William Optics UltraCat 131 WIFD — 131 mm Petzval astrograph The 5-element optical system provides a 648 mm focal length at f/4.9, with a 60 mm image circle. Its Petzval design does not require an external field flattener.
Manufacturer model: William Optics UltraCat 131 WIFD · Specifications checked against manufacturer data on 2026-11-03
William Optics UltraCat 131 WIFD — 131 mm Petzval astrograph
The 5-element optical system provides a 648 mm focal length at f/4.9, with a 60 mm image circle. Its Petzval design does not require an external field flattener.
Focus and camera alignment
The WIFD rack-and-pinion focuser provides 20 mm travel and 1:10 fine adjustment. A camera rotator controls framing, while the Sensor Tilt Xterminator allows sensor alignment. The lens cover incorporates a Bahtinov focusing mask.
Imaging connections
STX-R tilt adjuster / rotator; M79 to M68, M54 and M48 adapters. Petzval focus depends on the complete camera train remaining within the telescope's focusing range.
Size and mounting
The tube length is 650–785.5 mm. OTA weight is 11.3 kg and the manufacturer lists 13.97 kg total weight. The mounting plate is 400 mm Losmandy-style.
William Optics specifies two Super ED elements, one ED element, one lanthanum element and one X-element in the five-element optical system. The internal focuser moves the focusing mechanism within the telescope, while the rear tilt and rotation adjustments support sensor alignment and framing.
Compatibility notes
Petzval focus range must accommodate the complete imaging train; do not interpret recommended camera spacing as an exact flattener backfocus. Confirm supplied adapters and revision.
90872
Data sheet
Aperture (mm)
131
Camera adapters
STX-R tilt adjuster / rotator; M79 to M68, M54 and M48 adapters
Camera format
Medium format, full frame, APS-C and Micro Four Thirds
Camera rotation
Camera angle rotator
Electronic focus
ZWO EAF compatible; use the manufacturer installation guide for the selected EAF model.
External field flattener required
No
Fine focus ratio
1:10
Focal length (mm)
648
Focal ratio
4.9
Focuser
WIFD rack-and-pinion
Focuser travel (mm)
20
Focusing mask
Bahtinov mask incorporated in lens cover
Image circle (mm)
60
Lens elements
2 Super ED, 1 ED, 1 lanthanum and 1 X-element (5 elements total)
Mounting plate
400 mm Losmandy-style
Optical design
5-element Petzval
OTA weight (kg)
11.3
Sensor tilt adjustment
Sensor Tilt Xterminator
Total weight (kg)
13.97
Tube length (mm)
650–785.5
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William Optics UltraCat 131 WIFD — 131 mm Petzval astrograph The 5-element optical system provides a 648 mm focal length at f/4.9, with a 60 mm image circle. Its Petzval design does not require an external field flattener.
Manufacturer model: William Optics UltraCat 131 WIFD · Specifications checked against manufacturer data on 2026-11-03
