Telescope Comparison
William Optics GT81 vs William Optics Zenithstar 73
The specs are close. The experience isn't.
First light
William Optics · 81mm · £699
The custom-rig optical tube
- 81mm refractor — optical tube only, no mount included
- 478mm focal length at f/5.9
- 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
William Optics · 73mm · £599
The custom-rig optical tube
- 73mm refractor — optical tube only, no mount included
- 430mm focal length at f/5.89
- 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
William Optics GT81 gathers 1.2× more light. On bright targets — Moon, Saturn, Jupiter — you won't notice. On fainter targets — dim galaxies, faint globular clusters — the gap is real.
Focal length
William Optics GT81's longer focal length reaches higher magnification with the same eyepiece — better reach for planetary detail. William Optics Zenithstar 73's shorter focal length gives a wider true field — better for large open clusters and extended nebulae.
Focal ratio
Same focal ratio — the same eyepiece gives equivalent magnification and true field in both scopes.
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
| Target | William Optics GT81 | William Optics Zenithstar 73 |
|---|---|---|
| Planets | ||
| Moon | Excellent 81mm aperture delivers sharp, high-contrast lunar detail; the triplet design keeps the terminator free of colour fringing, though the short focal length limits magnification without a Barlow | Moderate 73mm aperture shows good crater and terminator detail, but the short 430mm focal length limits useful magnification before the image softens. |
| Saturn | Moderate Rings clearly visible and colour-free, but 81mm aperture and 478mm focal length make the Cassini Division very difficult | Challenging Rings visible and Titan identifiable, but 73mm aperture and 430mm focal length can't reveal the Cassini Division or subtle banding. |
| Jupiter | Moderate Main equatorial belts visible in steady seeing; 81mm resolves limited banding detail and the Great Red Spot is marginal | Moderate Main equatorial belts visible; 73mm falls between the Good and Moderate tiers, and the short focal length makes it hard to push magnification for finer detail. |
| Mars | Challenging Small orange disc visible at opposition; 81mm aperture insufficient to resolve surface features reliably | Challenging Small disc visible near opposition with possible hint of polar cap, but 73mm aperture and short focal length offer very limited surface detail. |
Deep sky | ||
| Orion Nebula (M42) | Excellent Bright nebula easily visible; 478mm focal length at f/5.9 frames the full extent with surrounding nebulosity | Good Core nebulosity and Trapezium visible; the wide field at 430mm frames the full nebula complex nicely, but aperture is just under the 80mm Excellent threshold. |
| Andromeda Galaxy (M31) | Excellent 478mm focal length captures the core and dust lanes in a single wide field; aperture shows the inner halo structure | Excellent 430mm focal length frames the full galaxy with room to spare; visually the core and inner dust lanes are visible from dark skies. |
| Open clusters | Excellent Wide-field sweet spot — Pleiades, Double Cluster, and M35 are beautifully framed with colour-free stars | Excellent Wide true field at 430mm is ideal for the Double Cluster, Pleiades, and other large clusters — they sit beautifully in the field of view. |
| Globular clusters | Challenging 81mm aperture shows globulars like M13 as fuzzy balls with no individual star resolution | Challenging 73mm aperture shows M13 and M22 as fuzzy unresolved glows — no star resolution possible at this aperture. |
| Faint galaxies | Moderate Core of brighter galaxies like M81/M82 visible under dark skies, but 81mm gathers limited light for faint targets | Challenging 73mm gathers limited light; only the brightest galaxies like M81/M82 show as faint smudges visually. |
| Milky Way / wide field | Excellent 478mm at f/5.9 is ideal for sweeping rich star fields; low-power eyepieces deliver expansive true fields | Excellent 430mm focal length at f/5.9 delivers sweeping rich star fields — among the best use cases for this scope visually and with a camera. |
Other | ||
| Double stars | Good Clean optics split wider doubles cleanly with no false colour, but 81mm limits resolution on close pairs below about 1.4 arcseconds | Moderate 73mm resolves wide doubles like Albireo easily, but the short focal length and modest aperture limit splitting of closer pairs. |
| Astrophotography (deep sky) | Not recommended No mount or tracking included; however, when paired with a suitable equatorial mount this becomes an excellent deep-sky imaging platform at f/5.9 | Not recommended No mount or tracking included; the OTA is excellent for deep sky imaging but only when paired with an equatorial tracking mount purchased separately. |
| Astrophotography (planetary) | Challenging 81mm aperture and 478mm focal length produce a small planetary image scale; limited even with a Barlow | Challenging 73mm aperture and 430mm focal length produce a very small planetary image scale — a Barlow helps but aperture is the fundamental limit. |
| Large emission nebulae (imaging) | Excellent Fast f/5.9 triplet with flat, colour-free field excels on targets like the Veil, North America Nebula, and Heart Nebula when paired with a narrowband or one-shot colour camera on a tracking mount | Not applicable |
The real tradeoff
Both scopes are capable. The question is which one fits the way you actually observe.
William Optics GT81
- You'll spend your observing nights with a wider true field — M31's dust lanes and the full Orion Nebula fit in a single eyepiece view, rewarding patient star-field sweeping and casual widefield visual hunting.
- You gain 8mm of aperture over the 73, which translates to noticeably brighter views of open clusters and large nebulae, and gives you more light for visual work on moderately faint targets.
- Your imaging setup will demand more precision — the native field curvature requires a dedicated flattener, and shorter focal length eyepieces or a Barlow become necessary if you want to push magnification for lunar or planetary detail.
William Optics Zenithstar 73
- You'll build a more compact, lighter travel rig — 73mm and 430mm focal length pack down smaller than the GT81, making it genuinely portable for dark-sky trips or remote observing sites.
- Your imaging workflow favours speed — the marginally faster focal ratio and smaller aperture forgive minor tracking errors and produce usable results even from suburban skies when paired with narrowband filters.
- You'll accept visual limitations early: this scope is purpose-built for cameras, and visual deep-sky work beyond bright nebulae and clusters runs out of light quickly, steering you toward the eyepiece or back to the camera.
The dark side
Every scope has a personality. Here’s where each one gets difficult.
William Optics
William Optics GT81
Sold as OTA only — mount, diagonal, and eyepieces are not included, so the true system cost exceeds the quoted £699 price significantly.
Field curvature at the field edges of the native focal plane requires a separate dedicated flattener for serious astrophotography work.
Short 478mm focal length demands very short focal-length eyepieces or a Barlow for high magnifications, and both options compromise eye relief.
No built-in focuser lock on some production runs — checking for this feature is essential before using heavy imaging trains to prevent focuser slip.
William Optics
William Optics Zenithstar 73
Sold as OTA only — no mount, eyepiece, diagonal, or finder included, making the total cost of a working setup significantly higher than the bare OTA price of £599.
The dedicated Flat73A field flattener is essentially mandatory for imaging; without it, stars towards the field edges show noticeable coma and curvature.
As an ED doublet rather than a triplet, some residual chromatic aberration may be visible on very bright stars in images, though it remains well-controlled for the price point.
The 2-inch focuser rack-and-pinion can exhibit minor flexure under heavier camera and filter train loads without careful balancing.
Which is right for you?
Two different buyers. Two different right answers.
The custom-rig optical tube
William Optics · William Optics GT81
You'll love the GT81 if you're a visual observer who wants to sweep the Milky Way and frame entire nebulae in a single field, or if you're an astrophotographer with an equatorial mount ready to image large emission nebulae and galaxies at native speed without mandatory additional optics. This scope rewards patient widefield discovery and handles the Andromeda Galaxy and Veil Nebula beautifully. It's not for you if you're chasing planetary detail, expecting a ready-to-use package, or hunting faint galaxies — 81mm aperture simply won't pull visual detail from dim deep-sky targets.
The custom-rig optical tube
William Optics · William Optics Zenithstar 73
You'll love the Zenithstar 73 if you travel to dark-sky sites and want a genuinely compact imaging platform that produces sharp results on large nebulae and star fields despite its tiny footprint, or if you're willing to accept visual limitations in exchange for a lighter, more portable rig. This scope rewards camera-based observing from light-polluted locations and excels with narrowband filters. It's not for you if you want serious visual planetary work, expect to resolve globular clusters by eye, or need a grab-and-go visual scope — it's a dedicated imaging instrument masquerading as a general-purpose refractor.
Our verdict
These two are closer than most comparisons on this site. The spec differences are genuine — mount type, focal ratio — but neither is the wrong answer for a typical observer starting out.
If I had to choose between them: the William Optics GT81 is the scope most people will be using regularly six months from now. The William Optics Zenithstar 73 rewards you more once you know what you're doing — it's worth revisiting after your first year.
William Optics GT81
View William Optics GT81 →William Optics Zenithstar 73
View William Optics Zenithstar 73 →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 | William Optics GT81 | William Optics Zenithstar 73 |
|---|---|---|
Apertureⓘ The most important spec — bigger = more light = better views | 81mm | 73mm |
Focal Length Longer = more magnification potential | 478mm | 430mm |
Focal Ratio Lower f-number = wider field of view; higher = more magnification per eyepiece | f/5.9 | f/5.89 |
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 FMC ED triplet on all air-to-glass surfaces | Fully multi-coated FMC ED doublet on all air-to-glass surfaces |
How do you point it?
| Spec | William Optics GT81 | William Optics Zenithstar 73 |
|---|---|---|
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 | William Optics GT81 | William Optics Zenithstar 73 |
|---|---|---|
Focuser Size 2" accepts wider eyepieces and gives better low-power views | 2" / 1.25" | 2" / 1.25" |
Focuser Type Rack-and-pinion is standard; Crayford and dual-speed are smoother | Dual-speed Crayford 2" (10:1 reduction fine focus) | Dual-speed Crayford 2" (10:1 reduction fine focus) |
Size & weight
| Spec | William Optics GT81 | William Optics Zenithstar 73 |
|---|---|---|
OTA Weightⓘ Optical tube only — useful for comparing mount load capacity | 2.5kg | 1.75kg |
Tube Length | 380mm | 320mm |
Tube Material | Aluminium, anodised | Aluminium, anodised blue |
What's in the box?
| Spec | William Optics GT81 | William Optics Zenithstar 73 |
|---|---|---|
Diagonal Tilts the eyepiece 90° for comfortable viewing — useful on refractors |
Blue highlight: William Optics GT81 advantage · Amber highlight: William Optics Zenithstar 73 advantage · Greyed cells: equal or subjective.