William Optics Fluorostar 91 — Space Gray The Fluorostar 91 is a 91 mm triplet apochromatic refractor with a 540 mm focal length and f/5.9 focal ratio. William Optics lists a native 43 mm image circle for this optical tube, with imaging configurations supporting full-frame, APS-C and Micro Four Thirds cameras.
Manufacturer model: William Optics Fluorostar 91 — Space Gray · Specifications checked against manufacturer data on 2026-11-03
The Fluorostar 91 is a 91 mm triplet apochromatic refractor with a 540 mm focal length and f/5.9 focal ratio. William Optics lists a native 43 mm image circle for this optical tube, with imaging configurations supporting full-frame, APS-C and Micro Four Thirds cameras.
A 3.3-inch rack-and-pinion focuser provides 95 mm travel, 1:10 fine focusing and electronic-focuser compatibility. The M92 rotator allows camera framing, while the M92 extender and M92 to 2-inch RotoLock form the manufacturer-listed connection system.
Flat68III 1.0×, Flat6AIII 0.8× and Ultra Flat91 0.79× are separately listed corrector options. The first two require the appropriate adapter. Manufacturer backfocus settings are 60.0 mm for Flat68III, 60.4 mm for Flat6AIII and 56.18 mm for Ultra Flat91; the table below preserves their individual adjustment and image-circle values.
The tube measures 441–510 mm with its retractable dew shield. William Optics gives an optical-tube weight of 4.16 kg and total weight of 5.57 kg. Listed equipment includes tube rings, HC120 handle, Vixen PV-LII plate, rotator, soft case and a lens cover with an integrated Bahtinov mask.
William Optics manufacturer documentation
Manufacturer-published named-model content completed. Optional correctors require their individually specified adapters and backfocus settings.
| Native image circle (mm) | 43 |
|---|---|
| Connections | M92 rotator; M92 extender; M92 to 2-inch RotoLock |
| Optical tube weight (kg) | 4.16 |
| Lens cover | Integrated Bahtinov mask |
| Supported camera formats | Full frame; APS-C; Micro Four Thirds |
| Optional correctors | Flat68III 1.0× with adapter; Flat6AIII 0.8× with adapter; Ultra Flat91 0.79× |
| Flat6aiii backfocus (mm) | 60.4 |
| Flat6aiii adjustment (mm) | 5.4 |
| Flat6aiii image circle (mm) | 44.4 |
| Flat68iii backfocus (mm) | 60 |
| Flat68iii adjustment (mm) | 5 |
| Flat68iii image circle (mm) | 43.4 |
| Ultra flat91 backfocus (mm) | 56.18 |
| Ultra flat91 adjustment (mm) | 1.18 |
| Ultra flat91 image circle (mm) | 43.2 |
| Listed equipment | Tube rings; HC120 handle; soft case; rotator; Bahtinov lens cover |
Data sheet
William Optics Fluorostar 91 — Space Gray The Fluorostar 91 is a 91 mm triplet apochromatic refractor with a 540 mm focal length and f/5.9 focal ratio. William Optics lists a native 43 mm image circle for this optical tube, with imaging configurations supporting full-frame, APS-C and Micro Four Thirds cameras.
Manufacturer model: William Optics Fluorostar 91 — Space Gray · Specifications checked against manufacturer data on 2026-11-03
