William Optics RedCat 91 WIFD OTA — 91 mm Petzval astrograph The 5-element optical system provides a 448 mm focal length at f/4.9, with a 55 mm image circle. Its Petzval design does not require an external field flattener.
Manufacturer model: William Optics RedCat 91 WIFD OTA · Specifications checked against manufacturer data on 2026-11-03
William Optics RedCat 91 WIFD OTA — 91 mm Petzval astrograph
The 5-element optical system provides a 448 mm focal length at f/4.9, with a 55 mm image circle. Its Petzval design does not require an external field flattener.
Focus and camera alignment
The WIFD rack-and-pinion focuser provides 36 mm travel and 1:10 fine adjustment. A camera rotator controls framing, while a sensor tilt adjuster allows sensor alignment. The lens cover incorporates a Bahtinov focusing mask.
Imaging connections
Tilt adjuster; camera rotator; M63 to M48 and M63 to M54 adapters. The M48 adapter incorporates an internal filter thread. Petzval focus depends on the complete camera train remaining within the telescope's focusing range.
Size and mounting
The tube length is 556 mm. OTA weight is 4.72 kg and the manufacturer lists 6.3 kg total weight. The mounting plate is Vixen-style PV-LII.
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.
Additional specifications
Maximum m48 backspacing (mm)
60.8
Maximum m54 backspacing (mm)
65.8
85222
Data sheet
Aperture (mm)
91
Camera adapters
Tilt adjuster; camera rotator; M63 to M48 and M63 to M54 adapters
Camera format
Full frame, APS-C and Micro Four Thirds; medium format needs additional adapter
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)
448
Focal ratio
4.9
Focuser
WIFD rack-and-pinion
Focuser travel (mm)
36
Focusing mask
Bahtinov mask incorporated in lens cover
Image circle (mm)
55
Internal m48 filter thread
Yes
Lens elements
2 Super ED, 1 ED, 1 lanthanum and 1 X-element (5 elements total)
Mounting plate
Vixen-style PV-LII
Optical design
5-element Petzval
OTA weight (kg)
4.72
Sensor tilt adjustment
Sensor tilt adjuster
Total weight (kg)
6.3
Tube length (mm)
556
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William Optics RedCat 91 WIFD OTA — 91 mm Petzval astrograph The 5-element optical system provides a 448 mm focal length at f/4.9, with a 55 mm image circle. Its Petzval design does not require an external field flattener.
Manufacturer model: William Optics RedCat 91 WIFD OTA · Specifications checked against manufacturer data on 2026-11-03
