William Optics ZenithStar ZS61 II AP 61/360 OTA

Description

Packing down to just 230mm for airline carry-on transport and expedition stargazing, the William Optics ZenithStar ZS61 II AP 61/360 OTA is a compact wide-field apochromatic doublet refractor optical tube assembly. Operating at an f/5.9 focal ratio with a 360mm focal length, this 61mm (2.40 in) clear aperture instrument delivers a 1.90 arcseconds theoretical resolving power and reaches magnitude 11.5, collecting photons across extended emission nebulae, open star clusters, and daylight telephoto nature targets.

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 230mm retracted and 290mm extended with an 80mm outer dew shield diameter, the tube assembly supplies 135mm of back focus from the rear visual back (55mm standard from M48 threads on optional Flat61A or Flat61R correctors). Supported by a 2-inch dual-speed rack-and-pinion focuser with a 1:10 micro-reduction drive, an integrated 360-degree field rotator, and a precision thermometer, this 1.45 kg (3.20 lbs without rings / 1.80 kg with rings) refractor illuminates a 44mm full-frame flat field with dedicated correctors.

Key Features of William Optics ZenithStar ZS61 II AP 61/360 OTA

Built for traveling astrophotographers seeking high optical performance in an ultra-compact footprint, this 61mm doublet pairs certified FPL-53 optics with a 2-inch rack-and-pinion focuser. A CNC-machined mounting ring, a reversible Vixen and Arca-Swiss dovetail bar, a top saddle handle with a guiding slot, and an integrated Bahtinov mask cap provide an adaptable field platform.

• 61mm (2.40 in) clear aperture with 360mm focal length and fast f/5.9 optical speed
• Air-spaced ED doublet objective featuring Ohara FPL-53 and lanthanum optical glass
• Fully multi-coated STM broadband anti-reflection coatings with 0% central obstruction
• 2-inch dual-speed rack-and-pinion focuser 1:10 with built-in 360-degree field rotator and thermometer
• Generous 135mm back focus distance from visual back (55mm from M48 thread on Flat61A/R)
• Corrected 44mm full-frame flat field image circle when paired with dedicated Flat61A or Flat61R optics
• Reversible Vixen/Arca-Swiss dovetail bar with CNC mounting ring and top saddle handle
• Patented clear Bahtinov focus mask cap and top saddle handle with guiding slot included in package

Air-Spaced FPL-53 Doublet Optics & Full-Frame Field

The air-spaced doublet lens pairs an Ohara FPL-53 Super ED element with precision lanthanum glass to eliminate secondary spectrum chromatic aberration across visible wavelengths. The unobstructed 61mm optical path delivers high contrast on lunar crater rims and terrestrial targets, supporting practical visual magnifications from 8.7x up to 122x.

Two-Inch Rack-and-Pinion Focuser & Reversible Mounting Foot

Handling cooled astronomy cameras without drawtube flexure is delivered through a 2-inch rack-and-pinion focuser with a 1:10 micro-reduction gear, an integrated 360-degree camera field rotator, and an embedded thermometer to track ambient temperature shifts. Weighing just 1.80 kg with rings, handle, and dovetail, this compact optical assembly mounts effortlessly on travel tracker saddles.

• 2-inch dual-speed rack-and-pinion focuser with 1:10 micro-reduction and integrated temperature gauge
• 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 ZS61 Generation II Factory Warranty

The William Optics ZenithStar ZS61 II AP 61/360 OTA is backed by a 2-year limited warranty against material and manufacturing defects. The complete factory package contains the ZS61 II Doublet OTA, integrated patented Bahtinov focus mask cap, CNC mounting ring, dual-sided dovetail bar, and a top saddle handle with guiding slot.