Telescope Comparison
William Optics Zenithstar 61 vs William Optics Zenithstar 73
The specs are close. The experience isn't.
First light
William Optics · 61mm · £499
The custom-rig optical tube
- 61mm refractor — optical tube only, no mount included
- 360mm 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
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
William Optics Zenithstar 73 gathers 1.4× 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
William Optics Zenithstar 73's longer focal length reaches higher magnification with the same eyepiece — better reach for planetary detail. William Optics Zenithstar 61's shorter focal length gives a wider true field — better for large open clusters and extended nebulae.
Focal ratio
Same focal ratio — the same eyepiece gives equivalent magnification and true field in both scopes.
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 | William Optics Zenithstar 61 | William Optics Zenithstar 73 |
|---|---|---|
| Planets | ||
| Moon | Moderate 61mm aperture shows craters and maria, but the short 360mm focal length limits useful magnification for fine detail | Moderate 73mm aperture shows good crater and terminator detail, but the short 430mm focal length limits useful magnification before the image softens. |
| Saturn | Challenging Rings visible as distinct structure, but 61mm aperture and 360mm focal length cannot reveal Cassini Division or banding | Challenging Rings visible and Titan identifiable, but 73mm aperture and 430mm focal length can't reveal the Cassini Division or subtle banding. |
| Jupiter | Challenging Disc and two main equatorial belts visible, but small aperture limits cloud detail and the short focal length keeps the image very small | 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 | Not recommended Tiny orange disc at opposition; 61mm aperture and 360mm focal length cannot resolve surface 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) | Good Wide field frames the full nebula and surrounding region; 61mm shows the bright core and inner nebulosity but lacks aperture for fainter outer structure visually | 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 360mm focal length captures the full extent of M31 and companion galaxies in a single field — ideal framing for imaging | 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 360mm focal length gives wide true field, perfectly suited for large clusters like the Double Cluster, Pleiades, and Hyades | 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 | Challenging 61mm aperture shows fuzzy patches only; no star resolution possible even at the edges | Challenging 73mm aperture shows M13 and M22 as fuzzy unresolved glows — no star resolution possible at this aperture. |
| Faint galaxies | Not recommended 61mm aperture gathers too little light to reveal faint galaxy detail visually | Challenging 73mm gathers limited light; only the brightest galaxies like M81/M82 show as faint smudges visually. |
| Milky Way / wide field | Excellent 360mm focal length at f/5.9 delivers sweeping star fields — one of this scope's strengths both visually and for imaging | 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 | Moderate Dawes limit of ~1.9 arcseconds; wide pairs split cleanly but close doubles are beyond reach, and short focal length makes high-power splitting impractical | 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 or tracking included; on a suitable equatorial mount this would rate Excellent — f/5.9, APO glass, and 360mm focal length are ideal for wide-field imaging | 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) | Challenging 61mm aperture and 360mm focal length produce a very small planetary image scale; no tracking included | Challenging 73mm aperture and 430mm focal length produce a very small planetary image scale — a Barlow helps but aperture is the fundamental limit. |
| Large emission nebulae (imaging) | Excellent With a tracking mount, the wide f/5.9 field frames targets like the North America Nebula, Veil Nebula, and Heart/Soul complex superbly | Not applicable |
The real tradeoff
Both scopes are capable. The question is which one fits the way you actually observe.
William Optics Zenithstar 61
- You'll frame the entire Orion Nebula with breathing room on an APS-C sensor, making wide-field nebulae your sweet spot — but you'll sacrifice planetary detail, where even Saturn shows only its ring structure without fine divisions.
- Your observing sessions behind a camera will reward patience with fast exposures and clean star colours across the field; behind an eyepiece, you'll hit the magnification ceiling around 120× and watch planetary discs remain frustratingly small.
- You're buying the lightest, most portable wide-field imaging platform in this comparison, which means you can travel with it easily — but you're also committing to the Flat6A field flattener (£250) before your sensor can see flat stars at the edges.
William Optics Zenithstar 73
- You'll capture the full Andromeda Galaxy or Heart and Soul Nebulae side by side on APS-C, gaining 12mm more aperture and slightly longer focal length — but you're still constrained to wide-field targets and won't resolve planetary detail that a longer refractor or Newtonian can deliver.
- Your camera sessions will benefit from 20% more light-gathering than the 61mm, shortening exposures in moderately light-polluted skies and forgiving minor mount imperfections; your visual observing will still plateau at large, bright nebulae and open clusters.
- You're investing £100 more in the OTA and roughly the same amount again in the mandatory Flat73A flattener, but you gain a 73mm aperture that tips the balance slightly toward serious wide-field imaging rather than visual compromise.
The dark side
Every scope has a personality. Here’s where each one gets difficult.
William Optics
William Optics Zenithstar 61
Sold as OTA only — no mount, diagonal, eyepiece, or finder included, so a functioning setup requires additional investment of several hundred pounds beyond the £499 OTA price.
Field curvature at the edges is visible on APS-C and full-frame sensors without the Flat6A field flattener, which costs approximately £250 and is a near-essential accessory.
61mm aperture severely limits visual performance — unsuitable as a primary visual telescope, with maximum useful magnification around 120× and minimal planetary or lunar detail.
William Optics
William Optics Zenithstar 73
Sold as OTA only — no mount, eyepiece, diagonal, or finder included; total cost of a working setup is significantly higher than the £599 OTA price.
The Flat73A field flattener is essentially mandatory for imaging; without it, stars towards field edges show noticeable coma and curvature.
As an ED doublet rather than a triplet, some residual chromatic aberration may be visible on very bright stars in images; the 2-inch rack-and-pinion focuser can show minor flexure under heavier camera and filter train loads without careful balancing.
Which is right for you?
Two different buyers. Two different right answers.
The custom-rig optical tube
William Optics · William Optics Zenithstar 61
You're an intermediate astrophotographer who prioritises portability and light weight, willing to spend an extra £250 on the field flattener because you're already committed to wide-field nebula and Milky Way imaging; you'll never use this visually beyond sweeping the Orion Nebula with binoculars-level views, and you're comfortable with the smallest aperture in this class because your images prove its worth.
The custom-rig optical tube
William Optics · William Optics Zenithstar 73
You want the widest possible field of view for large emission nebulae and galaxy groups without abandoning visual capability entirely — the 73mm aperture gives you enough light to enjoy the Moon and bright nebulae at the eyepiece, and you're willing to pay £600 for the OTA because you know the Flat73A flattener will complete the system; you're not a planetary observer, but you want a telescope that doesn't feel like a purely one-dimensional tool.
Our verdict
These two are closer than most comparisons on this site. The spec differences are genuine — mount type, focal ratio — but neither is the wrong answer for a typical observer starting out.
If I had to choose between them: the William Optics Zenithstar 61 is the scope most people will be using regularly six months from now. The William Optics Zenithstar 73 rewards you more once you know what you're doing — it's worth revisiting after your first year.
William Optics Zenithstar 61
View William Optics Zenithstar 61 →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 | William Optics Zenithstar 61 | William Optics Zenithstar 73 |
|---|---|---|
Apertureⓘ The most important spec — bigger = more light = better views | 61mm | 73mm |
Focal Length Longer = more magnification potential | 360mm | 430mm |
Focal Ratio Lower f-number = wider field of view; higher = more magnification per eyepiece | f/5.9 | 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 FMC on all air-to-glass surfaces, including ED element | Fully multi-coated FMC ED doublet on all air-to-glass surfaces |
How do you point it?
| Spec | William Optics Zenithstar 61 | 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 | William Optics Zenithstar 61 | William Optics Zenithstar 73 |
|---|---|---|
Focuser Size 2" accepts wider eyepieces and gives better low-power views | 2" / 1.25" | 2" / 1.25" |
Focuser Type Rack-and-pinion is standard; Crayford and dual-speed are smoother | Dual-speed Crayford 2" (10:1 reduction fine focus) | Dual-speed Crayford 2" (10:1 reduction fine focus) |
Size & weight
| Spec | William Optics Zenithstar 61 | William Optics Zenithstar 73 |
|---|---|---|
OTA Weightⓘ Optical tube only — useful for comparing mount load capacity | 1.35kg | 1.75kg |
Tube Length | 270mm | 320mm |
Tube Material | Aluminium, anodised red | Aluminium, anodised blue |
What's in the box?
| Spec | William Optics Zenithstar 61 | William Optics Zenithstar 73 |
|---|---|---|
Diagonal Tilts the eyepiece 90° for comfortable viewing — useful on refractors |
Blue highlight: William Optics Zenithstar 61 advantage · Amber highlight: William Optics Zenithstar 73 advantage · Greyed cells: equal or subjective.