William Optics Zenithstar 61II APO

Apochromatic FPL-53 Doublet

The objective is a doublet APO with an Ohara FPL-53 ED element. It delivers clear, high-contrast images with minimal color aberration.

A doublet focuses a curved field, so for imaging the matching Flat61A flattener is used; for visual use the eyepiece goes straight into the focuser through the supplied adapters.

61mm Aperture at a Fast f/5.9

The combination of a 61mm aperture and a 360mm focal length results in a fast f/5.9 focal ratio. This quick f/5.9 focal ratio suits broad, detailed sky scans.

With a full-frame sensor, 360mm gives about 5.7 x 3.8 degrees; with APS-C (about 23.5 x 15.6mm) about 3.7 x 2.5 degrees. Either frames the Andromeda Galaxy (M31) or the North America Nebula (NGC 7000) in one shot. A 61mm objective has a Dawes limit of about 1.9 arcseconds.

Flat61A Flattener for Full-Frame Coverage

The manual names the FLAT61A as the recommended flattener, and it is included. It flattens the field for imaging; the manual gives the image circle as greater than full frame and lists APS-C and full frame as camera formats. To fit it, twist off the adapter and thread the flattener onto the M54 telescope thread. Do not retract the focuser drawtube all the way in, as it could damage the lens.

2-inch 10:1 Dual-Speed Rack & Pinion Focuser

The 2-inch dual-speed rack-and-pinion focuser has a coarse knob and a 10:1 fine knob. Tension screws (2mm hex wrench) set its tension, and a thumb screw fixes the focuser. William Optics asks you not to adjust any unmarked screws, especially on the focuser retrofit, before contacting them.

The lens cap carries what the manual calls the W.O. patented diffraction spikes Bahtinov focusing mask, three times brighter than the traditional type. Aim at a bright star and focus until the central spike is centred.

67.9mm Back Focus and M54 Imaging Train

The manual gives the backfocus as 67.9 mm from the end of the Flat61A flattener to the camera sensor. The flattener threads onto the telescope's M54 thread and has an adjustable part.

• For DSLR/Mirrorless Cameras: The manual takes the T-mount to sensor distance as 55 mm, so the flattener's adjustable part is set to 12.9 mm (67.9 minus 55).
• For Dedicated Astro Cameras: Subtract the camera, filter wheel, OAG and adapter distance (B) from 67.9 mm; the adjustable part makes up the difference (C).

CNC-Machined Body, Ring, Handle Bar and Dovetail

The body is CNC machined aluminium, with CNC made accessories included: a mounting ring, a CAT saddle handle bar and a 210 mm dual side dovetail plate suitable for Vixen and Arca Swiss type saddle plates. The dew shield retracts.

• Handle bar: a Vixen finder type base that the manual says suits the W.O. UniGuide, with M4 thumb screws for a guiding scope, plus 1/4 and 3/8 threads.
• Ring and focuser: M4 screws (2.5mm hex) hold the finder or guider scope base, and M6 screws attach the mounting ring.

Adapters for 2-inch and 1.25-inch Visual Accessories

The package includes an M54 to 2-inch adapter and a 2-inch to 1.25-inch adapter, each with M4 thumb screws. The visual path is a 2-inch diagonal with a 2-inch eyepiece, or a 1.25-inch diagonal or erecting prism with a 1.25-inch eyepiece. Its uses include wildlife viewing. At 360mm, a 20mm eyepiece gives 18x.

The 2.2 kg Portable Imaging System

The manual gives a tube weight of 2.2 kg with accessories, and a tube about 24 cm long with the dew shield retracted. A soft carry case is included. Add your camera and compare the total with your mount's rated payload.

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.