William Optics UltraCat 131
William Optics
William Optics UltraCat 131 is a 131 mm Advanced Petzval APO astrograph for deeper refractor imaging, smaller galaxies, and high-quality visual observing.
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Specifications
Optics
- Optical design
- Advanced Petzval APO astrograph
- Aperture
- 131 mm
- Focal length
- 648 mm
- Focal ratio
- f/4.9
- Dawes limit
- 0.89"
- Glass / elements
- 5-element advanced Petzval with Super ED, ED, lanthanum, and X-element glass
Imaging
- Image circle
- 60 mm
- Camera format
- Medium format / full frame / APS-C / Micro 4/3
- Full-frame coverage
- Yes
- Back focus
- No flattener required
- Corrected field
- Native medium-format flat field
Mechanical
- Focuser
- WIFD rack-and-pinion internal focuser
- Weight
- 11.3 kg / 24.91 lb
- Tube length
- 650-785.5 mm
- Dovetail
- Losmandy-style dovetail
Astrophotography Fit
Best for deeper refractor imaging, smaller galaxies, and high-quality visual observing when the mount, camera format, and correction hardware are chosen as one system.
William Optics UltraCat 131 sits in the telescope slot for deeper refractor imaging, smaller galaxies, and high-quality visual observing. The useful headline specs are aperture 131 mm, focal length 648 mm, f/4.9.
Camera coverage: Medium format / full frame / APS-C / Micro 4/3. Image circle: 60 mm. Back focus: No flattener required.
Fast astrographs collect signal quickly, but they are unforgiving: collimation, tilt, filter behavior, and back-focus spacing show up immediately in star shapes.
Weight matters here: 11.3 kg / 24.91 lb. Leave payload headroom for the camera, guider, focuser, dew control, and cables instead of judging the mount by bare tube weight alone.
Best paired with
- Rigid focuser, tilt adjuster, and exact corrector spacing
- Cooled cameras with a sensor inside the corrected image circle
- High-throughput filters that behave well at fast focal ratios
Not recommended for
- Users who do not want to tune collimation, tilt, and spacing
- Thick or narrow filters that show strong bandpass shift at fast focal ratios
- Loose camera adapters or focusers that cannot hold the imaging train square
Where To Buy
Some links on this page are affiliate links. Prices and availability are pulled automatically from each retailer and updated daily.
| Retailer | Price | Availability | View Offer |
|---|---|---|---|
| Cloud Break Optics Cloud Break Optics | $5,398.00 | In stock Updated 8/5/2026 | View Offer - $5,398.00 |
Price History
Chronological retailer pricing with out-of-stock points shown at reduced opacity.