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William Optics Zenithstar 81 WIFD Telescope
William Optics Zenithstar 81 WIFD Telescope

William Optics Zenithstar 81 WIFD Telescope

by William Optics
SKU WO-T-ZS-81GD-WIFD
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$1,098.00 USD
$999.00 USD - $1,098.00 USD
$1,098.00 USD
$1,043.10 USD
$949.05 USD - $1,043.10 USD
$1,043.10 USD
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  • 81mm Aperture, 559mm Focal Length (f/6.9)
  • 2-Element APO Objective with FPL-53 Glass
  • Patented William Optics Internal Focus Design (WIFD)
  • Greater-than-Full-Frame Image Circle
  • 210mm Vixen-Style Mounting Plate Included
  • Gold or Red, with Soft Carry Case
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  • Description
  • FAQ
  • Specifications
  • In the Box
  • Downloads
  • Warranty

  • William Optics Zenithstar 81 WIFD Telescope

    The William Optics Zenithstar 81 WIFD Telescope is an 81mm, 559mm, f/6.9 doublet APO with FPL-53 glass, for astrophotography and visual observing. It focuses with the patented William Optics Internal Focus Design (WIFD) and a 2 speed 10:1 fine knob, retracts to 526mm, and the manual gives an image circle greater than full frame. It weighs 3.545 kg as a tube and 4.56 kg with ring, dovetail and saddle.

    FPL-53 APO Doublet

    A new lens cell design includes FPL-53 glass in the WO two element designed APO refractor (Zenithstar series). This design is perfect for temperature compensation to better prevent pinched optics, which can occur in cold weather.

    The manual includes spot diagram and longitudinal aberration plots for the telescope with the 0.8x reducer, across 0.436 to 0.680 micrometres.

    81mm Aperture at f/6.9

    An 81mm objective has a Dawes limit of about 1.43 arcseconds. At 559mm, a full-frame sensor covers about 3.7 x 2.5 degrees and APS-C (about 23.5 x 15.6mm) about 2.4 x 1.6 degrees.

    That frames M31 on full frame (it is about 3 degrees long), and gives a useful scale on smaller targets such as the Hercules Cluster (M13) or the Dumbbell Nebula (M27). With the optional Flat6AIII 0.8x reducer the focal length becomes about 447mm (f/5.5) and full frame covers about 4.6 x 3.1 degrees, enough for the North America Nebula (NGC 7000).

    Image Circle

    The manual gives the image circle as greater than full frame, so a 36 x 24mm sensor (43.3mm diagonal) is covered. A doublet focuses a curved field; for flat stars to the edge of a large sensor, William Optics' accessories map puts the Flat6AIII in the imaging train.

    Patented WIFD Focuser with 10:1 Fine Knob

    William Optics' page describes a patented internal focusing mechanism that keeps the optics aligned, even with heavy cameras, and gives smooth, precise adjustments. The manual lists it 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.

    • Tension: do not over-tighten the focuser tension screw, and release it when focusing.
    • Smoothness: the rack-and-pinion gear spacing screws are 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 of the tube threads at M92. The included M92 to M63 adapter is the one for the FLAT6AIII.

    A camera angle rotation device, with a rotator knob and a locking screw, lets you turn the camera for framing and lock it.

    Optional Flat6AIII 0.8x Reducer

    The Flat6AIII, a 0.8x reducer, is an optional accessory, fitted through the included M92 to M63 adapter. With it the focal length is about 447mm and the focal ratio about f/5.5, so each sub-frame needs about 64% of the native exposure.

    The imaging train runs M92 to M63 adapter, Flat6AIII, T-mount and DSLR or mirrorless camera; for an astronomy camera, use an adapter so that back focus is 55mm to the sensor. The 50mm UniGuide is also an optional guide scope.

    Visual Use: 2-inch and 1.25-inch

    The ZenithStar 81 WIFD is made for astrophotography and visual observations, including planetary detail. At 559mm, a 25mm eyepiece gives about 22x and a 7mm about 80x.

    The manual's accessories map shows the visual path: a 2-inch RotoLock on the focuser, then a 2-inch diagonal and 2-inch eyepiece, or a 2-inch to 1.25-inch adapter with a 1.25-inch diagonal or erecting prism and 1.25-inch eyepiece. These are not in the box.

    Build: Bahtinov Cover, Rings, Handle Bar and Dovetail

    • Bahtinov mask: in the front dust cover, which threads on at M121.
    • Mounting: mounting rings with knobs on a 210mm Vixen style dovetail plate, which fits a Vixen-style saddle.
    • Saddle handle bar: 120mm, with M4 locking screws and M4 and M6 screw holes for a finder bracket.
    • Dew shield: retractable, 121mm across, with a lock screw.

    Weight, Length and Colours

    The optical tube weighs 3.545 kg (7.82 lbs), and 4.560 kg (10.05 lbs) with ring, dovetail and saddle. Length is 526mm retracted and 607mm extended. A soft carry case is included. This page offers gold and red.

    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 of dew or moisture. 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.

  • What does the WIFD focuser offer?

    The focuser uses a patented internal focusing mechanism that keeps the optics aligned even with heavy cameras. It has a 2 speed 10:1 fine knob, a thermometer on the adjusting knob, and is compatible with EAF.

    Can I use the Zenithstar 81 WIFD for visual observing?

    Yes. Add a 2-inch RotoLock and a 2-inch diagonal, or a 2-inch to 1.25-inch adapter with a 1.25-inch diagonal or erecting prism, plus eyepieces, as the manual's accessories map shows.

    How do I attach an EAF to the Zenithstar 81 WIFD?

    Remove the top screw with a 2mm Allen key, release the two inner knob screws, remove the knob, align the bracket screw holes, lock the two set screws on the shaft and tighten both bracket screws. Do not over-tighten the tension screw with a motor fitted.

    What do I need to connect a DSLR for the Orion Nebula (M42)?

    The included M92 to M63 adapter, the optional Flat6AIII reducer and a T-mount for your camera. With the Flat6AIII, full frame covers about 4.6 x 3.1 degrees, which holds M42 and the Running Man Nebula with room around them.

    How do I use the Bahtinov mask?

    Fit the front dust cover with the mask, aim at a bright star, and adjust focus until the central diffraction spike sits centred between the other two.

    What does the FPL-53 glass do?

    It is the ED glass in William Optics' two-element APO design, which reduces false colour; the manual pairs it with a lens cell designed for temperature compensation against pinched optics in the cold.

    How portable is the Zenithstar 81 WIFD?

    It is 526mm long retracted and weighs 4.56 kg (10.05 lbs) with ring, dovetail and saddle. A soft carry case is included.

    Will the Zenithstar 81 WIFD cover my full-frame sensor?

    The manual gives an image circle greater than full frame. For flat stars to the corners, William Optics' imaging path uses the Flat6AIII.

    How does the Flat6AIII change the field for the North America Nebula (NGC 7000)?

    At 0.8x the focal length is about 447mm (f/5.5). Full frame then covers about 4.6 x 3.1 degrees, enough for NGC 7000 and the Pelican Nebula together, and each sub-frame needs about 64% of the native exposure.

    What is the Camera Angle Rotation Device for?

    Turning the camera to frame your target, then locking it with the rotation locking screw.

    Does the Zenithstar 81 WIFD come with mounting hardware?

    Yes: mounting rings, a 120mm saddle handle bar and a 210mm Vixen style dovetail plate, for a Vixen-style saddle. No mount is included.

    How should I store the Zenithstar 81 WIFD?

    Leave it uncovered with the tube open until it reaches room temperature, make sure it is completely dry, and store it in a dry place with a consistent temperature.

  • Available Options Gold, Red
    Optical design APO Doublet
    Aperture 81 mm
    Focal length 559 mm
    Focal ratio f/6.9
    Rear / camera-side connection M92 (M63 via included adapter)
    Focuser type WIFD Rack & Pinion (EAF Compatible)
    Elements / glass 2 Elements, FPL-53
    Tube length (retracted) 526 mm (retracted), 607 mm (extended)
    OTA weight 3.545 kg / 7.82 lbs
    Dovetail included 210 mm Vixen-Style Plate
    Mount included No
    Matching flattener / reducer Flat6AIII (0.8x)
    Weight 4.56 kg / 10.05 lbs (with ring, dovetail and saddle)

    • Dust Cover with Bahtinov Mask

      × 1

    • Main Tube

      × 1

    • 120mm Saddle Handle Bar

      × 1

    • Camera Angle Rotation Device

      × 1

    • M92 to M63 Adapter (for FLAT6AIII)

      × 1

    • 210mm Vixen Style Dovetail Plate

      × 1

    • Soft Carry Case

      × 1

  • William Optics 2-Year Warranty