Telescope Comparison
William Optics GT81 vs William Optics Zenithstar 61
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 · 61mm · £499
The custom-rig optical tube
- 61mm refractor — optical tube only, no mount included
- 360mm 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
The full picture
The numbers that separate these two scopes — and what they mean at the eyepiece.
Aperture
William Optics GT81 gathers 1.8× 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 61'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)
William Optics Zenithstar 61's optical tube is 1.1kg lighter. Relevant if you plan to use it on multiple mounts or carry the tube to dark-sky sites separately.
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 61 |
|---|---|---|
| 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 61mm aperture shows craters and maria, but the short 360mm focal length limits useful magnification for fine detail |
| Saturn | Moderate Rings clearly visible and colour-free, but 81mm aperture and 478mm focal length make the Cassini Division very difficult | Challenging Rings visible as distinct structure, but 61mm aperture and 360mm focal length cannot reveal Cassini Division or banding |
| Jupiter | Moderate Main equatorial belts visible in steady seeing; 81mm resolves limited banding detail and the Great Red Spot is marginal | Challenging Disc and two main equatorial belts visible, but small aperture limits cloud detail and the short focal length keeps the image very small |
| Mars | Challenging Small orange disc visible at opposition; 81mm aperture insufficient to resolve surface features reliably | Not recommended Tiny orange disc at opposition; 61mm aperture and 360mm focal length cannot resolve surface features |
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 Wide field frames the full nebula and surrounding region; 61mm shows the bright core and inner nebulosity but lacks aperture for fainter outer structure visually |
| 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 360mm focal length captures the full extent of M31 and companion galaxies in a single field — ideal framing for imaging |
| Open clusters | Excellent Wide-field sweet spot — Pleiades, Double Cluster, and M35 are beautifully framed with colour-free stars | Excellent 360mm focal length gives wide true field, perfectly suited for large clusters like the Double Cluster, Pleiades, and Hyades |
| Globular clusters | Challenging 81mm aperture shows globulars like M13 as fuzzy balls with no individual star resolution | Challenging 61mm aperture shows fuzzy patches only; no star resolution possible even at the edges |
| Faint galaxies | Moderate Core of brighter galaxies like M81/M82 visible under dark skies, but 81mm gathers limited light for faint targets | Not recommended 61mm aperture gathers too little light to reveal faint galaxy detail 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 360mm focal length at f/5.9 delivers sweeping star fields — one of this scope's strengths both visually and for imaging |
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 Dawes limit of ~1.9 arcseconds; wide pairs split cleanly but close doubles are beyond reach, and short focal length makes high-power splitting impractical |
| 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; on a suitable equatorial mount this would rate Excellent — f/5.9, APO glass, and 360mm focal length are ideal for wide-field imaging |
| Astrophotography (planetary) | Challenging 81mm aperture and 478mm focal length produce a small planetary image scale; limited even with a Barlow | Challenging 61mm aperture and 360mm focal length produce a very small planetary image scale; no tracking included |
| 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 | Excellent With a tracking mount, the wide f/5.9 field frames targets like the North America Nebula, Veil Nebula, and Heart/Soul complex superbly |
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 sessions framing entire nebulae and star clusters in a single eyepiece view, rewarding patient wide-field sweeping over the Milky Way.
- Your imaging camera will capture expansive targets like M31 and the Veil Nebula at native focal length without cropping, but you'll need to invest in a flattener to eliminate edge curvature.
- You'll justify the larger aperture's modest planetary performance by recognising the GT81 prioritises light-gathering for faint nebulae over high-magnification lunar detail.
William Optics Zenithstar 61
- You'll pack this scope into a travel bag knowing it's genuinely portable, but accept that your camera will frame only the largest emission nebulae before needing multiple panels.
- Your first deep-sky imaging session will reveal how much the smaller aperture limits faint-galaxy work, pushing you toward bright-nebula and Milky Way photography instead.
- You'll find yourself comparing the Zenithstar 61's cost-per-aperture unfavourably to visual Dobsonians, but appreciate the optical quality and compactness if astrophotography is your only goal.
The dark side
Every scope has a personality. Here’s where each one gets difficult.
William Optics
William Optics GT81
Sold as OTA only with no mount, diagonal, or eyepieces — the £699 price tag omits the £300–800+ needed for a basic equatorial mount and accessories.
81mm aperture cannot resolve planetary detail (Cassini Division on Saturn is difficult; Mars shows no surface features except near opposition) despite the fast focal ratio.
Short 478mm focal length forces you to use very short eyepieces or a Barlow for magnification, reducing eye relief and comfort during high-power viewing.
Field curvature at the edges of the native focal plane requires a dedicated flattener for serious astrophotography, adding cost and complexity.
Some production runs lack a built-in focuser lock, risking focuser slip when mounting heavy imaging trains.
William Optics
William Optics Zenithstar 61
OTA only with no mount, diagonal, or eyepiece — the £499 price omits £300–800+ for a basic equatorial mount and essential accessories.
61mm aperture is severely limiting for visual observation, making it unsuitable as a primary visual telescope; a 6-inch Dobsonian costs similar money and shows vastly more.
Short 360mm focal length restricts maximum useful magnification to roughly 120×, eliminating meaningful planetary and lunar detail work.
Field curvature across APS-C and full-frame sensors requires a dedicated Flat6A field flattener/reducer, adding approximately £250 to achieve flat-field imaging.
Small aperture severely limits deep-sky visual performance on faint galaxies and globular clusters, making it unsuitable for non-imaging observation.
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 committed to wide-field astrophotography of large nebulae and already own (or plan to buy) an equatorial mount — the 81mm aperture and f/5.9 speed give you genuine light-gathering advantage for bright extended objects. This scope rewards patient imaging of targets spanning a degree or more, from the Veil Nebula to M31. You're not for the GT81 if you expect a ready-to-use package, want to chase fine planetary detail, or plan to observe faint galaxies visually — 81mm simply won't deliver.
The custom-rig optical tube
William Optics · William Optics Zenithstar 61
You'll love the Zenithstar 61 if you prioritise portability and lightweight astrophotography, accepting that you'll photograph only the brightest and largest emission nebulae, Milky Way regions, and wide naked-eye clusters. This scope rewards travellers and apartment dwellers who can't justify a bulky 6-inch refractor but need genuine optical quality. You're not for the Zenithstar 61 if you want visual performance, expect a complete ready-to-observe package, or need aperture to pull detail from faint galaxies and deep-sky objects — at 61mm, you'll be frustrated by what you cannot see.
Our verdict
At £499 versus £699, the William Optics GT81 costs 40% more. It delivers 20mm more aperture — a real and visible advantage on faint targets.
If budget is a genuine constraint, the William Optics Zenithstar 61 will make you a happy observer. The William Optics GT81's optical advantage on faint targets is real and you are unlikely to regret it if you can stretch. If I had to choose without knowing your situation: start with the William Optics Zenithstar 61, use it for a year, then upgrade knowing exactly what you want.
William Optics GT81
View William Optics GT81 →William Optics Zenithstar 61
View William Optics Zenithstar 61 →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 61 |
|---|---|---|
Apertureⓘ The most important spec — bigger = more light = better views | 81mm | 61mm |
Focal Length Longer = more magnification potential | 478mm | 360mm |
Focal Ratio Lower f-number = wider field of view; higher = more magnification per eyepiece | f/5.9 | f/5.9 |
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 FMC ED triplet on all air-to-glass surfaces | Fully multi-coated FMC on all air-to-glass surfaces, including ED element |
How do you point it?
| Spec | William Optics GT81 | William Optics Zenithstar 61 |
|---|---|---|
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 61 |
|---|---|---|
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 61 |
|---|---|---|
OTA Weightⓘ Optical tube only — useful for comparing mount load capacity | 2.5kg | 1.35kg |
Tube Length | 380mm | 270mm |
Tube Material | Aluminium, anodised | Aluminium, anodised red |
What's in the box?
| Spec | William Optics GT81 | William Optics Zenithstar 61 |
|---|---|---|
Diagonal Tilts the eyepiece 90° for comfortable viewing — useful on refractors |
Blue highlight: William Optics GT81 advantage · Amber highlight: William Optics Zenithstar 61 advantage · Greyed cells: equal or subjective.