William Optics
William Optics ZenithStar 81 APO Doublet
An 81mm FPL-53 doublet apo with William Optics' signature build, clean, colour-controlled stars and a wide flat field with the matched flattener, a beautiful mid-range imaging refractor.
85/ 100
Aperture
81mm
Focal Length
559mm
Focal Ratio
f/6.9
Mount Type
None (OTA only)
Optical Design
ED Doublet Refractor
Weight
3.6 kg
From$999
Lowest in 90 daysLowest ever tracked
Price tracked by PolarisObserved 99 timesTracking since Jul 2026
Where to buy
Verified 14h ago · prices tracked daily
01What it's good atWhat it's best at — and the trade-offs.
An FPL-53 ED doublet controls false colour for clean, largely colour-free stars.
01What it's good at
What it's best at — and the trade-offs.
An FPL-53 ED doublet controls false colour for clean, largely colour-free stars.
Best at
✓An FPL-53 ED doublet controls false colour for clean, largely colour-free stars.
✓81mm and 559mm suit a wide range of large nebulae and galaxy fields.
✓Gorgeous, functional build with a rotating focuser and included rings.
✓Light enough for small trackers, capable enough for serious deep-sky imaging.
Trade-offs
!Needs the matched Flat81 flattener for corner-sharp stars (sold separately).
!A doublet leaves a trace of colour on the very brightest stars versus a triplet.
!OTA only; a tracking mount and camera are extra.
!81mm is a wide-field aperture, not a light bucket for small faint targets.
02In plain EnglishWhat These Numbers Mean
81mm · f/6.9 · 559mm FL
02In plain English
What These Numbers Mean
81mm · f/6.9 · 559mm FL
Aperture
81 mm
Three and a bit inches of ED glass, about 23 percent more light than a 73 mm. Enough for pleasant lunar and planetary views at moderate power alongside its main job of deep-sky imaging.
Focal length
559 mm
A useful middle field. The Orion Nebula, M31's core and the larger galaxies gain real scale while many of the big nebulae still fit an APS-C sensor.
Focal ratio
f/6.9
Moderate speed. Exposures run longer than a fast astrograph needs, in exchange for gentler tolerances on focus, spacing and tilt. A reducer is the usual upgrade for faster work.
Optical design
ED doublet apochromat
Two elements with ED glass give tight, essentially colour-free stars at this ratio, and the doublet cools quickly and holds alignment indefinitely. Pair it with the matching flattener for round corners.
Weight
3.6 kg tube
About 8 lb, comfortably carried by a small equatorial once a camera and guide scope are attached. Still easy to transport in a single case.
Mount
OTA only
You supply the mount. At 559 mm autoguiding starts to pay off, so favour a mount with a guider port over a bare tracker.
03The Polaris ScoreHow it scores, and why.
85/100 · #5 of 8 imaging APO refractors
03The Polaris Score
How it scores, and why.
85/100 · #5 of 8 imaging APO refractors
85/ 100
William Optics ZenithStar 81 APO Doublet
#5 of 8 imaging APO refractors
Performance86
Ease of use81
Portability83
Upgrade potential80
Polaris weighs optical capability, how easy the scope is to live with, how portable it is, and how far it takes you before you outgrow it — not list price or brand.
04Who it's forAn honest fit check.
Imagers who want quality doublet optics and a lovely mechanical package.
04Who it's for
An honest fit check.
Imagers who want quality doublet optics and a lovely mechanical package.
Buy it if you
✓Imagers who want quality doublet optics and a lovely mechanical package.
✓Travellers pairing a light apo with a small tracker.
✓Wide-field shooters after large nebulae and star fields.
Look elsewhere if you
!Imagers chasing the last few percent of star colour, a triplet edges it.
!Small-target shooters who need long focal length.
!Deep-sky visual observers; it is an imaging tube first.
05Build your rigWhat to buy next — and where it leads.
7 pieces of gear, ordered by what changes the view most
05Build your rig
What to buy next — and where it leads.
7 pieces of gear, ordered by what changes the view most
Recommended upgrades
No fogging on long sessions★ Top pickPegasus Astro Multi-Channel Dew Controller
Stops dew from ending an imaging night.
$259.90View details →
Make nebulae popOptolong UHC Filter 2″
Lifts nebula contrast out of light-polluted skies.
Observe all night, no AA batteriesCelestron PowerTank Lithium
Runs the mount and heaters all night off one pack.
$99.95View details →
Growth path
Better tracking & light imagingCelestron Advanced VX (AVX) Equatorial Mount✓ Compatible
Commonly paired with this telescope
$1,099View details →
Portable astrophotographyZWO AM5N Harmonic Equatorial Mount✓ Compatible
Start capturing images of the Moon and planets
$2,299View details →
Permanent-setup astrophotographySky-Watcher EQ6-R Pro✓ Compatible
Commonly paired with this telescope
$2,250View details →
Start imagingZWO ASI2600MC Pro
Start capturing images of the Moon and planets
$1,499View details →
06The numbersFull specifications.
6 measured and stated values
06The numbers
Full specifications.
6 measured and stated values
Optics
Optical DesignED Doublet Refractor
Aperture81mm
Focal Length559mm
Focal Ratiof/6.9
Mount & drive
Mount TypeNone (OTA only)
Physical
Weight3.6 kg
07AlternativesFor a different goal, start here.
2 alternatives, each mapped to the goal it wins at
07Alternatives
For a different goal, start here.
2 alternatives, each mapped to the goal it wins at
