William Optics ZenithStar 81 AP 81/559 Gold OTA

Description

Operating at a 559mm focal length with 1.43 arcseconds theoretical resolving power, the William Optics ZenithStar 81 AP 81/559 Gold OTA is a color-free apochromatic doublet refractor optical tube assembly built for lunar, planetary, and deep-sky imaging. Sized with an unobstructed 81mm (3.19 in) clear aperture and an f/6.9 focal ratio, this instrument gathers light to visual magnitude 12.2, splitting close double stars and collecting photons for high-definition portable deep-sky astrophotography.

The optical system features an air-spaced extra-low dispersion doublet objective combining Ohara FPL-53 Super ED glass with a matched lanthanum element, treated with fully multi-coated STM broad-band anti-reflection coatings with an unobstructed 0% central profile. Built inside an aluminum chassis measuring 445mm retracted and 520mm extended with a 102mm outer dew shield diameter, the tube assembly supplies 150mm of back focus from the rear visual back (55mm standard from M48 threads on optional Flat6AIII reducers). Supported by a 2.5-inch dual-speed rack-and-pinion focuser with a 1:10 reduction gear, a 360-degree field rotator, and gold anodized accents, this 3.53 kg (7.78 lbs) refractor illuminates a 44mm full-frame flat field when paired with dedicated correctors.

Key Features of William Optics ZenithStar 81 AP 81/559 Gold OTA

Engineered for high-resolution wide-field imaging and visual planetary auditing, this 81mm doublet couples Ohara FPL-53 optics with an oversized 2.5-inch rack-and-pinion focuser. CNC-machined split tube rings, a reversible Vixen and Arca-Swiss dovetail bar, a gold top saddle handle with a guiding slot, and an integrated Bahtinov mask cap deliver a rigid imaging platform.

• 81mm (3.19 in) clear aperture with 559mm focal length and versatile f/6.9 optical speed
• Air-spaced ED doublet objective featuring Ohara FPL-53 and lanthanum glass elements
• Fully multi-coated STM broadband anti-reflection coatings with 0% central obstruction
• 2.5-inch dual-speed rack-and-pinion focuser 1:10 with gold accents and 360-degree field rotator
• Generous 150mm back focus distance from visual back (55mm from M48 thread on Flat6AIII)
• Corrected 44mm full-frame flat field image circle when paired with dedicated Flat6AIII reducer
• Reversible Vixen/Arca-Swiss dovetail bar with CNC tube rings and gold top saddle handle
• Patented clear Bahtinov focus mask cap and top saddle handle with guiding slot included

Extended f/6.9 Lanthanum-ED Doublet Optical Performance

The moderate f/6.9 focal ratio pairs an Ohara FPL-53 Super ED center element with high-refraction lanthanum mating glass to control chromatic dispersion, delivering clean star profiles and high-contrast planetary details. The unobstructed 81mm optical path maximizes light throughput, supporting practical visual magnifications from 11.6x up to 162x.

Dual-Speed Rack-and-Pinion Focuser & Bahtinov Mask Cap

Holding camera equipment without drawtube slip is delivered through a 2.5-inch rack-and-pinion focuser with a 1:10 micro-reduction gear, an integrated 360-degree field rotator, and an engraved millimeter scale. The front lens cap integrates a patented clear Bahtinov focus mask for rapid focus confirmation, while the reversible dovetail bar secures the 3.53 kg tube onto tracking saddles.

• 2.5-inch dual-speed rack-and-pinion focuser with 1:10 micro-reduction to eliminate drawtube flexure
• Built-in 360-degree camera field rotator for rapid target orientation without losing focus
• Patented clear Bahtinov focus mask integrated into the metal lens cap for precise focus acquisition

William Optics ZenithStar 81 Limited Warranty

The William Optics ZenithStar 81 AP 81/559 Gold OTA is backed by a 2-year limited warranty against material and manufacturing defects. The complete factory package contains the ZenithStar 81 Gold OTA, integrated patented Bahtinov focus mask cap, CNC tube rings, dual-sided dovetail bar, and a top saddle handle with guiding slot.