Focus Scientific

William Optics Zenithstar 103mm f/6.9 Apochromat Refractor OTA (Blue)

The new William Optics Zenithstar 103, at a stowed length of just 615mm, is a quite compact telescope, and thus suited to travel where bulk and weight are considerations. Some airlines may permit its being carried into the cabin. The relatively short focal length, particularly in conjunction with the optional 0.8X focal reducer (yielding f/5.5), 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 43mm covers a full-frame DSLR camera sensor, a diagonal angular coverage of just about 3.5° (4.3° at f/5.5). And the guaranteed Strehl ratio of 0.92 or better assures diffraction limited performance.

For the wide field astro-imager particularly, William Optics offers an optimised 0.8X focal reducer/flattener, yielding a fast f/5.5. Other of their field flatteners are compatible when adjusted appropriately, if one does not wish to shorten the focal length. 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 2.5" rack-and-pinion focuser employs a helical rack that will not slip under heavy loads, nor will the focuser sag or otherwise shift.

Features and Specifications

  • Aperture: 103mm (4.06")
  • Focal Ratio: F/6.9
  • Focal Length: 710 mm
  • Objective Type: Synthetic Flourite FPL-53 Doublet, Air Spaced, APO
  • Fully Multi-Coated, SMC coating
  • Image Circle Diameter: 43mm
  • 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
  • Tube and Guidescope Rings
  • Long Vixen style 44mm dovetail mount
  • Integrated Carry Handle
  • Tube Length: 615 mm (24.2") Fully Retracted
  • Tube with Rings and Dovetail Weight: 4.36kg (9.6 lbs)
  • Two year warranty