William Optics
William Optics Ultra-Cat 56
William Optics Ultra-Cat 56
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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.
Specification ;
- Lens Type
- 5-Element Advanced Petzval
- Lens Elements
- Super ED Glass x2 ED Glass x1 Lanthanum Glass x1 X-Element x1
- 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
- Focuser 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
- 269 mm
- Diameter
- 56 mm
- Focal Ratio
- f/4.8
- Image Circle
- 50 mm
- Focuser
- WIFD R&P Focuser Compatible with EAF
- Focuser 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
Diagram Reports;
What's in the box
Main Tube
Bahtinov Cap
M48, M54 Adapter with Wrench
Dual-side Plate
Mounting Ring & Handle bar
Soft Carry Case
Allen Key Set
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Yes. On top of the manufacturer's warranty, we also offer a Replacement Guarantee on all online orders. In the event that the product is defective or damaged during shipping, you will receive a replacement.
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We'll refund you 90% if you cancel before dispatch. Cancellations after dispatch are complicated. Take a look at the our Policy here
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We have strict return/refund policies as a company that deals with fragile products. Casual returns are not permitted. Items that are not damaged in transit or defective cannot be returned.So please take a moment to review our Refund Policy.
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