Astronomy Specialist



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The William Optics ZenithStar 73 III is a two element apochromatic refractor with a focal ratio of f/5.9 and a focal length of 430mm. Utilizing FPL-53 synthetic fluorite glass, this telescope provides excellent aberration correction.

The 2.5" 10:1 rack & pinion focuser is smooth, stable and reliable, providing no slippage over long imaging sessions. The new CNC-machined and anodized handle bar with finder-style saddle is perfect for attaching a guide scope or other accessories. The ZenithStar 73 III also includes William Optics' innovative clear Bahtinov mask integrated into the lens cap.

For best imaging results, it is recommended to pair the ZenithStar 73 III with its matching Flat73A field flattener or the Flat73R 0.8x reducer.



Optical design

Apochromatic doublet

Objective diameter

73 mm

Focal length

430 mm

Focal ratio


Image circle

43 mm (with Flat73A field flattener)

Compatible flattener

William Optics FLAT73A, FLAT73R

Mount attachment

210 mm Vixen-style/Arca-Swiss dovetail bar


Rack & pinion dual speed (10:1) 2.5" diameter

Camera connection

M63 x 1, 2"


66.2 mm (with flattener/reducer)

Tube weight

2.83 kg (including accessories)

Tube length

330 mm (fully retracted), 420 mm (fully extended)

Tube diameter


Dew shield diameter



WO-A-Z73IIIGD (Gold), WO-A-Z73IIIRD (Red)


William Optics ZenithStar 73 III backfocus William Optics ZenithStar 73 III backfocus with FLAT73A William Optics ZenithStar 73 III backfocus with FLAT73R


  • ZenithStar 73 III
  • Lens cap with Bahtinov mask
  • Finder saddle handle bar
  • 210 mm Vixen-style dovetail bar
  • Soft carry case

Customer Reviews

Based on 2 reviews
Robert Freeman
ZENITHSTAR 73 III F/5.9 Review

A great compact fast refractor, easy to set up and small enough to handle large wind gusts without affecting the guiding. Having a light polluted garden, I needed something fast to get detail from a duo band filter and I have not been disappointed. The build and optical quality are excellent, and I have bought many Williams Optics products in the past. As yet I have not got a field flattener as the camera I am using, an ASI 294MC is not full frame and I can get round any problems with the Lens module in StarTools 1.8.
The telescope was despatched within 24 hours and arrived quickly.

All Class - ZenithStar 73 III

After deciding to get into Astrophotography or EAA to be more precise. Having never looked through a telescope prior, I had no idea as to what I was after. I started watching as many videos and reading as many reviews and as I could find on the subject. One name that kept coming up was William Optics, in particular the Z73. It seems about the perfect focal length for those beautiful Nebula shots you see all over the internet and once mated to the Flat73 reducer/flattener it will only get better. Although I have only been doing this for approximately a month; I know I made the right decision. The build quality is second to none and the images it can produce are nothing less than stunning. I only hope I can learn to use it to its full potential. (I have yet to attach the flattener as I'm waiting for the M48 T-ring)

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