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William Optics
William Optics SpaceCat 51
The SpaceCat 51 is a 51mm f/4.9 astrograph with a field flattener built in to the focuser drawtube — point it at the sky, attach a camera, and you get flat, sharp stars across the full frame from the first session. At 250mm focal length, the field of view is wide: the Orion Nebula and running man nebula fit together, the North America Nebula fills the frame nicely, and the Milky Way core is sweeping and dramatic. Weighing 1.5kg, it pairs naturally with compact star trackers like the Sky-Watcher Star Adventurer or iOptron SkyGuider Pro. For travel astrophotographers and Milky Way landscape imagers, the SpaceCat 51 is one of the best small dedicated astrographs available.
Product image
What you'll see
No data available. The forum threads provided do not contain visual observing reports for the William Optics SpaceCat 51. Thread 1 discusses electronic focuser compatibility and technical specifications. Thread 2 (RedCat 51, not SpaceCat) focuses on astrophotography imaging results, optical sharpness comparisons to camera lenses, and equipment adapters—not observational targets or viewing experiences. To extract what owners see through the SpaceCat 51's eyepiece would require discussions specifically about eyepieces, visual targets, and observational impressions, which are not present in these threads.
Worth knowing before you buy
The focuser can be stiff out of the box, though it may loosen with use over time.
Canon EOS camera adapters have excessive play with approximately 15 degrees of movement once mounted, unlike the well-fitting Sony E-mount adapter.
Some users report coma-like star aberrations and slight diffraction spikes in images, with concerns about whether the lens lives up to marketing claims.
Head to head
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Full Specifications
Optics
| Aperture | 51mm |
| Focal Length | 250mm |
| Focal Ratio | f/4.9 |
| Optical Design | Refractor |
| Coatings | Fully multi-coated apochromatic triplet (FPL-53) |
Mount & Tracking
| GoTo (Computerised) | No |
| Tracking | No |
Focuser
| Focuser Size | 2" |
| Focuser Type | Dual-speed Crayford with field flattener integrated |
Physical
| OTA Weight | 1.5kg |
| Tube Length | 222mm |
| Tube Material | Aluminium |
Included Accessories
| Diagonal | No |