Focus Scientific

William Optics Zenithstar 73mm f/5.9 Apochromat Refractor OTA (Blue)

The new William Optics Zenithstar 73, at a stowed length of just 310mm, is a very compact telescope, and thus suited to travel where bulk and weight are considerations. Its diminutive 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/5.9 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 5.7 degrees. And the guaranteed Strehl ratio of 0.92 or better assures diffraction limited performance.

For the wide field astro-imager particularly, William Optics offers a dedicated field flattener--the Flattener 73--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.

Specifications

  • Aperture: 73mm (2.87")
  • Focal Ratio: F/5.9
  • Focal Length: 430 mm
  • Objective Type: Synthetic Flourite FPL 53 Doublet, Air Spaced, APO
  • Fully Multi-Coated, SMC coating
  • Image Circle Diameter: 44mm
  • Lens Shade: Retractable
  • Focuser: 2.5" R&P Focuser with 1:10 Dual Speed micro focuser
  • 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: 310 mm (12.2") Fully Retracted
  • Tube Weight:2.5kg (5.5 lbs)
  • Two year warranty