Content

ZenithStar 73 III

Aug 21, 2025 by WO Team

Lens Type
Doublet Apochromatic Prime Lens
Focal Length
430 mm
Diameter
73 mm
Focal Ratio
f/5.9
Image Circle
45 mm
Focuser
2" R&P Focuser
Adapter Included
2" → 1.25" Visual Back
Optional Flattener
FLAT 73A 1.0x
FLAT 73R 0.8x
Camera Format
Full Frame
APS-C
M4/3
OTA Weight
2.83 kg / 6.24 lbs
Tube Length
330 mm - 420 mm

Diagram Reports


Hardware Parts

Code Name
Image
Quantity
Usage
img.notionusercontent.com
2
Secure the guider to the handlebar
img.notionusercontent.com
1
Secure the dew shield
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2
Secure the handlebar to the mounting ring

Adaption Guide

graph LR A(Focuser) -.->M54(M54 → 2” Visual Back) ===>2Dig(<img src='https://williamoptics.com/cdn/shop/files/DIG2-90-1.jpg'>2” Diagonal) ==>2E(<img src='https://williamoptics.com/cdn/shop/files/E-SWA40-1.jpg'>2” Eyepiece) M54 -.-> 2(2” → 1.25” Visual Back) ==> 1Dig(<img src='https://williamoptics.com/cdn/shop/files/DIG125-90-1.jpg'>1.25” Diagonal) ==>1E(1.25”Eyepiece) A ==> subgraph1==> T & AC F73A(<img src='https://williamoptics.com/cdn/shop/files/P-FLAT73A-2.jpg'>Flat 73A<br>1.0x) F73R(<img src='https://williamoptics.com/cdn/shop/files/P-FLAT73R-2.jpg'>Flat 73R<br>0.8x) T(<img src='https://williamoptics.com/cdn/shop/files/TM-NK-F-M48-1.jpg'>T-Mount)==>C(Camera) AC(Astro-Camera) subgraph subgraph1 [M48 Flattener] F73A F73R end click 2Dig "https://williamoptics.com/products/dura-bright-2-90-dielectric-diagonal" click 1Dig "https://williamoptics.com/products/dura-bright-125-90-dielectric-diagonal" click F73A "https://williamoptics.com/products/flat73A" click 2E "https://williamoptics.com/products/swan-40mm-2-eyepiece" click 1E "https://williamoptics.com/collections/eye-pieces" click T "https://williamoptics.com/collections/adapters"

Operation Guide

Flattener Back Focus Adjustment

  1. Find the recommended back focus adjustment distance for your flattener.
    1. Back Focus
      Adjust Distance
      Image Circle
      Flat 73A
      66.2 mm
      11.4 mm
      ∅ 43 mm
      Flat 73R
      56.8 mm
      1.8 mm
      ∅ 43 mm
  1. No matter how complicated the structure of the front part is, please find the back adapter connected to the camera.
    1.  invert
  1. If your setup requires additional back extender, please remove the original part and install the extender.
    1.  invert
  1. Adjust the back adapter to the required distance.
    1.  invert
  1. Tighten the ring against the back adapter.
    1.  invert
  1. Fine Tuning Take a few test shots. According to the aberration pattern of the surrounding star points of your image, you can determine whether to extend or shorten the Flattener back focus.
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▲ Camera Sensor Too Close
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▲ Camera Sensor Too Far Away

Focuser Tension Adjustment

William Optics focusers are typically smooth and ready to use without any adjustment. For larger telescopes, tightening the Quick Tension Adjustment Knob can help prevent tube slippage. Other than this, we recommend not touching any screws on the focuser. If adjustment is necessary, please consult a qualified technician.
 
There’s two side to build R&P focuser tension:

BOTTOM / Rack & Pinion Gear

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  • Quick tension adjustment knob
  • Smoothness adjustment - Pull in (3mm Hex)
  • Smoothness adjustment - Push out (2mm Hex)
  • Top cover screw Can be removed to attach the electronic devices such as EAF (refer to Installing EAF)
  • Please Don’t Touch

TOP / Focuser Draw Tube

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  • Focuser Draw Tube Tension Screw - Please Don’t Touch
  • Guider/Finder bracket mounting screws (2.5mm Hex)
 
Notice
  • When operating the telescope with a motorized focuser, do not overtighten the focuser tension. Excessive tension may damage the focuser mechanism.
  • Only adjust focuser smoothness if necessary. Incorrect adjustments may affect focusing accuracy or cause mechanical issues.
  • Make small and slow adjustments. Do not turn anything too much at once.

Installing EAF

  1. Turn the telescope over and remove the Tension screw.
 
  1. Remove the top cover screw near the temperature gauge with 2mm Allen key.
Starting from June 2025, new telescope models no longer include the top cover screw. (Skip Step 2)
 
  1. Release (don’t remove) two inner knob screws.
  1. Remove the temperature gauge knob.
  1. Find the flat side on the shaft and turn it to face the hole.
  1. Put the 5-6mm Flexible coupling on the shaft and lock the two inner screws.
  1. Put the EAF body on and lock the two inner screws.
On Step 6 & 7:
Please make sure one of the screws is lock on the flat side (D-cut profile)
 invert
  1. Align the screw holes.
  1. Lock the four M4 bracket screws.

Care & Safety Instructions

Solar Safety Warning

Never point your telescope or finder scope at the Sun without a proper solar filter securely mounted to the front of the telescope.
Even a brief, unfiltered glimpse of the Sun can permanently damage your eyes or your camera’s image sensor.
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Use only front-mounted solar filters made by trusted manufacturers. These filters must cover the front of the telescope’s dew shield securely.
Do not use solar eyepiece filters. They are unsafe and can fail under sunlight.
With the correct solar filter in place, you can safely observe and photograph the Sun without damaging your eyes, your camera, or your telescope.

Telescope Care & Storage

To preserve the optical quality and lifespan of your William Optics telescope, please follow these essential care and storage practices:
  • Allow time to acclimate After use, let the telescope adjust to room temperature before storage. During this period, keep the optical tube open and uncovered to allow moisture to evaporate naturally.
  • Ensure dryness Make sure the entire telescope is completely dry, especially the optics and interior tube, and free of dew or moisture before putting it away.
  • Use proper storage conditions Store your telescope in a dry, dust-free environment with a stable temperature. Avoid high heat or humidity. Dehumidifiers or silica gel are recommended for long-term protection.
  • Prevent dust accumulation Always place dust caps on both ends when not in use, or cover the telescope with a clean plastic sheet if it’s not stored in a case.
Following these steps will help maintain optimal performance and prevent damage to sensitive components such as lenses, mirrors, and coatings.
For a detailed cleaning and maintenance guide, please refer to the full article: Cleaning and Maintenance of Telescopes

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