/5.9 Optics

The GT71 II has a 71mm objective, a 419mm focal length and an f/5.9 focal ratio, which is ideal for capturing detailed celestial images. A 71mm objective has a Dawes limit of about 1.63 arcseconds.

With a full-frame sensor, 419mm gives about 4.9 x 3.3 degrees; with APS-C (about 23.5 x 15.6mm) about 3.2 x 2.1 degrees. That frames the Andromeda Galaxy (M31), the North America Nebula (NGC 7000) or the Orion Nebula region.

FPL-53 Triplet APO

The lens is a triplet APO prime lens, with an objective of 3 elements air-spaced with FPL53 glass. The FPL-53 glass provides exceptional color correction and clarity.

The body is CNC-machined aluminium with a retractable dew shield.

44mm Image Circle and Camera Formats

The image circle is 44mm, and supported formats are full frame, APS-C and M4/3. A full-frame sensor has a 43.3mm diagonal, so it sits just inside the circle.

Like any triplet, the GT71 II focuses a curved field. For flat stars to the edge of a large sensor, there are two optional correctors, below.

2.5-inch R&P Focuser with 93mm Travel

It has a robust 2.5-inch dual-speed focuser, rack-and-pinion, compatible with EAF (an electronic automatic focuser), with 93mm of travel. The rear takes the included M63 to 2-inch visual back.

• Dual-speed: a fine knob for small adjustments alongside the main knob.
• EAF: The focuser is compatible with EAF.
• CAA63: the included camera angle adjuster lets you turn the camera to frame your target.

Optional FLAT GT 1.0x and FLAT 6AIII 0.8x

Two optional flatteners are available for the GT71 II.

• FLAT GT 1.0x: keeps the native 419mm and f/5.9.
• FLAT 6AIII 0.8x: a reducer that gives about 335mm and f/4.7. Full frame then covers about 6.2 x 4.1 degrees, and each sub-frame needs about 64% of the native exposure.

Ask us for the back focus and adapters for your camera with either one.

Weight, Length and Dovetail

The optical tube weighs 2.95 kg (6.5 lbs) and is 392mm long with the dew shield retracted, 470mm extended. The dovetail type is Vixen and Arca-Swiss, so it fits either style of saddle. Add your camera and compare the total with your mount's rated payload.

Visual Use and Care

The box includes an M63 to 2-inch visual back and a 2-inch to 1.25-inch visual back. Add a diagonal and eyepieces for visual use; at 419mm a 20mm eyepiece gives about 21x and a 7mm about 60x. The GT71 suits both seasoned astronomers and beginners.

Make sure the lens is dry before storing the telescope, and store it away from humidity. 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.