Telescope Comparison
William Optics RedCat 51 vs William Optics SpaceCat 51
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 · 51mm · £449
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
The full picture
The numbers that separate these two scopes — and what they mean at the eyepiece.
Aperture
Equal light-gathering. Aperture won't settle this comparison — the mount, focal ratio, and observing experience are what differ.
Focal length
Same focal length — identical magnification with any given eyepiece. Differences come from optical design and coatings.
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
Both scopes · same aperture
Both refractors share essentially the same aperture — views through each will be very similar on all standard targets. The hallmarks of good refractor optics are sharp stars and good contrast on planetary targets, with no false colour on ED or apochromatic glass. Saturn's rings are distinct from the disk; Jupiter shows two equatorial bands. The Orion Nebula (M42) is bright and well-defined. Open clusters are a strength — the Double Cluster in Perseus and the Pleiades look good at low power. The differences between these two scopes show up in focal ratio, focal length, and what they're optimised for, not in fundamental light-gathering capability.
The real tradeoff
Both scopes are capable. The question is which one fits the way you actually observe.
Both scopes are solving a similar problem in a similar way. The differences are real — build quality and optical refinement — but these show up after several months of regular use, not on the first night. Pick the one whose design best matches how you actually plan to observe.
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 cannot observe until you buy a separate compatible mount — add at least £100–300 before you have a working telescope.
Nothing to look through on day one
Until a mount arrives, the optical tube is a piece of glass you cannot point at the sky.
William Optics
William Optics SpaceCat 51
No mount included
You cannot observe until you buy a separate compatible mount — add at least £100–300 before you have a working telescope.
Nothing to look through on day one
Until a mount arrives, the optical tube is a piece of glass you cannot point at the sky.
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 this if…
- You already own a compatible equatorial or alt-az mount — this is the optical tube you've specifically chosen to put on it
- You're building an imaging rig piece by piece and know exactly what you need at the end of a focuser
- Choosing an optical tube independently of the mount gives you more flexibility over your overall system
This will frustrate you if…
- You buy it without fully accounting for the mount — add at least £100–300 to the purchase price before you have a working telescope
- You expected a complete package and didn't realise this is a bare optical tube that cannot be used without a separate mount
The custom-rig optical tube
William Optics · William Optics SpaceCat 51
You’ll love this if…
- You already own a compatible equatorial or alt-az mount — this is the optical tube you've specifically chosen to put on it
- You're building an imaging rig piece by piece and know exactly what you need at the end of a focuser
- Choosing an optical tube independently of the mount gives you more flexibility over your overall system
This will frustrate you if…
- You buy it without fully accounting for the mount — add at least £100–300 to the purchase price before you have a working telescope
- You expected a complete package and didn't realise this is a bare optical tube that cannot be used without a separate mount
Our verdict
Same aperture, same light-gathering, £150 price difference. The extra cost of the William Optics RedCat 51 buys a different mount — not better optics.
For most beginners, the William Optics SpaceCat 51 is the right starting point — the optics are identical and the savings are better spent on a quality eyepiece or a dark-sky trip. The William Optics RedCat 51 makes sense if the mount it comes with is specifically what you want to learn. If I had to choose: the William Optics SpaceCat 51 — same sky, less money.
William Optics RedCat 51
View William Optics RedCat 51 →William Optics SpaceCat 51
View William Optics SpaceCat 51 →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 SpaceCat 51 |
|---|---|---|
Aperture The most important spec — bigger = more light = better views | 51mm | 51mm |
Focal Length Longer = more magnification potential | 250mm | 250mm |
Focal Ratio Lower f-number = wider field of view; higher = more magnification per eyepiece | f/4.9 | f/4.9 |
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 apochromatic triplet (FPL-53) |
How do you point it?
| Spec | William Optics RedCat 51 | William Optics SpaceCat 51 |
|---|---|---|
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 SpaceCat 51 |
|---|---|---|
Focuser Size 2" accepts wider eyepieces and gives better low-power views | 2" / 1.25" | 2" |
Focuser Type Rack-and-pinion is standard; Crayford and dual-speed are smoother | Dual-speed Crayford with micro-focuser | Dual-speed Crayford with field flattener integrated |
Size & weight
| Spec | William Optics RedCat 51 | William Optics SpaceCat 51 |
|---|---|---|
OTA Weightⓘ Optical tube only — useful for comparing mount load capacity | 1.35kg | 1.5kg |
Tube Length | 232mm | 222mm |
Tube Material | Aluminium, anodised red | Aluminium |
What's in the box?
| Spec | William Optics RedCat 51 | William Optics SpaceCat 51 |
|---|---|---|
Diagonal Tilts the eyepiece 90° for comfortable viewing — useful on refractors |
Blue highlight: William Optics RedCat 51 advantage · Amber highlight: William Optics SpaceCat 51 advantage · Greyed cells: equal or subjective.