Telescope Comparison
Sky-Watcher Esprit 100ED vs William Optics Zenithstar 73
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 · 73mm · £599
The custom-rig optical tube
- 73mm refractor — optical tube only, no mount included
- 430mm focal length at f/5.89
- 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 1.9× 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 Zenithstar 73'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 Zenithstar 73's f/5.89 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 Zenithstar 73's optical tube is 2.1kg 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 Zenithstar 73 |
|---|---|---|
| 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 | Moderate 73mm aperture shows good crater and terminator detail, but the short 430mm focal length limits useful magnification before the image softens. |
| Saturn | Good Rings clearly separated, Cassini Division visible in good seeing; 550mm focal length limits image scale at the eyepiece | Challenging Rings visible and Titan identifiable, but 73mm aperture and 430mm focal length can't reveal the Cassini Division or subtle banding. |
| 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; 73mm falls between the Good and Moderate tiers, and the short focal length makes it hard to push magnification for finer 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 near opposition with possible hint of polar cap, but 73mm aperture and short focal length offer very limited surface detail. |
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 | Good Core nebulosity and Trapezium visible; the wide field at 430mm frames the full nebula complex nicely, but aperture is just under the 80mm Excellent threshold. |
| Andromeda Galaxy (M31) | Excellent 550mm captures the full extent of M31 including companion galaxies; 100mm aperture shows outer halo hints visually | Excellent 430mm focal length frames the full galaxy with room to spare; visually the core and inner dust lanes are visible from dark skies. |
| Open clusters | Excellent 550mm focal length provides generous framing — the Double Cluster, Pleiades, and similar targets are beautifully presented | Excellent Wide true field at 430mm is ideal for the Double Cluster, Pleiades, and other large clusters — they sit beautifully in the field of view. |
| Globular clusters | Moderate M13 and M5 appear granular with hints of edge resolution; core remains unresolved at 100mm | Challenging 73mm aperture shows M13 and M22 as fuzzy unresolved glows — no star resolution possible at this aperture. |
| Faint galaxies | Moderate 100mm shows brighter Messier galaxies as fuzzy patches; NGC targets require dark skies and are at the limit visually | Challenging 73mm gathers limited light; only the brightest galaxies like M81/M82 show as faint smudges visually. |
| 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 430mm focal length at f/5.9 delivers sweeping rich star fields — among the best use cases for this scope visually and with a camera. |
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 | Moderate 73mm resolves wide doubles like Albireo easily, but the short focal length and modest aperture limit splitting of closer pairs. |
| 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; the OTA is excellent for deep sky imaging but only when paired with an equatorial tracking mount purchased separately. |
| 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 73mm aperture and 430mm focal length produce a very small planetary image scale — a Barlow helps but aperture is the fundamental limit. |
| 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're investing in a complete imaging optical system — the integrated field flattener means you nail your 55mm back-focus distance once and get pinpoint stars to the corners of a full-frame sensor without buying a separate corrector.
- You'll frame targets like the Leo Triplet and Rosette Nebula at a scale that actually resolves structure, not just captures blobs — 550mm at f/5.5 hits a sweet spot where medium-sized deep-sky objects fill the frame with detail worth processing.
- You'll also find that on nights when imaging conditions aren't cooperating, dropping in an eyepiece gives you genuinely enjoyable views — 100mm of triplet APO resolves the Trapezium cleanly, shows Saturn's Cassini division, and makes globular clusters like M13 look granular and alive in a way the smaller scope simply can't.
William Optics Zenithstar 73
- You'll pack this into a carry-on bag and set up at a dark-sky site in minutes — the compact OTA weighs dramatically less than the Esprit, which means your mount requirements are lighter and cheaper too.
- You're shooting the widest targets in the sky — Heart and Soul side by side, the full Orion molecular cloud complex — at 430mm, and the fast f/5.9 ratio keeps exposures short enough that a modest mount tracks well without guiding heroics.
- You'll spend less upfront on the OTA, but you need to budget honestly: the Flat73A flattener is an essential add-on, not an optional extra, so factor that into your real cost before you compare prices.
The dark side
Every scope has a personality. Here’s where each one gets difficult.
Sky-Watcher
Sky-Watcher Esprit 100ED
The OTA is £1099 before you own a single accessory — by the time you add a mount capable of handling 6–7kg of imaging payload, a guidescope, and a camera, you're realistically looking at £2500–£3000+ for a working system.
The integrated flattener demands exactly 55mm of back-focus spacing; get it wrong and your corner stars turn to streaks, which means you'll spend time researching spacer combinations for your specific camera and filter setup.
At f/5.5, some narrowband filters — particularly Ha — can produce halos or uneven illumination across full-frame sensors, pushing you toward APS-C or careful filter selection.
William Optics
William Optics Zenithstar 73
The dedicated Flat73A field flattener is essentially mandatory for imaging — without it, edge stars show obvious coma and curvature, so your real OTA cost is the £599 plus the flattener price.
As an ED doublet rather than a triplet, residual chromatic aberration can show on bright stars in images — it's well controlled for the price, but it's there, and you'll notice it if you pixel-peep Vega or Sirius.
The 2-inch rack-and-pinion focuser can flex slightly under heavier camera and filter train loads, so you'll need to balance carefully and may find yourself wishing for a more robust focuser if you're running a full-frame camera with a filter wheel.
Which is right for you?
Two different buyers. Two different right answers.
The custom-rig optical tube
Sky-Watcher · Sky-Watcher Esprit 100ED
You've already been imaging with an 80mm refractor or a DSLR on a star tracker, and you know you want more — more resolution on galaxy detail, more signal from faint nebulae, and a flat field you don't have to fight for. You're ready to invest in a serious mount and you want an OTA that won't be the weak link in your imaging chain. You also appreciate that on visual nights you can drop in an eyepiece and see something genuinely satisfying through 100mm of triplet APO glass. This isn't for you if you're just getting started in astrophotography and don't yet own a mount — the total system cost will shock you — or if your primary targets are planets, where the 550mm focal length leaves you wanting more image scale.
The custom-rig optical tube
William Optics · William Optics Zenithstar 73
You want to get into deep-sky astrophotography without committing to a heavy, expensive rig, and you value portability above all — you'll travel to dark skies, set up quickly, and shoot enormous wide-field targets that bigger scopes can't even frame. You already own or plan to buy a lightweight equatorial mount, and you're comfortable with APS-C sensors where the smaller aperture and doublet design are least limiting. This isn't for you if you want to do any serious visual observing — 73mm of aperture simply won't show you faint galaxies or resolve globulars — or if you expect sharp edge-to-edge imaging stars out of the box without purchasing the separate field flattener.
Our verdict
At £599 versus £1,099, the Sky-Watcher Esprit 100ED costs 83% more. It delivers 27mm more aperture — a real and visible advantage on faint targets.
If budget is a genuine constraint, the William Optics Zenithstar 73 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 Zenithstar 73, use it for a year, then upgrade knowing exactly what you want.
Sky-Watcher Esprit 100ED
View Sky-Watcher Esprit 100ED →William Optics Zenithstar 73
View William Optics Zenithstar 73 →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 Zenithstar 73 |
|---|---|---|
Apertureⓘ The most important spec — bigger = more light = better views | 100mm | 73mm |
Focal Length Longer = more magnification potential | 550mm | 430mm |
Focal Ratio Lower f-number = wider field of view; higher = more magnification per eyepiece | f/5.5 | f/5.89 |
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 doublet on all air-to-glass surfaces |
How do you point it?
| Spec | Sky-Watcher Esprit 100ED | William Optics Zenithstar 73 |
|---|---|---|
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 Zenithstar 73 |
|---|---|---|
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 Zenithstar 73 |
|---|---|---|
OTA Weightⓘ Optical tube only — useful for comparing mount load capacity | 3.9kg | 1.75kg |
Tube Length | 535mm | 320mm |
Tube Material | Aluminium, white powder coat | Aluminium, anodised blue |
What's in the box?
| Spec | Sky-Watcher Esprit 100ED | William Optics Zenithstar 73 |
|---|---|---|
Diagonal Tilts the eyepiece 90° for comfortable viewing — useful on refractors |
Blue highlight: Sky-Watcher Esprit 100ED advantage · Amber highlight: William Optics Zenithstar 73 advantage · Greyed cells: equal or subjective.