Telescope Comparison
William Optics RedCat 51 vs William Optics Zenithstar 61
The specs are close. The experience isn't.
First light
William Optics · 51mm · £599
The custom-rig optical tube
- 51mm refractor — optical tube only, no mount included
- 250mm focal length at f/4.9
- 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 · 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
The full picture
The numbers that separate these two scopes — and what they mean at the eyepiece.
Aperture
William Optics Zenithstar 61 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 61's longer focal length reaches higher magnification with the same eyepiece — better reach for planetary detail. William Optics RedCat 51's shorter focal length gives a wider true field — better for large open clusters and extended nebulae.
Focal ratio
William Optics RedCat 51's faster f/4.9 delivers wider fields with any eyepiece — better for open clusters and large nebulae. William Optics Zenithstar 61'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 | William Optics RedCat 51 | William Optics Zenithstar 61 |
|---|---|---|
| Planets | ||
| Moon | Moderate 51mm aperture shows craters and maria, but 250mm focal length means very small image scale with no high-magnification reach | Moderate 61mm aperture shows craters and maria, but the short 360mm focal length limits useful magnification for fine detail |
| Saturn | Challenging Rings barely distinguishable from the disc at 250mm focal length; too small and short for any useful detail | Challenging Rings visible as distinct structure, but 61mm aperture and 360mm focal length cannot reveal Cassini Division or banding |
| Jupiter | Challenging Small disc visible with no cloud band detail at this aperture and focal length | Challenging Disc and two main equatorial belts visible, but small aperture limits cloud detail and the short focal length keeps the image very small |
| Mars | Not recommended At 51mm aperture and 250mm focal length, Mars is a tiny featureless orange dot even at opposition | Not recommended Tiny orange disc at opposition; 61mm aperture and 360mm focal length cannot resolve surface features |
Deep sky | ||
| Orion Nebula (M42) | Moderate Core visible visually but 51mm aperture limits nebulosity; as an imaging target at f/4.9 it is superb with wide-field framing | 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 |
| Andromeda Galaxy (M31) | Excellent 250mm focal length frames the entire galaxy with companion galaxies — an ideal imaging target at this field of view | Excellent 360mm focal length captures the full extent of M31 and companion galaxies in a single field — ideal framing for imaging |
| Open clusters | Excellent Very wide field at 250mm captures large clusters like the Double Cluster and Pleiades beautifully in full context | Excellent 360mm focal length gives wide true field, perfectly suited for large clusters like the Double Cluster, Pleiades, and Hyades |
| Globular clusters | Challenging 51mm aperture cannot resolve individual stars; globulars appear as small fuzzy patches | Challenging 61mm aperture shows fuzzy patches only; no star resolution possible even at the edges |
| Faint galaxies | Not recommended 51mm aperture gathers too little light; visually invisible, and even imaged they are tiny at 250mm focal length | Not recommended 61mm aperture gathers too little light to reveal faint galaxy detail visually |
| Milky Way / wide field | Excellent 250mm at f/4.9 is an ideal combination for sweeping Milky Way panoramas and large-scale star fields | Excellent 360mm focal length at f/5.9 delivers sweeping star fields — one of this scope's strengths both visually and for imaging |
Other | ||
| Double stars | Moderate 51mm aperture limits resolving power to about 2.3 arcseconds; only wide doubles splittable, and short focal length makes them hard to see | 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 |
| Large emission nebulae (imaging) | Excellent The RedCat 51's core strength — Heart, Soul, California, North America, and Veil nebulae all fit the wide field at f/4.9 | 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 |
| Astrophotography (planetary) | Challenging 51mm aperture and 250mm focal length produce tiny planetary image scales unsuitable for meaningful detail | Challenging 61mm aperture and 360mm focal length produce a very small planetary image scale; no tracking included |
| Astrophotography (deep sky) | Not applicable | 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 |
The real tradeoff
Both scopes are capable. The question is which one fits the way you actually observe.
William Optics RedCat 51
- You're buying a specialist wide-field imaging lens that demands a separate equatorial mount and astronomy camera before you can observe anything — budget accordingly for the full system.
- Your observing sessions reward speed and portability: at 1.35kg, you'll pack this OTA into a travel rig with a lightweight tracker and capture sweeping Milky Way fields or entire nebula complexes in single exposures without worrying about guiding precision.
- You'll appreciate the built-in field flattener that works across the full frame immediately, but you'll spend time stacking focus adapters carefully — get the back-focus spacing wrong and field curvature ruins your images.
William Optics Zenithstar 61
- You're buying a 61mm imaging refractor that can theoretically show you the Moon's craters and Saturn's rings visually, but you'll quickly realize the short focal length limits detail and the small aperture punishes higher magnifications — it's really an imaging scope that happens to accept an eyepiece.
- Your observing sessions reward patience with wide-field targets: the enormous true field of view makes it effortless to frame entire nebula complexes or Milky Way sweeps, and the apochromatic optics deliver clean star colours without chromatic fringing.
- You'll need to budget for a dedicated field flattener (roughly £250 more) to eliminate edge curvature on modern sensors, and you'll appreciate the slightly longer 360mm focal length gives you more imaging scale and slightly better visual performance than the RedCat — but you're still fundamentally limited by aperture.
The dark side
Every scope has a personality. Here’s where each one gets difficult.
William Optics
William Optics RedCat 51
No mount included — you must purchase a separate equatorial mount or star tracker, adding £300–£800+ to your total cost before first light.
The 51mm aperture is entirely unsuitable for visual observing; the Moon appears bright but tiny, planets are featureless discs, and deep-sky objects are invisible.
The 48mm imaging circle vignettes on full-frame sensors in the extreme corners; the helical focuser has limited travel and requires a focus motor or Bahtinov mask for precise camera focusing; back-focus spacing is critical and incorrect adapter stacking introduces field curvature.
William Optics
William Optics Zenithstar 61
OTA only — no mount, no diagonal, no eyepiece included; significant additional investment required before any observing is possible.
Field curvature is visible at the edges of APS-C and full-frame sensors without a dedicated field flattener (the Flat6A costs roughly £250 and is nearly essential for imaging).
The 61mm aperture severely limits visual performance; the maximum useful magnification of ~120× restricts planetary and lunar detail, and the scope is entirely unsuitable as a primary visual telescope for any observer seeking meaningful resolution.
Which is right for you?
Two different buyers. Two different right answers.
The custom-rig optical tube
William Optics · William Optics RedCat 51
You'll love the RedCat 51 if you're an experienced astrophotographer who already owns an equatorial mount and camera, travel frequently to dark skies, and want an ultraportable wide-field imaging platform optimized for large nebulae and Milky Way mosaics without the weight penalty of a longer focal length. This scope rewards photographers willing to prioritize speed and portability over aperture and flexibility.
The custom-rig optical tube
William Optics · William Optics Zenithstar 61
This scope is right for you if you're an intermediate astrophotographer seeking a dedicated, high-quality imaging OTA with slightly more focal length and aperture than the RedCat, don't mind the extra cost of a field flattener, and value apochromatic optics that eliminate chromatic fringing. This isn't for you if you expect a ready-to-use telescope, want meaningful visual performance, or need aperture for faint-object detection — you'll find a 6-inch Dobsonian a far better value for either goal.
Our verdict
At similar price points, these scopes offer different amounts of aperture per pound. The William Optics Zenithstar 61 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 William Optics Zenithstar 61 is the stronger pick. The William Optics RedCat 51 compensates with other features — decide whether those trade-offs justify the premium. If I had to choose: the William Optics Zenithstar 61 — more aperture per pound means more sky.
William Optics RedCat 51
View William Optics RedCat 51 →William Optics Zenithstar 61
View William Optics Zenithstar 61 →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 RedCat 51 | William Optics Zenithstar 61 |
|---|---|---|
Apertureⓘ The most important spec — bigger = more light = better views | 51mm | 61mm |
Focal Length Longer = more magnification potential | 250mm | 360mm |
Focal Ratio Lower f-number = wider field of view; higher = more magnification per eyepiece | f/4.9 | 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 Petzval design on all surfaces | Fully multi-coated FMC on all air-to-glass surfaces, including ED element |
How do you point it?
| Spec | William Optics RedCat 51 | William Optics Zenithstar 61 |
|---|---|---|
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 RedCat 51 | William Optics Zenithstar 61 |
|---|---|---|
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 with micro-focuser | Dual-speed Crayford 2" (10:1 reduction fine focus) |
Size & weight
| Spec | William Optics RedCat 51 | William Optics Zenithstar 61 |
|---|---|---|
OTA Weight Optical tube only — useful for comparing mount load capacity | 1.35kg | 1.35kg |
Tube Length | 232mm | 270mm |
Tube Material | Aluminium, anodised red | Aluminium, anodised red |
What's in the box?
| Spec | William Optics RedCat 51 | William Optics Zenithstar 61 |
|---|---|---|
Diagonal Tilts the eyepiece 90° for comfortable viewing — useful on refractors |
Blue highlight: William Optics RedCat 51 advantage · Amber highlight: William Optics Zenithstar 61 advantage · Greyed cells: equal or subjective.