William Optics Fluorostar FLT 91 Apochromatic Refractor - Checked & Tuned
William Optics Fluorostar FLT 91 Apochromatic Refractor - Checked & TunedWilliam Optics Fluorostar FLT 91 Apochromatic Refractor - Checked & TunedWilliam Optics Fluorostar FLT 91 Apochromatic Refractor - Checked & Tuned
William Optics Fluorostar FLT 91 Apochromatic Refractor - Checked & TunedWilliam Optics Fluorostar FLT 91 Apochromatic Refractor - Checked & TunedWilliam Optics Fluorostar FLT 91 Apochromatic Refractor - Checked & Tuned

William Optics Fluorostar FLT 91 Apochromatic Refractor - Checked & Tuned

  (3 Reviews)
✓ 2 year warranty


1x in stock shipped 1-2 working days

Charlotte Daniels writing for BBC Sky @ Night Magazine

"We were particularly keen to get imaging with the FLT 91 and so reached for the Flat 6AIll. The rack and pinion focuser allows for 40mm of back focus, which we found with the flattener-reducer attached was sufficient. With our CCD camera on board, we swung over to the Sadr region in Cygnus and set our sights on IC 1318 (sometimes known as the Butterfly Nebula). We were pleased to see the Flat 6AIll did its job and provided pin-sharp stars from edge to edge.

Opting for a bi-colour image, we captured about three hours of data to process. Next, we slewed to the Tulip Nebula and gathered a further couple of hours on this target.

Curious to see performance without a flattener attached, we removed the Flat 6AIll focal reducer. To achieve focus, we needed to use the additional 40mm extension tube supplied, plus an additional 40mm extension. However, we were pleased to note that we could only observe a slight curvature of the stars towards the edge of the field of view.

The 3.3-inch barrel of the focuser also enables the use of a full-frame DSLR. Attaching ours and firing off some flat calibration frames, we observed only minimal vignetting that could easily be removed in processing. For imagers, the FLT 91 is therefore suitable for a range of CCD and CMOS cameras.

Returning to our images, we found processing a breeze thanks to the clean data; no calibration frames were used for either target. With absolutely no vignetting and lovely round stars, we ended up with two very pleasing images to enjoy. As winter approaches, we imagine the FLT 91 would be an excellent deep-sky workhorse, and a reliable option that won't let its user down during those clear and long dark nights.

The William Optics Fluorostar FLT 91 granted us a stunning suite of astro images to enjoy and is small and light enough to remain portable for travelling astronomers. Its optics and performance will appeal to both imagers and visual users alike." 

About this product

Model:  wo_flt_91_gd
Part Number:  T-FLT-91GD-RP33

Optical Bench Test

FLO tests all William Optics triplet refractor telescopes before dispatch. 

You can be confident your telescope will, from the outset, have well figured optics, negligible false colour and spherical aberration, and no astigmatism (the bane of any optical system!).

Star shapes will be round, not triangular, oval or egg-shaped!

New for 2021 - William Optics Fluorostar / FLT 91

An Apochromatic triplet refractor for visual astronomers and astrophotographers who want very high performance from a midsize telescope. 

The three-element optical design features a 91mm aperture using FPL-53 glass that delivers high contrast colour-free images and an impressive 0.95+ Strehl Ratio. 

Thoughtful design features like the integrated saddle handle, 360º field rotator, 2" Rotolock, large focuser knob and built-in Bahtinov mask are more than just nice touches. They genuinely help make this an excellent choice for astrophotographers. 

To use with the Flat6A III 0.8x Reducer / Flattener, set the adjustable distance at 5.4mm for 55mm back focus. For the Flat68 III, set the adjustable length at 10mm, which leaves 65mm back focus. 


  • 91mm FPL-53 Apochromatic Triplet Lens with 0.95+ Strehl Ratio
  • 540mm, f/5.9 Focal Length
  • Exquisite William Optics Fit & Finish
  • Dual Speed 3.3" Rack & Pinion Focuser
  • 2" Rotolock for Secure Attachment
  • Bahtinov Mask Integrated into the Lens Cover
  • Versatile Saddle Bar Handle for Guider / Finder Mounting
  • Built-in 360º Field Rotator
  • OTA Weight 4.2kg / 5.6kg with Rings / Dovetail / Accessories
  • Supplied with a Soft Carry Case


The retractable dew shield features a removable lens cover, with our patented Bahtinov Mask revealed by unscrewing the face plate. The transparent design allows for brighter star patterns and precise focusing for astrophotography.

Versatile Saddle Bar

The Saddle Bar Handle offers a solid handle with a built-in dovetail chamber for use with the WO Uniguide scopes or other Vixen-style guiding/finder scopes.

New Focuser Knob

A larger fit designed for better ergonomics.

Field Rotator

The super smooth field rotator includes markings for every degree and a solid thumbscrew position lock.

Soft Carry Case

The standard package comes with a soft carry case.

Field of View Simulator

Product Videos

What's in the box

  • FLT91 Telescope OTA
  • Vixen-style Dovetail & Tube Rings
  • Saddle Handle Bar
  • 2" Rotolock
  • Integrated Bahtinov Mask
  • Soft Carry Case


Lens Type Apochromatic Triplet Prime Lens
Focal Length 540mm
Diameter 91mm
Focal Ratio f/5.9
Focuser Dual Speed 3.3" R&P Focuser
Colour Red | Gold | Space grey
Lens Material FPL-53
Strehl Ratio 0.95+
Camera Format Full Frame DSLR / Mirrorless / CMOS / CCD
Test Report Zygo test report available at additional cost
T-Mounts Supported Canon EF, RF Nikon F, Z Sony E Pentax K Micro Four Thirds (M4/3) FUJI X
Retracted Length 441mm without rings (485mm with rings fitted)
Extended Length 510mm
Extension Tube Length 40mm
OTA Weight 4.165Kg
Total Weight 5.575Kg
Extension Tube Weight 0.105Kg

Customer reviews

Average Rating (3 Reviews):  
Write a Review and share your opinions!

Rating (max 5):  
A Beautiful Piece of Kit
05 April 2022  | 

I bought the William Optics Fluorostar FLT 91 Apochromatic Refractor recently (March 2022) along with the William Optics Adjustable Flat6A III 0.8x Reducer/Flattener, as a treat to myself. Yes, there are cheaper products on the market and yes you could say that this was an extravagant purchase, but I have excuses. Primarily this was upgrade and replacement to a 13-year old Celestron refractor to support my growing astrophotography hobby. But above all, this was an investment in the future and Iím so glad that I did it.

OK, so Iíve only had a handful of nights with the FLT 91 so far, but, gosh, it is a beauty. The build quality is first class, it is heavy (very heavy), the focuser is precise and the whole look is stunning (I chose the red rings/dovetail). I was a little concerned about historical issues with the build quality, having read the initial reviews on forums when the FLT 91 first shipped a couple of years ago (the proximity of the focuser to the dovetail) but I am pleased to say that whatever issues William Optics may have had back then have been resolved now. I was also aware that a number of early adopters had also struggled to achieve the correct back focus with their cameras, but again I had no problem at all (with the ZWO ASI294MC-PRO). In fact, the FLT 91 ships with an extension tube that simply needs to be removed before attaching a camera.

The Flat6A III 0.8x Reducer/Flattener is a quality instrument too with some beautiful optics. Again, installation was a piece of cake and no adjustment was necessary (although the Flat6A is adjustable). It brings the focal length of the FLT 91 down to a speedy f/4.7 and the end results are stunning: perfect round stars in all four corners plus the centre, with absolutely no noticeable chromatic aberrations. Amazing. So all in all, Iím really pleased with my purchase, and top marks to FLO for shipping so quickly to me. Yes, the FLT 91 is more expensive than the GT-81 but you are paying for a bigger and better product. If you are in the market for a new refractor I would highly recommend the FLT 91 with Flat6A III 0.8x Reducer/Flattener as the combination of optical quality and build quality is a winner.

Rating (max 5):  
Beautiful Scope
14 December 2021  | 

I very recently bought this scope, nearly fell off my seat to learn there were 4 in stock after the way things have been during these lockdowns, lol. Hats off to FLO for getting it to me within 24 hours I think it was, amazing, thank you guys! This scope just oozes class, beautifully made triplet apo and sits well on the EQ6R Pro mount. Had to mess about moving the focuser knobs away from the the mounting plate as they were sitting right over the end of it preventing the use of the rotary part of the focuser, a mere faff and nothing else. Can highly recommend this scope, amazing! Thanks once again to the FLO team who go above and beyond with not only sorting everyones orders out but for decent advice also which is invaluable. THANKS GUYS!!

Rating (max 5):  
William Optics FLT91
27 October 2021  | 

Marvellous piece of kit, that I'm hoping for great things from. First Light also worked their usual magic with this one..


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