William Optics Fluorostar 120

A 120mm FPL-53 Triplet

The Fluorostar 120's triplet lens system uses FPL-53 glass for superior aberration correction and crystal-clear imaging.

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.

780mm at f/6.5

A 120mm objective has a Dawes limit of about 0.97 arcseconds. At 780mm a full-frame sensor covers about 2.6 x 1.8 degrees and APS-C (about 23.5 x 15.6mm) about 1.7 x 1.1 degrees. That scale suits galaxies, the brighter parts of nebulae and detailed lunar landscapes.

Image Circle

The manual gives the image circle as 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, use one of the flatteners in the manual (below).

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

This heavy-duty 3.3-inch rack-and-pinion focuser has a built-in rotator. It is dual speed, 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 (2mm Allen key) and pull (3mm) gear spacing screws, adjusted only when necessary.
• EAF: some of 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, Rotator and Visual Backs

The focuser ends in an M92 thread. In the box are an M92 rotator, an M92 to 2-inch RotoLock adapter and a 2-inch to 1.25-inch visual back.

• Camera angle rotator: release the camera angle rotation locking screw, turn the camera to frame your target, and lock it again.
• 2-inch RotoLock: holds a 2-inch diagonal or accessory.
• 2-inch to 1.25-inch visual back: for 1.25-inch accessories.

Optional Flat7A 0.8x and Flat68III 1x

The manual shows spot diagrams for two flatteners, neither included:

• P-FLAT68III (1x): keeps 780mm and f/6.5.
• P-FLAT7A (0.8x): about 624mm and f/5.2; full frame then covers about 3.3 x 2.2 degrees and each sub-frame needs about 64% of the native exposure.

The accessories map runs flattener, T-mount, DSLR or mirrorless camera; or flattener, the adapter that comes with your astronomy camera, camera.

Visual Use

The accessories map shows a 2-inch RotoLock with 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 780mm, a 25mm eyepiece gives about 31x and a 7mm about 111x.

Dew Shield, Bahtinov Mask, Handle Bar and Plate

• Bahtinov mask: in the lens cover; it is patented and made from a material three times lighter than traditional masks. The front cover threads on at M156.
• Dew shield: retractable, 155mm across, with a lock screw.
• Saddle handle bar: CNC-anodized aluminium, and a mount for the William Optics UniGuide and other Vixen-type finder bases; M4 locking screws and M4 and M6 screw holes.
• Mounting: mounting rings with knobs on a Losmandy Style Type D mounting plate.

Weight and Length

The optical tube weighs 7.105 kg (15.66 lbs), and 9.455 kg (20.84 lbs) with ring, dovetail and saddle. Length is 680mm retracted and 850mm 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.