Telescope Comparison
Sky-Watcher Evostar 100ED vs William Optics GT81
The specs are close. The experience isn't.
First light
Sky-Watcher · 100mm · £449
The custom-rig optical tube
- 100mm refractor — optical tube only, no mount included
- 900mm focal length at f/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
Sky-Watcher Evostar 100ED gathers 1.5× 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
Sky-Watcher Evostar 100ED'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
William Optics GT81's faster f/5.9 delivers wider fields with any eyepiece — better for open clusters and large nebulae. Sky-Watcher Evostar 100ED's f/9 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
| Target | Sky-Watcher Evostar 100ED | William Optics GT81 |
|---|---|---|
| Planets | ||
| Moon | Excellent 100mm aperture and f/9 focal ratio reward high magnification with sharp, high-contrast lunar detail | 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 900mm focal length and clean ED optics show rings, Cassini Division in good seeing, and subtle disc banding | Moderate Rings clearly visible and colour-free, but 81mm aperture and 478mm focal length make the Cassini Division very difficult |
| Jupiter | Good 100mm resolves two or more cloud belts, GRS, and moon shadow transits; f/9 handles high power well | Moderate Main equatorial belts visible in steady seeing; 81mm resolves limited banding detail and the Great Red Spot is marginal |
| Mars | Moderate Disc visible with polar cap and large albedo features at opposition, but 100mm limits fine surface detail | Challenging Small orange disc visible at opposition; 81mm aperture insufficient to resolve surface features reliably |
Deep sky | ||
| Orion Nebula (M42) | Good Bright nebula core and trapezium well shown, but 900mm focal length crops the outer wings | Excellent Bright nebula easily visible; 478mm focal length at f/5.9 frames the full extent with surrounding nebulosity |
| Andromeda Galaxy (M31) | Good Bright core and inner halo visible; 900mm frames only the central region, missing the full extent | Excellent 478mm focal length captures the core and dust lanes in a single wide field; aperture shows the inner halo structure |
| Open clusters | Good Compact clusters like M35 frame well; larger groups like the Double Cluster fill the low-power field | Excellent Wide-field sweet spot — Pleiades, Double Cluster, and M35 are beautifully framed with colour-free stars |
| Globular clusters | Moderate M13 and M5 appear granular at high power but the core remains unresolved at 100mm | Challenging 81mm aperture shows globulars like M13 as fuzzy balls with no individual star resolution |
| Faint galaxies | Moderate Brighter Messier galaxies detectable as smudges; 100mm lacks the aperture for structure or faint targets | Moderate Core of brighter galaxies like M81/M82 visible under dark skies, but 81mm gathers limited light for faint targets |
| Milky Way / wide field | Not recommended 900mm focal length produces too narrow a field for sweeping Milky Way star fields | Excellent 478mm at f/5.9 is ideal for sweeping rich star fields; low-power eyepieces deliver expansive true fields |
Other | ||
| Double stars | Excellent Clean ED optics at f/9 produce tight diffraction patterns; Dawes limit around 1.2 arcseconds | 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 applicable | 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) | Not applicable | Challenging 81mm aperture and 478mm focal length produce a small planetary image scale; limited even with a Barlow |
| 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.
Sky-Watcher Evostar 100ED
- You'll spend your observing sessions locked onto bright targets—the Moon, Saturn, Jupiter—where the 100mm aperture and f/9 focal ratio reward you with crisp, high-magnification views that demand steady seeing and patience.
- Your first eyepiece purchase will be a short focal-length piece or a Barlow; the 900mm tube length means you're hunting for magnification, not wide fields.
- If you image, you'll accept longer exposures and plan your sessions around tracking stability; the f/9 ratio is unforgiving of poor polar alignment or mount vibration, but it repays discipline with tight, colour-free star fields.
William Optics GT81
- You'll reach for this scope on nights when you want to sweep—the Orion Nebula whole in one eyepiece, the Pleiades resolved and contextual, large nebulae framed with their surroundings rather than cropped.
- Your observing experience is portable and field-friendly; at 81mm, you're trading deep planetary detail for the ability to actually see what you're after without hunting through magnified darkness.
- If you image, you'll buy it precisely because f/5.9 is fast enough to gather light quickly and the triplet design keeps stars clean across a wide sensor; you're optimised for nebula-sized targets, not planetary close-ups.
The dark side
Every scope has a personality. Here’s where each one gets difficult.
Sky-Watcher
Sky-Watcher Evostar 100ED
Sold as OTA only—you'll need to budget separately for an equatorial mount, diagonal, finder, and eyepieces, pushing actual system cost well above £449.
The ED doublet retains some residual chromatic aberration on the brightest stars, visible under high magnification in perfect conditions—a triplet would eliminate it entirely.
At 900mm focal length, you cannot image wide-field nebulae without a reducer, and even reduced it demands longer exposures than dedicated f/5–f/7 astrographs.
William Optics
William Optics GT81
Sold as OTA only—mount, diagonal, and eyepieces must be purchased separately, bringing true ownership cost to several times the £699 OTA price.
81mm aperture is undersized for planetary and double-star work; Saturn's Cassini Division is difficult to resolve, and Mars remains featureless except near opposition.
Field curvature at the edges requires a dedicated flattener for serious astrophotography, and some production runs lack a focuser lock, risking slip under heavy imaging trains.
Which is right for you?
Two different buyers. Two different right answers.
The custom-rig optical tube
Sky-Watcher · Sky-Watcher Evostar 100ED
You'll love this if you're drawn to lunar and planetary observing, enjoy splitting double stars, and have the patience to build a permanent or semi-permanent setup around a good equatorial mount. You're comfortable investing in quality eyepieces and tracking, and you want the crisp, low-colour optics that reward high magnification.
The custom-rig optical tube
William Optics · William Optics GT81
This is for you if you want to photograph nebulae and star fields, travel with your telescope, or simply spend nights sweeping large, rewarding targets without fiddling with magnification. You're willing to accept that 81mm won't show you Saturn's cloud belts in detail because you'd rather see the entire Orion Nebula in one field.
Our verdict
At £449 versus £699, the William Optics GT81 costs 56% more. The extra money buys a more capable mount and better build quality, not larger optics.
For most buyers starting out, the Sky-Watcher Evostar 100ED is the sensible choice — put the savings into a better eyepiece. The William Optics GT81 makes sense once you know exactly why you need what it offers. If I had to choose: the Sky-Watcher Evostar 100ED, and spend the difference on a quality eyepiece.
Sky-Watcher Evostar 100ED
View Sky-Watcher Evostar 100ED →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 | Sky-Watcher Evostar 100ED | William Optics GT81 |
|---|---|---|
Apertureⓘ The most important spec — bigger = more light = better views | 100mm | 81mm |
Focal Length Longer = more magnification potential | 900mm | 478mm |
Focal Ratio Lower f-number = wider field of view; higher = more magnification per eyepiece | f/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 ED doublet | Fully multi-coated FMC ED triplet on all air-to-glass surfaces |
How do you point it?
| Spec | Sky-Watcher Evostar 100ED | 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 | Sky-Watcher Evostar 100ED | William Optics GT81 |
|---|---|---|
Focuser Size 2" accepts wider eyepieces and gives better low-power views | 2" | 2" / 1.25" |
Focuser Type Rack-and-pinion is standard; Crayford and dual-speed are smoother | Dual-speed Crayford (with 1.25" adapter) | Dual-speed Crayford 2" (10:1 reduction fine focus) |
Size & weight
| Spec | Sky-Watcher Evostar 100ED | William Optics GT81 |
|---|---|---|
OTA Weightⓘ Optical tube only — useful for comparing mount load capacity | 2.6kg | 2.5kg |
Tube Length | 720mm | 380mm |
Tube Material | Aluminium | Aluminium, anodised |
What's in the box?
| Spec | Sky-Watcher Evostar 100ED | William Optics GT81 |
|---|---|---|
Diagonal Tilts the eyepiece 90° for comfortable viewing — useful on refractors |
Blue highlight: Sky-Watcher Evostar 100ED advantage · Amber highlight: William Optics GT81 advantage · Greyed cells: equal or subjective.