Telescope Comparison
William Optics FluoroStar 91 vs William Optics GT81
The price gap is real. The question is whether the extra capability is worth it at your stage.
First light
William Optics · 91mm · £1,299
The custom-rig optical tube
- 91mm refractor — optical tube only, no mount included
- 537mm 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 · 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
The full picture
The numbers that separate these two scopes — and what they mean at the eyepiece.
Aperture
William Optics FluoroStar 91 gathers 1.3× 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 FluoroStar 91's longer focal length reaches higher magnification with the same eyepiece — better reach for planetary detail. William Optics GT81'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 FluoroStar 91 | William Optics GT81 |
|---|---|---|
| Planets | ||
| Moon | Excellent 91mm aperture and fluorite correction deliver sharp, high-contrast lunar detail with no false colour on the limb | 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 |
| Saturn | Good Ring structure and Cassini Division visible in good seeing, though short focal length requires high-power eyepieces to push magnification | Moderate Rings clearly visible and colour-free, but 81mm aperture and 478mm focal length make the Cassini Division very difficult |
| Jupiter | Good Main cloud belts and GRS visible; 91mm resolves some detail but the 537mm focal length limits comfortable high-power use | Moderate Main equatorial belts visible in steady seeing; 81mm resolves limited banding detail and the Great Red Spot is marginal |
| Mars | Challenging Disc visible at opposition with hints of albedo features, but 91mm aperture and short focal length make surface detail very difficult | Challenging Small orange disc visible at opposition; 81mm aperture insufficient to resolve surface features reliably |
Deep sky | ||
| Orion Nebula (M42) | Excellent 91mm aperture and 537mm focal length at f/5.9 frame the full nebula complex with bright, detailed nebulosity and resolved Trapezium | Excellent Bright nebula easily visible; 478mm focal length at f/5.9 frames the full extent with surrounding nebulosity |
| Andromeda Galaxy (M31) | Excellent 537mm focal length captures the full extent of M31 including companion galaxies; 91mm aperture shows hints of outer halo structure | Excellent 478mm focal length captures the core and dust lanes in a single wide field; aperture shows the inner halo structure |
| Open clusters | Excellent Wide field at 537mm beautifully frames large clusters like the Double Cluster and Pleiades with tight, colour-free stars | Excellent Wide-field sweet spot — Pleiades, Double Cluster, and M35 are beautifully framed with colour-free stars |
| Globular clusters | Moderate 91mm shows globulars as granular, concentrated balls — M13 has a bright core but individual stars remain unresolved | Challenging 81mm aperture shows globulars like M13 as fuzzy balls with no individual star resolution |
| Faint galaxies | Challenging 91mm gathers limited light for faint galaxies visually; brighter Messier galaxies visible as faint smudges, but detail is minimal | Moderate Core of brighter galaxies like M81/M82 visible under dark skies, but 81mm gathers limited light for faint targets |
| Milky Way / wide field | Excellent 537mm at f/5.9 is ideal for rich Milky Way sweeps — star fields through Cygnus and Sagittarius are stunning | Excellent 478mm at f/5.9 is ideal for sweeping rich star fields; low-power eyepieces deliver expansive true fields |
Other | ||
| Double stars | Good 91mm resolves wide and moderate doubles cleanly with excellent colour correction, though close pairs need very short eyepieces at this focal length | Good Clean optics split wider doubles cleanly with no false colour, but 81mm limits resolution on close pairs below about 1.4 arcseconds |
| Astrophotography (deep sky) | Not recommended No mount or tracking included — optically superb for deep-sky imaging but requires a separate equatorial mount to realise that potential | 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 |
| Astrophotography (planetary) | Moderate 91mm and 537mm focal length are limited for planetary imaging; usable with a 2–3× Barlow on a tracking mount, but aperture constrains resolution | Challenging 81mm aperture and 478mm focal length produce a small planetary image scale; limited even with a Barlow |
| Emission nebulae (imaging) | Excellent Fast f/5.9 fluorite triplet excels at narrowband and broadband emission nebula imaging — Heart, Soul, North America, and Veil nebulae are ideal targets with a matched flattener | Not applicable |
| Large emission nebulae (imaging) | Not applicable | 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 |
The real tradeoff
Both scopes are capable. The question is which one fits the way you actually observe.
William Optics FluoroStar 91
- You're investing in natural fluorite glass specifically to squeeze the last fraction of chromatic aberration out of your stacked astrophotography data — the visual difference over ED glass is subtle, but your imaging files will show it after hours of integration.
- Your observing sessions centre on camera work: you'll spend setup time configuring a field flattener or reducer, checking sensor alignment, and framing wide targets like Andromeda or Cygnus on an equatorial mount, rewarded by edge-to-edge star flatness and colour-free pinpoint data.
- You're trading aperture (91mm) for optical purity and portability — you can pack this OTA and a mid-range mount into a travel rig, but you'll accept that Saturn's Cassini Division and Jupiter's detail require perfect seeing and quality short-focal-length eyepieces to resolve.
William Optics GT81
- You're buying a serious widefield imaging platform at half the price of the FluoroStar, accepting a 10mm aperture loss and slightly tighter field in exchange for £600 that goes directly toward a quality mount or camera.
- Your observing rhythm splits cleanly: visual nights are leisurely sweeps of the Milky Way and large nebulae framed whole in the eyepiece, while imaging nights pair this OTA with an equatorial mount to capture extended targets like the Veil or North America Nebula across a degree of sky.
- You're constrained by aperture (81mm) for faint galaxies and planetary detail, but you'll find the triplet's colour-free field and fast f/5.9 make widefield targets pop — M31, M42, and open clusters appear more vivid than they would at longer focal lengths, and you won't need a flattener for visual work.
The dark side
Every scope has a personality. Here’s where each one gets difficult.
William Optics
William Optics FluoroStar 91
Sold as OTA only with no mount, diagonal, finder, or eyepieces — total system cost exceeds the £1299 price tag by £500–£1500+ depending on your mount choice.
Requires a properly matched field flattener or reducer for imaging; without it, field curvature produces elongated stars at the edges of larger sensors.
91mm aperture limits high-power planetary and double-star performance compared to 130mm+ refractors or larger reflectors in the same total budget; the short 537mm focal length means Saturn's Cassini Division requires perfect seeing and a quality Barlow or very short eyepiece.
William Optics
William Optics GT81
Sold as OTA only with no mount, diagonal, or eyepieces included — total system cost significantly exceeds the £699 price tag.
Field curvature at the edges of the native focal plane requires a dedicated flattener for serious astrophotography; uncorrected imaging will show elongated stars beyond the central field.
No built-in focuser lock on some production runs — check your unit when using heavy imaging trains to prevent focuser slip; 81mm aperture limits planetary and double-star performance, and is too small to resolve meaningful detail on faint galaxies.
Which is right for you?
Two different buyers. Two different right answers.
The custom-rig optical tube
William Optics · William Optics FluoroStar 91
You'll love this if you're an experienced deep-sky imager who has already learned that sub-arc-second colour correction compounds across stacked data, and you're willing to invest the full system cost (OTA plus quality equatorial mount, field flattener, and camera) to chase pixel-perfect results on widefield targets like Andromeda and large nebulae. You accept that the visual premium of fluorite over ED glass is subtle, but you've calculated that the difference justifies the price over a multi-night imaging campaign. This isn't for you if you need a ready-to-observe package, want to chase planetary detail on a budget, or are a visual-only observer.
The custom-rig optical tube
William Optics · William Optics GT81
You'll love this if you want a serious widefield imaging scope without spending £2000+ on the full system — the GT81's £699 price leaves room in your budget for a proper equatorial mount and camera. You're equally happy sweeping the Milky Way visually or framing extended nebulae for imaging, and you don't need the extra 10mm of aperture or the fluorite glass to achieve your goals. This isn't for you if you're chasing planetary detail, hunting faint galaxies that demand aperture, or you need everything in one box.
Our verdict
At £699 versus £1,299, the William Optics FluoroStar 91 costs 86% more. It delivers 10mm more aperture — a real and visible advantage on faint targets.
If budget is a genuine constraint, the William Optics GT81 will make you a happy observer. The William Optics FluoroStar 91'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 GT81, use it for a year, then upgrade knowing exactly what you want.
William Optics FluoroStar 91
View William Optics FluoroStar 91 →William Optics GT81
View William Optics GT81 →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 FluoroStar 91 | William Optics GT81 |
|---|---|---|
Apertureⓘ The most important spec — bigger = more light = better views | 91mm | 81mm |
Focal Length Longer = more magnification potential | 537mm | 478mm |
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 fluorite triplet on all air-to-glass surfaces | Fully multi-coated FMC ED triplet on all air-to-glass surfaces |
How do you point it?
| Spec | William Optics FluoroStar 91 | William Optics GT81 |
|---|---|---|
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 FluoroStar 91 | William Optics GT81 |
|---|---|---|
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 FluoroStar 91 | William Optics GT81 |
|---|---|---|
OTA Weightⓘ Optical tube only — useful for comparing mount load capacity | 3.2kg | 2.5kg |
Tube Length | 430mm | 380mm |
Tube Material | Aluminium, anodised | Aluminium, anodised |
What's in the box?
| Spec | William Optics FluoroStar 91 | William Optics GT81 |
|---|---|---|
Diagonal Tilts the eyepiece 90° for comfortable viewing — useful on refractors |
Blue highlight: William Optics FluoroStar 91 advantage · Amber highlight: William Optics GT81 advantage · Greyed cells: equal or subjective.