Telescope Comparison
Sky-Watcher Esprit 100ED vs William Optics FluoroStar 91
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
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
The full picture
The numbers that separate these two scopes — and what they mean at the eyepiece.
Aperture
Sky-Watcher Esprit 100ED gathers 1.2× 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 FluoroStar 91'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 FluoroStar 91'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)
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 Esprit 100ED | William Optics FluoroStar 91 |
|---|---|---|
| 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 91mm aperture and fluorite correction deliver sharp, high-contrast lunar detail with no false colour on the limb |
| Saturn | Good Rings clearly separated, Cassini Division visible in good seeing; 550mm focal length limits image scale at the eyepiece | Good Ring structure and Cassini Division visible in good seeing, though short focal length requires high-power eyepieces to push magnification |
| Jupiter | Good Two main cloud belts and GRS visible; 100mm aperture resolves belt detail but the short focal length caps useful magnification | Good Main cloud belts and GRS visible; 91mm resolves some detail but the 537mm focal length limits comfortable high-power use |
| Mars | Moderate Disc and polar cap visible at opposition; 100mm and 550mm focal length are limiting for surface albedo features | Challenging Disc visible at opposition with hints of albedo features, but 91mm aperture and short focal length make surface detail very difficult |
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 91mm aperture and 537mm focal length at f/5.9 frame the full nebula complex with bright, detailed nebulosity and resolved Trapezium |
| Andromeda Galaxy (M31) | Excellent 550mm captures the full extent of M31 including companion galaxies; 100mm aperture shows outer halo hints visually | Excellent 537mm focal length captures the full extent of M31 including companion galaxies; 91mm aperture shows hints of outer halo structure |
| Open clusters | Excellent 550mm focal length provides generous framing — the Double Cluster, Pleiades, and similar targets are beautifully presented | Excellent Wide field at 537mm beautifully frames large clusters like the Double Cluster and Pleiades with tight, colour-free stars |
| Globular clusters | Moderate M13 and M5 appear granular with hints of edge resolution; core remains unresolved at 100mm | Moderate 91mm shows globulars as granular, concentrated balls — M13 has a bright core but individual stars remain unresolved |
| Faint galaxies | Moderate 100mm shows brighter Messier galaxies as fuzzy patches; NGC targets require dark skies and are at the limit visually | Challenging 91mm gathers limited light for faint galaxies visually; brighter Messier galaxies visible as faint smudges, but detail is minimal |
| 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 537mm at f/5.9 is ideal for rich Milky Way sweeps — star fields through Cygnus and Sagittarius are stunning |
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 91mm resolves wide and moderate doubles cleanly with excellent colour correction, though close pairs need very short eyepieces at this focal length |
| 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 — optically superb for deep-sky imaging but requires a separate equatorial mount to realise that potential |
| 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 91mm and 537mm focal length are limited for planetary imaging; usable with a 2–3× Barlow on a tracking mount, but aperture constrains resolution |
| 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 | 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 |
| 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 unbox a scope with the field flattener already built in — no hunting for the right flattener, no second purchase, no spacing guesswork beyond nailing the 55mm back-focus distance. Your first imaging session starts with one fewer variable to get wrong.
- You're getting 100mm of aperture at f/5.5, which means slightly faster exposures and a touch more signal per sub than the 91mm FluoroStar — not a night-and-day difference, but over a long imaging session you'll accumulate roughly 20% more light, and that compounds across dozens of subs.
- You'll pay £200 less for the OTA and get a scope that's heavier when fully loaded (potentially 6–7kg with guide gear and camera), so you need to be honest about whether your mount can handle that payload for imaging — if you're on an HEQ5-class mount, you're near the edge.
William Optics FluoroStar 91
- You're paying a premium for natural fluorite glass, and the return on that investment shows up in your stacked data — virtually zero chromatic aberration means tighter, cleaner stars with less colour bloat in long exposures, especially on bright stars where ED triplets can still show faint halos.
- You'll need to buy a matched field flattener or reducer separately, which adds cost and another spacing variable to dial in — but it also gives you flexibility to choose a reducer for faster f/ratios or a flattener optimised for your specific sensor size.
- The lighter, more compact OTA makes this a genuinely portable imaging rig — if you're driving to dark-sky sites and packing carefully, you'll notice the difference in the car and on the mount, and a lighter payload gives your mount more headroom for guiding accuracy.
The dark side
Every scope has a personality. Here’s where each one gets difficult.
Sky-Watcher
Sky-Watcher Esprit 100ED
The integrated field flattener demands exactly 55mm of back-focus — get the spacing wrong and you'll see elongated stars at the edges of your frames, which means you need spacer rings and careful measurement before your first real imaging run.
At f/5.5, some narrowband filters can produce halos or uneven illumination across full-frame sensors — if you're shooting Ha or OIII on a full-frame camera, test for filter artefacts early; APS-C users are less affected.
No mount, finder, diagonal, or eyepieces are included — budget £2,500–£3,000+ for a complete imaging system once you add an EQ mount, guide scope, and camera.
William Optics
William Optics FluoroStar 91
Sold as a bare OTA with no mount, finder, diagonal, or eyepieces — and the field flattener is also a separate purchase, so the real cost of a complete imaging system climbs well beyond the £1,299 sticker price.
The visual difference between natural fluorite and quality FPL-53 ED glass is marginal at the eyepiece — you're paying a premium that only genuinely pays off in demanding long-exposure astrophotography, not casual visual use.
At 91mm aperture with a 537mm focal length, high-magnification planetary or double-star work requires very short focal-length eyepieces or a Barlow, and even then you'll hit the aperture's resolution limit well before a 100mm or larger scope does.
Which is right for you?
Two different buyers. Two different right answers.
The custom-rig optical tube
Sky-Watcher · Sky-Watcher Esprit 100ED
You want to image deep-sky targets and you'd rather have one less thing to buy and calibrate — the integrated field flattener means you're solving for back-focus distance, not shopping for a separate flattener. You're stepping up from an 80mm refractor and you want the extra light grasp of 100mm without breaking your mount's payload budget. You'll occasionally put an eyepiece in and appreciate decent lunar and planetary views, but the camera is your primary tool. You're cost-conscious enough to notice the £200 OTA savings, especially knowing the mount and accessories will dwarf the tube price.
The custom-rig optical tube
William Optics · William Optics FluoroStar 91
You're an experienced imager chasing the cleanest possible colour correction in your stacked data, and you know from hard-won experience that even small chromatic aberration compounds across hundreds of subs. You value portability — you travel to dark sites regularly, and every gram on the mount matters for guiding precision. You're comfortable buying and spacing a separate field flattener because you want the flexibility to match it to your exact sensor and imaging train. The higher OTA price doesn't faze you because you've already invested in a capable mount, guide system, and cooled camera — this is a considered upgrade, not your first scope.
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 William Optics FluoroStar 91 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 →William Optics FluoroStar 91
View William Optics FluoroStar 91 →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 FluoroStar 91 |
|---|---|---|
Apertureⓘ The most important spec — bigger = more light = better views | 100mm | 91mm |
Focal Length Longer = more magnification potential | 550mm | 537mm |
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 fluorite triplet on all air-to-glass surfaces |
How do you point it?
| Spec | Sky-Watcher Esprit 100ED | William Optics FluoroStar 91 |
|---|---|---|
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 FluoroStar 91 |
|---|---|---|
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 FluoroStar 91 |
|---|---|---|
OTA Weightⓘ Optical tube only — useful for comparing mount load capacity | 3.9kg | 3.2kg |
Tube Length | 535mm | 430mm |
Tube Material | Aluminium, white powder coat | Aluminium, anodised |
What's in the box?
| Spec | Sky-Watcher Esprit 100ED | William Optics FluoroStar 91 |
|---|---|---|
Diagonal Tilts the eyepiece 90° for comfortable viewing — useful on refractors |
Blue highlight: Sky-Watcher Esprit 100ED advantage · Amber highlight: William Optics FluoroStar 91 advantage · Greyed cells: equal or subjective.