William Optics ZenithStar 81 81/559 Apochromatic Refractor OTA (Red)
The ZenithStar 81 is an apochromatic doublet telescope with a 559 mm focal length and an f/6.9 focal ratio. The optical design uses FPL53 glass, and every lens is machined with CNC ultra-high precision. The result is outstanding colour correction - chromatic aberration is reduced to a minimum, and stars remain tight points of light right to the edge of the field of view. Add an optional 0.8x reducer and the focal ratio drops to f/5.5, opening the door to fast wide-field astrophotography. The ZenithStar 81 delivers top-level APO colour correction in a compact, travel-ready body.
Robust 2.5″ rack-and-pinion focuser
The 2.5″ rack-and-pinion focuser is one of the most rigid in its class. Dual-speed focusing with a 1:10 reduction ratio lets you achieve precise focus without the slightest image shift - an essential feature for astrophotography. The camera-side M63 thread is compatible with most popular adapters and camera systems. The 64 mm clear aperture of the focuser ensures that the light cone is not clipped at the edges, even with full-frame sensors.
Compact design, serious performance
An 81 mm aperture puts the ZS81 firmly among capable instruments for deep-sky objects and planets. Yet the tube measures just 456 mm when retracted, and the total weight stays under 3.5 kg. Many airlines accept this telescope as carry-on luggage - a genuine advantage for the travelling observer. The tube comes with a removable dew shield, a carry handle, tube rings, and a dual Arca-Swiss and Vixen dovetail bar, so you can mount it on almost any mount without additional accessories. The William Optics ZenithStar 81 apochromatic refractor combines travel dimensions with performance that will satisfy even an experienced observer.
What can you observe?
Solar observation is only safe with a dedicated solar filter - never point the telescope at the Sun without one, as permanent eye damage will result. For everything else, the ZenithStar 81 excels. The telescope reaches a limiting magnitude of 11.3, delivers a maximum useful magnification of 162x, and resolves detail down to 1.70 arc seconds. That is enough to separate close double stars and reveal fine detail on planetary discs. The Moon, planets, nebulae, and galaxies all make rewarding targets.
Bahtinov mask included
The ZenithStar 81 comes with an innovative Bahtinov mask that makes precise focusing dramatically easier. Instead of searching slowly for the best focuser position, simply watch the diffraction pattern the mask creates. When the central spike is perfectly centred between the two outer spikes, the star is in sharp focus. It is a practical accessory that anyone who photographs star clusters or uses bright stars as reference objects will appreciate immediately. Its inclusion shows that the William Optics designers had real-world observers in mind - both at the eyepiece and behind the camera.
Key specifications
| Parameter |
Value |
| Optical design |
Apochromatic doublet (FPL53) |
| Aperture |
81 mm |
| Focal length |
559 mm |
| Focal ratio |
f/6.9 |
| Focal ratio with 0.8x reducer |
f/5.5 |
| Focuser type |
Rack-and-pinion, dual-speed 1:10 |
| Focuser diameter |
2.5″ |
| Focuser clear aperture |
64 mm |
| Camera thread |
M63 |
| Tube length (retracted) |
456 mm |
| Weight |
3.5 kg |
| Limiting magnitude |
11.3 |
| Maximum useful magnification |
162x |
| Resolving power |
1.70″ |
| Dovetail |
Dual Arca-Swiss and Vixen |
| Colour |
Red |
What is included
- William Optics ZenithStar 81 OTA (Red)
- Removable dew shield
- Carry handle
- Tube rings
- Dual Arca-Swiss and Vixen dovetail bar
- Bahtinov mask
Specifications
Type of telescope:
ED & Apochromats
Optical scheme:
Apochromat Doublet
Focal length:
559 mm
Aperture:
81 mm (3.2″)
Aperture ratio:
6.9
Resolving capacity:
1.70"
Limit value:
11.3 magnitude
Light gathering capacity:
133x
Product range:
ZenithStar
Max. useful magnification:
162x
Connection type:
2″
Mounting:
No mount
Recommended for:
Advanced
Application area:
Deep Sky observation, Planets observation
Package contents of "Apochromatic Refractor William Optics 81/559 ZenithStar 81 Red OTA":
✅ Carrying handle: yes
✅ Miscellaneous: Bahtinov Mask
✅ Tube clamps: yes
✅ Prism rail: Arca-Swiss & Vixen-Style
✅ Dew Shield: yes
Typical magnifications
The recommended magnification is highlighted in yellow.
D = telescope aperture in millimeters
