Telescope Comparison
Sky-Watcher Esprit 100ED vs William Optics GT81
The price gap is real. The question is whether the extra capability is worth it at your stage.
First light
Sky-Watcher · 100mm · £1,099
The custom-rig optical tube
- 100mm refractor — optical tube only, no mount included
- 550mm focal length at f/5.5
- 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 Esprit 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 Esprit 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
Sky-Watcher Esprit 100ED's faster f/5.5 delivers wider fields with any eyepiece — better for open clusters and large nebulae. William Optics GT81's f/5.9 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)
William Optics GT81's optical tube is 1.4kg 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 | Sky-Watcher Esprit 100ED | William Optics GT81 |
|---|---|---|
| Planets | ||
| Moon | Excellent 100mm aperture delivers sharp crater detail and clean terminator views; the fast focal ratio means lower magnification per eyepiece but detail is still crisp | 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 Rings clearly separated, Cassini Division visible in good seeing; 550mm focal length limits image scale at the eyepiece | Moderate Rings clearly visible and colour-free, but 81mm aperture and 478mm focal length make the Cassini Division very difficult |
| Jupiter | Good Two main cloud belts and GRS visible; 100mm aperture resolves belt detail but the short focal length caps useful magnification | Moderate Main equatorial belts visible in steady seeing; 81mm resolves limited banding detail and the Great Red Spot is marginal |
| Mars | Moderate Disc and polar cap visible at opposition; 100mm and 550mm focal length are limiting for surface albedo features | Challenging Small orange disc visible at opposition; 81mm aperture insufficient to resolve surface features reliably |
Deep sky | ||
| Orion Nebula (M42) | Excellent 100mm gathers ample light, 550mm frames the full nebula with surrounding nebulosity; f/5.5 rewards both visual and imaging use | Excellent Bright nebula easily visible; 478mm focal length at f/5.9 frames the full extent with surrounding nebulosity |
| Andromeda Galaxy (M31) | Excellent 550mm captures the full extent of M31 including companion galaxies; 100mm aperture shows outer halo hints visually | Excellent 478mm focal length captures the core and dust lanes in a single wide field; aperture shows the inner halo structure |
| Open clusters | Excellent 550mm focal length provides generous framing — the Double Cluster, Pleiades, and similar targets are beautifully presented | 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 with hints of edge resolution; core remains unresolved at 100mm | Challenging 81mm aperture shows globulars like M13 as fuzzy balls with no individual star resolution |
| Faint galaxies | Moderate 100mm shows brighter Messier galaxies as fuzzy patches; NGC targets require dark skies and are at the limit visually | Moderate Core of brighter galaxies like M81/M82 visible under dark skies, but 81mm gathers limited light for faint targets |
| Milky Way / wide field | Good 550mm is slightly long for sweeping Milky Way panoramas but still delivers rich star fields; excellent for targeted regions like Cygnus | Excellent 478mm at f/5.9 is ideal for sweeping rich star fields; low-power eyepieces deliver expansive true fields |
Other | ||
| Double stars | Good 100mm resolves down to about 1.2 arcsec; chromatic correction is excellent but the fast focal ratio makes splitting tight pairs trickier than in a long-focus refractor | 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 included — on a suitable equatorial GoTo mount this scope would rate Excellent (f/5.5, 100mm, flat field to full-frame), but as sold it cannot track | 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 100mm aperture is workable with a Barlow and planetary camera, but 550mm native focal length requires significant amplification; needs a tracking mount | Challenging 81mm aperture and 478mm focal length produce a small planetary image scale; limited even with a Barlow |
| Emission nebulae (imaging) | Excellent Fast f/5.5 ratio and 550mm focal length are ideal for large emission targets like the Veil, Rosette, and Heart Nebulae on APS-C or full-frame sensors | Not applicable |
| Galaxy groups (imaging) | Good 550mm frames targets like the Leo Triplet and Markarian's Chain well on APS-C; 100mm gathers enough light for reasonable exposure times | 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.
Sky-Watcher Esprit 100ED
- You'll load your camera, nail the 55mm back-focus spacing, and spend the session collecting photons — the integrated field flattener means pinpoint stars to the corners of your full-frame sensor without buying a separate corrector or fiddling with spacer stacks.
- You'll frame the Veil Nebula complex, the full extent of Andromeda, or the Rosette Nebula at a focal length that actually resolves structure in medium-scale targets like the Leo Triplet — the extra 19mm of aperture over the GT81 pulls fainter detail out of galaxy arms and nebula filaments in shorter total integration times.
- You'll feel the weight when you load it up — 6–7kg fully rigged with guidescope and camera means you need a serious mount like an HEQ5 or EQ6-R, and your total outlay will push well past £2500 before you capture a single frame.
William Optics GT81
- You'll appreciate how light this OTA is on your mount — it pairs comfortably with a Star Adventurer or iOptron GEM28 for a grab-and-go imaging rig you can set up in fifteen minutes and carry in a single trip from the car.
- You'll be shooting enormous targets like the North America Nebula and Heart and Soul complex at f/5.9, but when you zoom in on your subs you'll notice you need a dedicated field flattener to tame edge star elongation — that's an extra purchase the Esprit doesn't require.
- You'll save £400 on the OTA, but on nights when you swing over to the Leo Triplet or a faint galaxy group, you'll wish for the extra aperture and focal length that the Esprit delivers — 81mm simply collects less light and renders smaller image scale on those targets.
The dark side
Every scope has a personality. Here’s where each one gets difficult.
Sky-Watcher
Sky-Watcher Esprit 100ED
At £1099 for the OTA alone — no mount, no finder, no diagonal, no eyepiece — your realistic imaging-ready system cost lands between £2500 and £3000+, making budget planning essential before you commit.
The integrated field flattener is optimised for exactly 55mm of back-focus; get the spacing wrong with your camera and adapter stack and you'll see elongated stars at the edges that no amount of post-processing can fix.
Some narrowband filters — particularly Hα and OIII — can produce halos or uneven illumination across a full-frame sensor at f/5.5; APS-C users are largely unaffected, but full-frame shooters should test filter compatibility.
William Optics
William Optics GT81
No mount, diagonal, or eyepieces are included at £699, and you'll still need a dedicated field flattener for imaging — factor in at least another £100–200 before the optical train is photography-ready.
Some production runs lack a focuser lock, so a heavy imaging train with guidescope and camera can cause the drawtube to slip during long exposures — check your unit and consider an aftermarket lock if needed.
At 81mm aperture and 478mm focal length, planetary image scale is tiny and detail is limited — the Cassini Division on Saturn is a struggle, and Mars remains a featureless orange dot except near opposition.
Which is right for you?
Two different buyers. Two different right answers.
The custom-rig optical tube
Sky-Watcher · Sky-Watcher Esprit 100ED
You already own a solid equatorial mount rated for 7kg+ imaging payloads and you're ready to step up from an 80mm-class refractor to something that pulls more signal from nebulae and galaxies on full-frame or APS-C sensors. You value the convenience of an integrated field flattener that eliminates spacer-stack guesswork, and you're willing to pay a premium for flat, sharp stars to the corners without buying a separate corrector. You're not looking for a planetary scope or a visual grab-and-go — you want a dedicated deep-sky imaging platform that rewards longer integration with genuinely more detail than a smaller aperture can deliver.
The custom-rig optical tube
William Optics · William Optics GT81
You want a lightweight, portable imaging refractor you can pair with a smaller tracking mount for nights when setup speed and mobility matter more than raw aperture. You're targeting sweeping, degree-wide nebulae and large star fields where the 478mm focal length perfectly frames your subjects, and you're comfortable buying a separate flattener to get imaging-grade corners. You're not chasing planetary detail or faint galaxy structure — you know 81mm has limits — but you want a colour-free, fast optical system that gets you clean widefield data at a significantly lower entry cost than the Esprit.
Our verdict
At £699 versus £1,099, the Sky-Watcher Esprit 100ED costs 57% more. It delivers 19mm 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 Sky-Watcher Esprit 100ED'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.
Sky-Watcher Esprit 100ED
View Sky-Watcher Esprit 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 Esprit 100ED | William Optics GT81 |
|---|---|---|
Apertureⓘ The most important spec — bigger = more light = better views | 100mm | 81mm |
Focal Length Longer = more magnification potential | 550mm | 478mm |
Focal Ratio Lower f-number = wider field of view; higher = more magnification per eyepiece | f/5.5 | 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 triplet with FMC on all air-to-glass surfaces | Fully multi-coated FMC ED triplet on all air-to-glass surfaces |
How do you point it?
| Spec | Sky-Watcher Esprit 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 Esprit 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 (10:1 reduction, with 1.25" adapter) | Dual-speed Crayford 2" (10:1 reduction fine focus) |
Size & weight
| Spec | Sky-Watcher Esprit 100ED | William Optics GT81 |
|---|---|---|
OTA Weightⓘ Optical tube only — useful for comparing mount load capacity | 3.9kg | 2.5kg |
Tube Length | 535mm | 380mm |
Tube Material | Aluminium, white powder coat | Aluminium, anodised |
What's in the box?
| Spec | Sky-Watcher Esprit 100ED | William Optics GT81 |
|---|---|---|
Diagonal Tilts the eyepiece 90° for comfortable viewing — useful on refractors |
Blue highlight: Sky-Watcher Esprit 100ED advantage · Amber highlight: William Optics GT81 advantage · Greyed cells: equal or subjective.