William Optics GreyCat 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 GreyCat 91 WIFD OTA · Specifications checked against manufacturer data on 2026-11-03
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.
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.
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.
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 manufacturer manual
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.
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.
| Maximum m48 backspacing (mm) | 60.8 |
|---|---|
| Maximum m54 backspacing (mm) | 65.8 |
Data sheet
William Optics GreyCat 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 GreyCat 91 WIFD OTA · Specifications checked against manufacturer data on 2026-11-03
