The new William Optics Zenithstar 81, at a stowed length of 460mm (18.1"), is a reasonably compact telescope, and thus suited to travel where bulk and weight are considerations. Its comparatively small size does not detract from its versatility. The short focal length delivers wide visual fields and deep-sky astro-images. At the same time the Ohara FPL-53 objective yields excellent control of false color for high power work.
The FPL-53 substrate used in its objective has dispersion characteristics not much different from that of the much vaunted fluorite crystal, and so is termed a synthetic fluorite. This permits the employment of a fairly fast f/6.9 while delivering superb chromatic aberration suppression.
The generously large illuminated circle of 44mm covers a full-frame DSLR camera sensor, a diagonal angular coverage of 4.5 degrees. And the guaranteed Strehl ratio of 0.92 or better assures diffraction limited performance.
For the wide field astro-imager particularly, William Optics recommends using its Flattener 6A III, with which to achieve the sharpest images to the sensor corners. Aiding in the determination of when to check for focus drift, the focuser knob has a handy built-in thermometer. Critical focus is assessed using the included Bhatinov mask cover, and is afforded by the co-axial 10:1 fine focus knob. The sturdy rack-and-pinion focuser employs a helical rack that will not slip under heavy loads in the way a Crayford type is more likely to.
For ease of carrying, and safer handling during setup and takedown, a handle bridges the two mounting rings. And it does double duty with its dovetail slot, for the mounting of a finder or guide scope.
- Aperture: 81mm (3.1")
- Focal Ratio: F/6.9
- Focal Length: 559 mm
- Objective Type: Synthetic Flourite FPL 53 Doublet, Air Spaced, APO
- Fully Multi-Coated, SMC coating
- Image Circle Diameter: 44mm (4.5°)
- Lens Shade: Retractable
- Focuser: 2.5" R&P Focuser with 1:10 Dual Speed micro focuser
- Features a dual purpose carrying handle/accessory dovetail
- Dial Type Thermometer Built into Focuser
- Bhatinov Mask Cover for Critical Focus
- 2" and 1.25" Adapter with Compression Rings
- 44mm Vixen style dovetail mount
- Tube Length Fully Retracted: 460 mm (18.1")
- Tube Length Fully Extended: 560 mm (22")
- Tube Weight:3.53kg (7.8 lbs)
- Two year warranty