Telescope Comparison
Sky-Watcher Esprit 100ED vs Vixen SD81S
The specs are close. The experience isn't.
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
Vixen · 81mm · £1,199
The custom-rig optical tube
- 81mm refractor — optical tube only, no mount included
- 625mm focal length at f/7.72
- 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
Vixen SD81S's longer focal length reaches higher magnification with the same eyepiece — better reach for planetary detail. Sky-Watcher Esprit 100ED'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. Vixen SD81S's f/7.72 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)
Vixen SD81S's optical tube is 1.9kg 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 | Vixen SD81S |
|---|---|---|
| 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 with superb colour correction delivers crisp, fringe-free lunar detail; f/7.7 supports rewarding high-magnification views |
| Saturn | Good Rings clearly separated, Cassini Division visible in good seeing; 550mm focal length limits image scale at the eyepiece | Good Rings clearly defined, Cassini Division visible in good seeing; 625mm focal length limits image scale but clean optics compensate |
| Jupiter | Good Two main cloud belts and GRS visible; 100mm aperture resolves belt detail but the short focal length caps useful magnification | Good Main equatorial belts and GRS visible with high contrast and no false colour; aperture limits finer belt detail |
| Mars | Moderate Disc and polar cap visible at opposition; 100mm and 550mm focal length are limiting for surface albedo features | Challenging Small disc visible at opposition with possible polar cap hint, but 81mm aperture cannot resolve surface albedo features |
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 625mm focal length frames the nebula well; 81mm gathers enough light to show core structure and nebulosity wings |
| Andromeda Galaxy (M31) | Excellent 550mm captures the full extent of M31 including companion galaxies; 100mm aperture shows outer halo hints visually | Excellent 625mm focal length captures the galaxy's full extent; core and dust lanes visible, though outer halo is faint at 81mm |
| Open clusters | Excellent 550mm focal length provides generous framing — the Double Cluster, Pleiades, and similar targets are beautifully presented | Excellent Wide true field at 625mm beautifully frames clusters like the Pleiades and Double Cluster with pinpoint stars |
| Globular clusters | Moderate M13 and M5 appear granular with hints of edge resolution; core remains unresolved at 100mm | Challenging 81mm cannot resolve individual stars — globulars appear as fuzzy, concentrated glows |
| Faint galaxies | Moderate 100mm shows brighter Messier galaxies as fuzzy patches; NGC targets require dark skies and are at the limit visually | Moderate Brighter Messier galaxies visible as diffuse patches; 81mm lacks the light grasp for structure or fainter NGC 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 | Good 625mm focal length is moderately wide; rich starfields are enjoyable but the scope is too narrow for grand sweeping views |
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 and near-zero chromatic aberration make this a satisfying double star scope; Dawes limit around 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; when paired with a suitable equatorial mount this would rate Excellent (81mm, f/7.7, superb correction) |
| 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 | Moderate Clean optics suit planetary capture, but 81mm aperture and 625mm focal length limit resolution and image scale |
| 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 |
The real tradeoff
Both scopes are capable. The question is which one fits the way you actually observe.
Sky-Watcher Esprit 100ED
- You'll spend your sessions collecting photons, not fighting optical compromises — the integrated field flattener means you skip the back-focus spacing guesswork that plagues most imaging refractors, and your stars are pinpoints to the corners of a full-frame sensor right out of the box.
- You'll frame the entire Veil Nebula on APS-C at 550mm f/5.5, pulling in signal roughly 50% faster than the Vixen at f/7.7 — that's the difference between a usable 3-minute sub and needing 5 minutes for the same signal-to-noise, which compounds across a whole night's imaging.
- You'll feel that extra weight though — fully loaded with guidescope and camera you're approaching 6–7kg, which means your mount choice matters and your grab-and-go fantasies stay fantasies. This is a scope you set up deliberately, not one you throw in the car on a whim.
Vixen SD81S
- You'll appreciate the 2.4kg OTA every time you mount up — an HEQ5 or even a Star Adventurer GTi handles this effortlessly, and the compact tube tucks into luggage for dark-sky trips where the Esprit would be a burden.
- You'll find yourself reaching for an eyepiece more often than you expected — the f/7.7 focal ratio and Vixen's SD glass deliver cleaner high-magnification lunar and planetary views than the faster Esprit, rewarding visual detours between imaging runs with crisp, nearly colour-free detail.
- You'll pay a hidden tax for imaging: the SD flattener is an extra £250–£350 on top of the £1199 OTA, and without it your edge stars will streak — so budget accordingly and accept that this scope's true imaging cost is closer to £1500 before you even think about a mount.
The dark side
Every scope has a personality. Here’s where each one gets difficult.
Sky-Watcher
Sky-Watcher Esprit 100ED
The £1099 price tag is just the opening bid — add a mount capable of carrying 6–7kg of imaging payload and you're looking at £2500–£3000+ for a complete working system.
The integrated field flattener demands exactly 55mm of back-focus; get the spacing wrong and your corner stars turn to seagulls, which means you'll need spacer rings and careful measurement before your first light frame.
Some narrowband filters produce halos or uneven illumination at f/5.5 across full-frame sensors — APS-C users are less affected, but full-frame Ha and OIII shooters should research filter compatibility before committing.
Vixen
Vixen SD81S
At £1199 for an 81mm doublet OTA with no accessories, you're paying a steep premium for Vixen's SD glass — competing ED doublets from other manufacturers offer similar aperture for significantly less money.
Vixen's proprietary dovetail and accessory system means you may need adapters to mate this tube with your existing third-party mount and accessories, adding cost and potential flex points.
This is a two-element SD doublet, not a triplet — while colour correction is excellent for a doublet, it is not truly apochromatic across the full visual spectrum, and demanding imagers may notice residual colour on bright stars that the Esprit's triplet design suppresses.
Which is right for you?
Two different buyers. Two different right answers.
The custom-rig optical tube
Sky-Watcher · Sky-Watcher Esprit 100ED
You're an intermediate or advancing astrophotographer who has outgrown an 80mm refractor and wants noticeably more light grasp and a faster focal ratio without jumping to a heavy Newtonian. You already own or are prepared to buy a serious equatorial mount like an HEQ5 or EQ6-R, and you want an imaging platform that delivers flat fields on APS-C or full-frame sensors without buying a separate flattener. You're not looking for a visual scope — you want photons on a sensor, and you want them fast. This isn't for you if you need a ready-to-go system, if your budget tops out at the OTA price, or if planetary imaging is your primary goal.
The custom-rig optical tube
Vixen · Vixen SD81S
You're an imager or visual observer who prizes portability and optical purity above all else, and you're willing to accept 81mm of aperture because you know your best images come from dark sites you can actually travel to. You value the flexibility of a scope that's genuinely rewarding at the eyepiece between imaging runs, and you're comfortable investing in Vixen's ecosystem — including the dedicated flattener — because optical quality matters to you more than value-per-millimetre-of-aperture. This isn't for you if you want to resolve faint galaxies or push planetary detail, if Vixen's proprietary accessories frustrate you, or if spending over £1200 on an 81mm OTA feels disproportionate when the Esprit gives you 100mm of triplet APO for less money.
Our verdict
At similar price points, these scopes offer different amounts of aperture per pound. The Sky-Watcher Esprit 100ED gives you more light-gathering for your money — and for visual observing, aperture per pound is the most useful single metric.
For pure optical value, the Sky-Watcher Esprit 100ED is the stronger pick. The Vixen SD81S compensates with other features — decide whether those trade-offs justify the premium. If I had to choose: the Sky-Watcher Esprit 100ED — more aperture per pound means more sky.
Sky-Watcher Esprit 100ED
View Sky-Watcher Esprit 100ED →Vixen SD81S
View Vixen SD81S →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 | Vixen SD81S |
|---|---|---|
Apertureⓘ The most important spec — bigger = more light = better views | 100mm | 81mm |
Focal Length Longer = more magnification potential | 550mm | 625mm |
Focal Ratio Lower f-number = wider field of view; higher = more magnification per eyepiece | f/5.5 | f/7.72 |
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 SD (Super Duplex) glass doublet |
How do you point it?
| Spec | Sky-Watcher Esprit 100ED | Vixen SD81S |
|---|---|---|
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 | Vixen SD81S |
|---|---|---|
Focuser Size 2" accepts wider eyepieces and gives better low-power views | 2" | 2" |
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 (with 1.25" adapter) |
Size & weight
| Spec | Sky-Watcher Esprit 100ED | Vixen SD81S |
|---|---|---|
OTA Weightⓘ Optical tube only — useful for comparing mount load capacity | 3.9kg | 2kg |
Tube Length | 535mm | 540mm |
Tube Material | Aluminium, white powder coat | Aluminium |
What's in the box?
| Spec | Sky-Watcher Esprit 100ED | Vixen SD81S |
|---|---|---|
Diagonal Tilts the eyepiece 90° for comfortable viewing — useful on refractors |
Blue highlight: Sky-Watcher Esprit 100ED advantage · Amber highlight: Vixen SD81S advantage · Greyed cells: equal or subjective.