William Optics Fluorostar 91

What the Fluorostar 91 Is For

The FLT91 is a precision-engineered apochromatic triplet refractor for discerning astronomers and astrophotographers, a mid-sized telescope whose features provide stability for both visual and photographic use.

At 540mm it suits medium-scale deep-sky imaging and, with its RotoLock visual back and extension tube in the box, visual observing.

FPL-53 Triplet with a Strehl Ratio Over 0.95

It is a 91mm apochromatic triplet with FPL-53 glass, delivering sharp, high-contrast images and a Strehl ratio exceeding 0.95.

The manual describes a new lens cell design with FPL-53 glass in the WO triplet series (GT and FLT), designed for temperature compensation to better prevent pinched optics in cold weather.

91mm Aperture and 540mm Focal Length

A 91mm objective has a Dawes limit of about 1.27 arcseconds. At 540mm a full-frame sensor covers about 3.8 x 2.5 degrees, and APS-C (about 23.5 x 15.6mm) about 2.5 x 1.7 degrees. With the Flat6AIII (0.8x, about 432mm and f/4.7) full frame covers about 4.8 x 3.2 degrees.

Image Circle

The image circle is greater than 43mm. A full-frame sensor's diagonal is 43.3mm, at the edge of that figure; APS-C sits well inside. For flat stars on a large sensor, use one of the matching flatteners (below).

The 3.3-inch Dual-Speed Rack-and-Pinion Focuser

The focuser is a dual speed 3.3-inch rack-and-pinion with a 10:1 ratio fine knob and cap, a focuser adjusting knob with a thermometer, and a focuser rotation locking screw.

• Lock: do not over-tighten the focuser lock screw, and release it when focusing.
• Smoothness: push and pull gear spacing screws (3mm and 2mm Allen keys), adjusted only when necessary.
• EAF: the gear spacing screws can be removed to attach a motor such as an EAF (electronic automatic focuser): remove the top screw (2mm Allen key), release the two inner knob screws, remove the knob, align the screw holes, lock the two set screws on the shaft and tighten both bracket screws. With a motor fitted, do not over-tighten the tension screw.

M92 Threads and Camera Angle Rotator

The focuser ends in an M92 thread. In the box, a visual extension tube and the 2-inch RotoLock visual back screw onto it. For imaging, unscrew the extension tube and screw the flattener onto the M92 thread.

The 360° field rotator turns the camera for framing: release the camera angle rotation locking screw, turn the camera, then lock it again.

Flatteners William Optics Names

The FLT91 is compatible with the Flat 6AIII, Flat 68III and Ultra Flat 91, and allows precise back-focus adjustments. The manual shows spot diagrams for two of them:

• P-FLAT6AIII-M92 (0.8x): about 432mm and f/4.7; screws onto the focuser's M92 thread.
• P-FLAT68III (1x): keeps 540mm and f/5.9; screws onto the M92 thread, with an optional longer adjusting adapter for the FLT91.
• Ultra Flat 91: named as compatible; ask us for details.

Visual Use: Extension Tube and 2-inch RotoLock

The manual's accessories map shows the visual path: extension tube and 2-inch RotoLock, then a 2-inch diagonal and eyepiece, or a 2-inch to 1.25-inch adapter with a 1.25-inch diagonal or erecting prism. At 540mm, a 25mm eyepiece gives about 22x and a 7mm about 77x.

Dew Shield, Bahtinov Mask and Mounting Hardware

• Bahtinov mask: in the lens cover, marked patented; the front cover threads on at M121.
• Dew shield: retractable, 122mm across, with a lock screw.
• Mounting: mounting rings with knobs on a long version Vixen-style dovetail plate.
• 120mm saddle handle bar: M4 locking screws and M4 and M6 screw holes; remove the two bracket holder screws (2.5mm Allen key) before fitting a finder bracket.

Weight and Length

The manual gives 4.165 kg (9.18 lbs) for the optical tube and 5.575 kg (12.29 lbs) with ring, dovetail and saddle; William Optics' page rounds these to 4.2 and 5.6 kg. Length is 441mm retracted and 510mm extended. A soft carry case is included. Add your camera and compare the total with your mount's rated payload.

Care and Storage

Before storing the telescope, leave it uncovered with the tube open long enough to adjust to room temperature, and make sure it is completely dry. Store it in a dry place with a consistent temperature.

Never aim the telescope or finder at the Sun without proper solar filters installed on the front of the telescope; doing so for even a moment may permanently damage your vision. Solar eyepiece filters are not considered safe.