Telescope Comparison
Askar FMA180 Pro vs William Optics SpaceCat 51
The specs are close. The experience isn't.
First light
Askar · 40mm · £299
The custom-rig optical tube
- 40mm refractor — optical tube only, no mount included
- 180mm focal length at f/4.5
- 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 · 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
William Optics SpaceCat 51 gathers 1.6× 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 SpaceCat 51's longer focal length reaches higher magnification with the same eyepiece — better reach for planetary detail. Askar FMA180 Pro's shorter focal length gives a wider true field — better for large open clusters and extended nebulae.
Focal ratio
Askar FMA180 Pro's faster f/4.5 delivers wider fields with any eyepiece — better for open clusters and large nebulae. William Optics SpaceCat 51's f/4.9 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)
Askar FMA180 Pro's optical tube is 1.1kg 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
Askar
Askar FMA180 Pro
Saturn's rings are clearly visible as a distinct shape around the planet; Jupiter shows a disc with two cloud bands. The Moon is an excellent target with clear crater and highland detail at moderate power. The Orion Nebula (M42) is visible as a bright, distinct patch with the Trapezium as a tight cluster. Open clusters are a strength — the Pleiades, the Beehive (M44), the Hyades fill a wide-field eyepiece well. The fast focal ratio delivers wide fields — good for large nebulae and extended star fields.
William Optics
William Optics SpaceCat 51
Saturn's rings are clearly visible as a distinct shape around the planet; Jupiter shows a disc with two cloud bands. The Moon is an excellent target with clear crater and highland detail at moderate power. The Orion Nebula (M42) is visible as a bright, distinct patch with the Trapezium as a tight cluster. Open clusters are a strength — the Pleiades, the Beehive (M44), the Hyades fill a wide-field eyepiece well. The fast focal ratio delivers wide fields — good for large nebulae and extended star fields. The William Optics SpaceCat 51 gathers 1.6× more light than the Askar FMA180 Pro — a difference that's marginal on bright targets but visible on fainter ones: dimmer galaxies, faint globular clusters, and extended nebulosity that sits below the threshold of the smaller aperture.
The real tradeoff
Both scopes are capable. The question is which one fits the way you actually observe.
The William Optics SpaceCat 51 costs 50% more. It delivers 11mm more aperture — a real and visible advantage on faint targets. For a first telescope, the Askar FMA180 Pro is the smarter entry point. Return to the William Optics SpaceCat 51 when you know from experience what you actually need.
The dark side
Every scope has a personality. Here’s where each one gets difficult.
Askar
Askar FMA180 Pro
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
Askar · Askar FMA180 Pro
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
At £299 versus £449, the William Optics SpaceCat 51 costs 50% more. It delivers 11mm more aperture — a real and visible advantage on faint targets.
If budget is a genuine constraint, the Askar FMA180 Pro will make you a happy observer. The William Optics SpaceCat 51'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 Askar FMA180 Pro, use it for a year, then upgrade knowing exactly what you want.
Askar FMA180 Pro
View Askar FMA180 Pro →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 | Askar FMA180 Pro | William Optics SpaceCat 51 |
|---|---|---|
Apertureⓘ The most important spec — bigger = more light = better views | 40mm | 51mm |
Focal Length Longer = more magnification potential | 180mm | 250mm |
Focal Ratio Lower f-number = wider field of view; higher = more magnification per eyepiece | f/4.5 | 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 ED triplet | Fully multi-coated apochromatic triplet (FPL-53) |
How do you point it?
| Spec | Askar FMA180 Pro | 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 | Askar FMA180 Pro | William Optics SpaceCat 51 |
|---|---|---|
Focuser Size 2" accepts wider eyepieces and gives better low-power views | 1.25" | 2" |
Focuser Type Rack-and-pinion is standard; Crayford and dual-speed are smoother | Helical focuser | Dual-speed Crayford with field flattener integrated |
Size & weight
| Spec | Askar FMA180 Pro | William Optics SpaceCat 51 |
|---|---|---|
OTA Weightⓘ Optical tube only — useful for comparing mount load capacity | 0.38kg | 1.5kg |
Tube Length | 115mm | 222mm |
Tube Material | Aluminium | Aluminium |
What's in the box?
| Spec | Askar FMA180 Pro | William Optics SpaceCat 51 |
|---|---|---|
Diagonal Tilts the eyepiece 90° for comfortable viewing — useful on refractors |
Blue highlight: Askar FMA180 Pro advantage · Amber highlight: William Optics SpaceCat 51 advantage · Greyed cells: equal or subjective.