Telescope Comparison
Sky-Watcher Evostar 100ED vs William Optics Zenithstar 73
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 · 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 Evostar 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 Evostar 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
William Optics Zenithstar 73's faster f/5.89 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 Zenithstar 73 |
|---|---|---|
| Planets | ||
| Moon | Excellent 100mm aperture and f/9 focal ratio reward high magnification with sharp, high-contrast lunar detail | Moderate 73mm aperture shows good crater and terminator detail, but the short 430mm focal length limits useful magnification before the image softens. |
| Saturn | Good 900mm focal length and clean ED optics show rings, Cassini Division in good seeing, and subtle disc banding | Challenging Rings visible and Titan identifiable, but 73mm aperture and 430mm focal length can't reveal the Cassini Division or subtle banding. |
| 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; 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 visible with polar cap and large albedo features at opposition, but 100mm limits fine surface detail | 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) | Good Bright nebula core and trapezium well shown, but 900mm focal length crops the outer wings | 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) | Good Bright core and inner halo visible; 900mm frames only the central region, missing the full extent | 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 | Good Compact clusters like M35 frame well; larger groups like the Double Cluster fill the low-power field | 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 at high power but the 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 Brighter Messier galaxies detectable as smudges; 100mm lacks the aperture for structure or faint targets | Challenging 73mm gathers limited light; only the brightest galaxies like M81/M82 show as faint smudges visually. |
| Milky Way / wide field | Not recommended 900mm focal length produces too narrow a field for sweeping Milky Way star fields | 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 | Excellent Clean ED optics at f/9 produce tight diffraction patterns; Dawes limit around 1.2 arcseconds | 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 applicable | 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) | Not applicable | Challenging 73mm aperture and 430mm focal length produce a very small planetary image scale — a Barlow helps but aperture is the fundamental limit. |
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 nights at the eyepiece chasing planetary and lunar detail — Saturn's Cassini Division, Jupiter's festoons, and crater rims sharp enough to cut — because this is where the 100mm aperture and f/9 focal length genuinely reward patience.
- Your typical session involves high magnification (150–200×) on bright targets where the ED glass's low chromatic aberration makes the difference between clean views and purple fringing, and where you'll resolve double stars down to 1.2 arcseconds that other scopes blur together.
- If you decide to image, you'll accept longer exposures than faster astrographs demand, but you'll get tight, round stars across the field because the f/9 design was optimized for optical correction, not speed.
William Optics Zenithstar 73
- You'll unlock this scope's potential only when you mount a camera on the back — the wide 430mm focal length frames entire nebula complexes (Orion, Heart and Soul, full Andromeda) in a single exposure, which is exactly what it was designed to deliver.
- Your observing sessions at the eyepiece will be pleasant but limited; the Moon and brightest nebulae show their beauty, but 73mm of aperture runs out of light-gathering power quickly on faint galaxies and globular clusters, making visual work feel more like a bonus than the main event.
- You'll spend time optimizing your imaging setup — the dedicated Flat73A flattener is essentially non-negotiable to control coma at the field edges — and balancing your camera and filter train to avoid flexure in the 2-inch focuser.
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 with no mount, diagonal, finder, or eyepieces included, so the true cost to first light is substantially more than the £449 price tag.
The 900mm focal length is poorly suited to wide-field deep-sky imaging without a reducer, and even with one it remains slower than dedicated f/5–f/7 astrographs, making deep-sky imaging feel inefficient.
ED doublet optics show some residual chromatic aberration on very bright stars, though considerably less than achromats — you may notice it on extremely bright targets under high magnification.
William Optics
William Optics Zenithstar 73
Sold as OTA only with no mount, diagonal, eyepiece, or finder included, so the actual cost to a working setup is significantly higher than the £599 OTA price.
The dedicated Flat73A field flattener is essentially mandatory for imaging; without it, stars toward the field edges will show noticeable coma and curvature that degrades the image.
73mm aperture limits both visual planetary detail and deep-sky performance — faint galaxies appear as dim smudges and globular clusters won't resolve into stars, making this a camera-first instrument with visual as an afterthought.
Which is right for you?
Two different buyers. Two different right answers.
The custom-rig optical tube
Sky-Watcher · Sky-Watcher Evostar 100ED
You're the observer who lives at the eyepiece and wants crisp, high-contrast views of the Moon, planets, and tight double stars — you'll love the 100mm aperture and f/9 focal ratio's ability to split difficult pairs and reveal subtle planetary detail that smaller or faster scopes can't match, and you're willing to pair it with your own equatorial mount and invest in quality eyepieces to do so. This scope isn't for you if you need a ready-to-observe package out of the box, or if you're chasing wide-field deep-sky sweeps across the Milky Way.
The custom-rig optical tube
William Optics · William Optics Zenithstar 73
You're the astrophotographer who wants to frame enormous nebulae, galaxy groups, and star fields in single exposures from dark-sky sites or portable setups — you'll thrive with the fast f/5.9 focal ratio, the compact size, and the wide field that a camera sensor loves, provided you commit to the Flat73A flattener and a solid equatorial mount. This scope isn't for you if visual planetary observing or resolving globular clusters matters to you, or if you want to grab it and look through an eyepiece without serious equipment investment.
Our verdict
At similar price points, these scopes offer different amounts of aperture per pound. The Sky-Watcher Evostar 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 Evostar 100ED is the stronger pick. The William Optics Zenithstar 73 compensates with other features — decide whether those trade-offs justify the premium. If I had to choose: the Sky-Watcher Evostar 100ED — more aperture per pound means more sky.
Sky-Watcher Evostar 100ED
View Sky-Watcher Evostar 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 Evostar 100ED | William Optics Zenithstar 73 |
|---|---|---|
Apertureⓘ The most important spec — bigger = more light = better views | 100mm | 73mm |
Focal Length Longer = more magnification potential | 900mm | 430mm |
Focal Ratio Lower f-number = wider field of view; higher = more magnification per eyepiece | f/9 | 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 doublet | Fully multi-coated FMC ED doublet on all air-to-glass surfaces |
How do you point it?
| Spec | Sky-Watcher Evostar 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 Evostar 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 (with 1.25" adapter) | Dual-speed Crayford 2" (10:1 reduction fine focus) |
Size & weight
| Spec | Sky-Watcher Evostar 100ED | William Optics Zenithstar 73 |
|---|---|---|
OTA Weightⓘ Optical tube only — useful for comparing mount load capacity | 2.6kg | 1.75kg |
Tube Length | 720mm | 320mm |
Tube Material | Aluminium | Aluminium, anodised blue |
What's in the box?
| Spec | Sky-Watcher Evostar 100ED | William Optics Zenithstar 73 |
|---|---|---|
Diagonal Tilts the eyepiece 90° for comfortable viewing — useful on refractors |
Blue highlight: Sky-Watcher Evostar 100ED advantage · Amber highlight: William Optics Zenithstar 73 advantage · Greyed cells: equal or subjective.