backfocus.tools

Fluorostar 91 back focus

Aperture
91 mm
Focal length
540 mm at f/5.9
Image circle
43 mm
Rear thread
M92
Corrector
William Optics FLAT6AIII

Checked against the manufacturer’s documentation in September 2026.

Where to buy the William Optics Fluorostar 91

BuyHigh Point Scientific · US

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Cameras

CameraBack focusSpacersResult
Ares-C Pro60.4 mm26.4 mmExact
Artemis-C Pro60.4 mm42.9 mmExact
Neptune-C II60.4 mm31.4 mmExact
Poseidon-C Pro60.4 mm42.9 mmExact
Saturn-C SQR60.4 mm31.4 mmExact
Uranus-C60.4 mm31.4 mmExact
QHY183C60.4 mm26.4 mmExact
QHY183M60.4 mm26.4 mmExact
QHY268C (SBFL - Jan 2023 onward)60.4 mm25.0 mmWithin tolerance
QHY268M (SBFL)60.4 mm25.0 mmWithin tolerance
QHY294M Pro60.4 mm26.4 mmExact
QHY533C60.4 mm26.9 mmExact
QHY533M60.4 mm46.4 mmExact
QHY600M (SBFL)60.4 mm25.0 mmWithin tolerance
ASI071MC Pro60.4 mm26.4 mmExact
ASI1600MM Pro60.4 mm37.4 mmExact
ASI183MC Pro60.4 mm37.4 mmExact
ASI183MM Pro60.4 mm37.4 mmExact
ASI2400MC Pro60.4 mm21.9 mmExact
ASI2600MC Duo60.4 mm21.9 mmExact
ASI2600MC Pro60.4 mm26.4 mmExact
ASI2600MM Duo60.4 mm21.9 mmExact
ASI2600MM Pro60.4 mm26.4 mmExact
ASI294MC Pro60.4 mm37.4 mmExact
ASI294MM Pro60.4 mm37.4 mmExact
ASI533MC Pro60.4 mm37.4 mmExact
ASI533MM Pro60.4 mm37.4 mmExact
ASI585MC Pro60.4 mm37.4 mmExact
ASI585MM Pro60.4 mm37.4 mmExact
ASI6200MC Pro60.4 mm21.9 mmExact
ASI6200MM Pro60.4 mm21.9 mmExact