William Optics Ultra-Cat 56
The Ultra-Cat 56 is a high-performance 56mm apochromatic refractor, engineered for astrophotographers who demand exceptional portability without sacrificing optical precision, color accuracy, and wide-field performance.
Featuring a 5-element advanced Petzval optical design with super ED and lanthanum glass, paired with a fast f/4.8 focal ratio, the Ultra-Cat 56 delivers razor-sharp, perfectly flat images across a 50mm image circle. With a 269mm focal length, it is optimized for Full Frame, APS-C, and M4/3 sensors, and remains fully compatible with Medium Format cameras, making it an ideal ultra-wide-field imaging platform for deep-sky and large-scale nebula photography.
Key Features:
Advanced 5-Element Petzval Optics
Eliminates field curvature, ensuring sharp, edge-to-edge stars without additional flatteners.
50mm Large Image Circle
Fully supports Full Frame cameras, APS-C and M4/3 sensors for optimal performance. Compatible with Medium Format.
Fast f/4.8 Optics
Superior light-gathering capability, enabling deep-sky imaging with shorter exposure times.
WIFD (William Optics Internal Focuser Design)
Patented internal rack-and-pinion focuser ensures stable and precise focusing.
No vignetting or balance issues — ideal for astrophotography.
Compatible with EAF (Electronic Automatic Focuser) for precision focus control.
Sensor Tilt Xterminator (STX) & Rotator
Integrated tilt adjuster eliminates sensor misalignment, ensuring uniform image sharpness.
No Flattener Required
The optical design naturally corrects field curvature, simplifying setup.
Anti-Shrink Lens Cell Technology
Reduces thermal expansion effects, maintaining perfect optical alignment in any climate.
Patented Bahtinov Mask for Focusing
Built-in focusing aid ensures pinpoint star sharpness with ease.
Premium Build & Ergonomic Design
Lightweight yet robust (4.31 kg / 9.5 lbs) construction with a sleek anodized finish.
Patented handle bar design for enhanced portability and ease of use.
Versatile Camera Compatibility
Supports Canon EF, RF, Nikon F,Z, Sony E, Pentax K, MFT, and FUJI X mounts.
Technical Specifications
| Lens Type |
5-Element Advanced Petzval |
| Lens Elements |
Super ED Glass x2 ED Glass x1 Lanthanum Glass x1 X-Element x1 |
| Focal Length |
269 mm |
| Diameter |
56 mm |
| Focal Ratio |
f/4.8 |
| Image Circle |
50 mm |
| Focuser |
WIFD R&P Focuser Compatible with EAF |
| Focus Travel |
0–30 mm |
| Adapter Included |
Sensor Tilt Xterminator Rotator M54, M48 Camera Adapters |
| Optional Flattener |
No Flattener Required |
| T-Mount Support |
Canon EF, RF Nikon F, Z Sony E Pentax K MFT (M4/3) FUJI X |
| Camera Format |
Full Frame APS-C M4/3 |
| Dovetail Type |
Vixen ↔ Arca-Swiss |
| Tube Length |
296.3 mm – 348.3 mm |
| OTA Weight |
2.56 kg / 5.64 lbs |
| Total Weight |
3.21 kg / 7.08 lbs |
In the Box
- Main tube
- Bahtinov Cap
- M48, M54 Adapter with wrench
- Dual Side Plate
- Mounting Ring and Handle bar
- Soft Carry case
- Allen Key Set
Warranty & Manual
William Optics 2-year Warranty
