Telescope Comparison
Vixen SD81S vs William Optics Zenithstar 81
The price gap is real. The question is whether the extra capability is worth it at your stage.
First light
Vixen · 81mm · £1,199
The custom-rig optical tube
- 81mm refractor — optical tube only, no mount included
- 625mm focal length at f/7.72
- Requires a compatible mount before you can observe anything
- Best for: observers who already own a suitable mount or are building a specific imaging rig
- Not a complete purchase — budget at least £100–300 extra for a mount before observing
William Optics · 81mm · £699
The custom-rig optical tube
- 81mm refractor — optical tube only, no mount included
- 559mm focal length at f/6.9
- Requires a compatible mount before you can observe anything
- Best for: observers who already own a suitable mount or are building a specific imaging rig
- Not a complete purchase — budget at least £100–300 extra for a mount before observing
The full picture
The numbers that separate these two scopes — and what they mean at the eyepiece.
Aperture
Equal light-gathering. Aperture won't settle this comparison — the mount, focal ratio, and observing experience are what differ.
Focal length
Vixen SD81S's longer focal length reaches higher magnification with the same eyepiece — better reach for planetary detail. William Optics Zenithstar 81's shorter focal length gives a wider true field — better for large open clusters and extended nebulae.
Focal ratio
William Optics Zenithstar 81's faster f/6.9 delivers wider fields with any eyepiece — better for open clusters and large nebulae. Vixen SD81S's f/7.72 provides more magnification per eyepiece — better for fine planetary detail.
Mount type
Neither scope includes a mount — both require a separate purchase before you can observe.
Weight (OTA)
Similar optical tube weight. Any portability difference between these setups comes from the mount, not the tube itself.
Optical design
Both are refractors — no mirrors to collimate, good contrast, colour-free stars with ED or APO glass. The differences between them are in aperture, focal ratio, and glass quality.
At the eyepiece
Both scopes · same aperture
Both refractors share essentially the same aperture — views through each will be very similar on all standard targets. The hallmarks of good refractor optics are sharp stars and good contrast on planetary targets, with no false colour on ED or apochromatic glass. Saturn's rings are distinct from the disk; Jupiter shows two equatorial bands. The Orion Nebula (M42) is bright and well-defined. Open clusters are a strength — the Double Cluster in Perseus and the Pleiades look good at low power. The differences between these two scopes show up in focal ratio, focal length, and what they're optimised for, not in fundamental light-gathering capability.
The real tradeoff
Both scopes are capable. The question is which one fits the way you actually observe.
The Vixen SD81S costs 72% more. The premium buys a more capable mount and better build quality, not larger optics. For a first telescope, the William Optics Zenithstar 81 is the smarter entry point. Return to the Vixen SD81S when you know from experience what you actually need.
The dark side
Every scope has a personality. Here’s where each one gets difficult.
Vixen
Vixen SD81S
No mount included
You cannot observe until you buy a separate compatible mount — add at least £100–300 before you have a working telescope.
Nothing to look through on day one
Until a mount arrives, the optical tube is a piece of glass you cannot point at the sky.
William Optics
William Optics Zenithstar 81
No mount included
You cannot observe until you buy a separate compatible mount — add at least £100–300 before you have a working telescope.
Nothing to look through on day one
Until a mount arrives, the optical tube is a piece of glass you cannot point at the sky.
Which is right for you?
Two different buyers. Two different right answers.
The custom-rig optical tube
Vixen · Vixen SD81S
You’ll love this if…
- You already own a compatible equatorial or alt-az mount — this is the optical tube you've specifically chosen to put on it
- You're building an imaging rig piece by piece and know exactly what you need at the end of a focuser
- Choosing an optical tube independently of the mount gives you more flexibility over your overall system
This will frustrate you if…
- You buy it without fully accounting for the mount — add at least £100–300 to the purchase price before you have a working telescope
- You expected a complete package and didn't realise this is a bare optical tube that cannot be used without a separate mount
The custom-rig optical tube
William Optics · William Optics Zenithstar 81
You’ll love this if…
- You already own a compatible equatorial or alt-az mount — this is the optical tube you've specifically chosen to put on it
- You're building an imaging rig piece by piece and know exactly what you need at the end of a focuser
- Choosing an optical tube independently of the mount gives you more flexibility over your overall system
This will frustrate you if…
- You buy it without fully accounting for the mount — add at least £100–300 to the purchase price before you have a working telescope
- You expected a complete package and didn't realise this is a bare optical tube that cannot be used without a separate mount
Our verdict
At £699 versus £1,199, the Vixen SD81S costs 72% more. The extra money buys a more capable mount and better build quality, not larger optics.
For most buyers starting out, the William Optics Zenithstar 81 is the sensible choice — put the savings into a better eyepiece. The Vixen SD81S makes sense once you know exactly why you need what it offers. If I had to choose: the William Optics Zenithstar 81, and spend the difference on a quality eyepiece.
Vixen SD81S
View Vixen SD81S →William Optics Zenithstar 81
View William Optics Zenithstar 81 →Deep field: Full specifications
Every data point, for those who want to go further.
Full specifications
Fields highlighted in blue or amber indicate the better value for that spec. Data is manufacturer-stated and may vary.
How much can it see?
| Spec | Vixen SD81S | William Optics Zenithstar 81 |
|---|---|---|
Aperture The most important spec — bigger = more light = better views | 81mm | 81mm |
Focal Length Longer = more magnification potential | 625mm | 559mm |
Focal Ratio Lower f-number = wider field of view; higher = more magnification per eyepiece | f/7.72 | f/6.9 |
Optical Design The type of optics — each design has different strengths | Refractor | Refractor |
Coatings Better coatings = more light transmission through the optics | Fully multi-coated SD (Super Duplex) glass doublet | Fully multi-coated apochromatic triplet (FPL-53) |
How do you point it?
| Spec | Vixen SD81S | William Optics Zenithstar 81 |
|---|---|---|
Mount Type The mechanical system that holds and moves the telescope | None (OTA only) | None (OTA only) |
GoTo Computer-controlled pointing — finds any of thousands of objects automatically | ||
Tracking Motor keeps objects centred as the Earth rotates — essential for astrophotography |
The focuser
| Spec | Vixen SD81S | William Optics Zenithstar 81 |
|---|---|---|
Focuser Size 2" accepts wider eyepieces and gives better low-power views | 2" | 2" |
Focuser Type Rack-and-pinion is standard; Crayford and dual-speed are smoother | Dual-speed Crayford (with 1.25" adapter) | Dual-speed Crayford (10:1) |
Size & weight
| Spec | Vixen SD81S | William Optics Zenithstar 81 |
|---|---|---|
OTA Weightⓘ Optical tube only — useful for comparing mount load capacity | 2kg | 1.86kg |
Tube Length | 540mm | 390mm |
Tube Material | Aluminium | Aluminium |
What's in the box?
| Spec | Vixen SD81S | William Optics Zenithstar 81 |
|---|---|---|
Diagonal Tilts the eyepiece 90° for comfortable viewing — useful on refractors |
Blue highlight: Vixen SD81S advantage · Amber highlight: William Optics Zenithstar 81 advantage · Greyed cells: equal or subjective.