FIRST LIGHT
William Optics Fluorostar FLT 91 apo triplet refractor
Portable, superb build and performance? This could be the one you’re looking for
VITAL STATS
• Price £1,949
• Optics FPL-53 glass apo triplet
• Aperture 91mm
• Focal length 540mm, f/5.9
• Focuser Dual-speed 3.3-inch rack and pinion focuser
• Extras Rotolock 2-inch adaptor, saddle bar, 40mm extension tube, Bahtinov mask, carry case
• Weight 5.6kg with tube rings, dovetail; 4.2kg without
• Supplier The Widescreen Centre
• Tel 01353 776199
• www.widescreencentre.co.uk
Arriving in a single box, the William Optics Fluorostar FLT 91 was protected in a robust and well-padded soft carry case. We were immediately impressed with the quality of the case and how securely the telescope was supported inside. With twin handles that double as shoulder straps, allowing the case to be used as a rucksack, and additional space for accessories, it’s a convenient prospect for astronomers and astrophotographers who like to travel.
As the name suggests, the FLT 91 is a 91mm (3.5 inch) aperture fluorite triplet telescope, with a focal length of 540mm. This provides a very useful f/5.9 focal ratio for both budding and more advanced deep-sky imagers. On inspection, this triple lens refractor appears very well built, with no hint of plastic and an excellent overall finish. The white OTA with red embellishments makes for a striking setup; it is also available with green or grey detail.
William Optics supplies numerous accessories as standard, including a 40mm extension tube, tube rings, Vixen saddle, carry handle and 2-inch Rotolock adaptor. We were also loaned the dedicated William Optics Flat 6AIII 0.8x focal reducer and field flattener, which can be purchased separately. Mounting the Fluorostar FLT 91 onto our Sky-Watcher EQ6-R, we found that obtaining balance was easy thanks to the long Vixen-style dovetail bar. In addition, we were able to mount our guide scope directly onto the carry handle that comes attached to the tube rings, which made setting up even easier.
Making connections
Popping a 32mm eyepiece in, the first thing we noted was that a 1.25-inch adaptor was not supplied. That’s not an issue as we had our own, but it’s something for visual users to be aware of. The Rotolock is an excellent addition that makes swapping accessories particularly easy. Slewing over to a setting Moon, we resolved some beautiful details and, later, a star test confirmed a flat field of view. This triplet definitely holds its own for visual astronomy.
We were particularly keen to get imaging with the FLT 91 and so reached for the Flat 6AIII. 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 6AIII 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 6AIII 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.
Superior optics
A 540mm focal length and 91mm aperture make the FLT 91 suitable for both viewing and capturing deep-sky targets while still being portable, and its high-quality optics ensure excellent results. The FPL-53 glass (a type of synthetic fluorite) and triplet design minimises distortions such as chromatic aberration. This is because all colour channels are focused evenly, removing halo effects. The high-transmission (STM) coatings on all lenses also help to boost contrast, maximise light transmission and reduce unwanted reflections inside the optical system.
While these qualities ensure the FLT 91 provides an excellent field of view, the addition of the Flat 6AIII 0.8x reducer (not included) allows the focal ratio to reduce from f/5.9 to f/4.75 while providing a wider field of view. This grants faster imaging to the user and works well for capturing larger nebulae. All these features contribute towards clean data that makes processing easier, particularly for those new to astrophotography.
VITAL STATS
1. William Optics flattener 6AIII with M92 thread for FLT 91
2. William Optics 50mm red guiding scope with Rotolock
3. William Optics 48mm T-mount for Canon/Nikon cameras
VERDICT
