Optics: Apochromatic fluorite objective lens
Focal Length: 300mm, f/5.5 native, or f/4.3 with reducer and flattener
Focuser: Dual speed rack and pinion
Tel: 01582 726522
Vixen has a fine pedigree of telescopes, and new models from the Japanese company always attract their fair share of interest.
This is the case with the FL55SS,
a compact and lightweight refractor that offers premium features in a portable package.
The FL55SS is a dual-purpose telescope suitable for visual astronomy, or with the addition of a dedicated lens kit, for fast, wide-field astrophotography.
The unit is supplied as an OTA,
or Optical Tube Assembly only, so to test it visually we used our own 1.25-inch diagonal.
An extension tube for the visual back is supplied, which allows the scope to be used in a straight through configuration if desired.
Without a diagonal, however, we found this was an uncomfortable way of viewing objects that were high overhead.
If you are considering the FL55SS as a visual instrument, then a high-quality, maximum light transmission diagonal would be a worthwhile investment, to ensure optimal results from the front end’s petite 55mm objective lens.
We began our visual session with basic checks for astigmatism, lens misalignment and mechanical issues.
As expected with Vixen equipment we found no problems.
The telescope has a native focal length of 300mm, so our 25mm Plössl eyepiece, giving just 12x magnification, provided a wide overview – useful for orientation as there is no finderscope included with the FL55SS.
The view proved to be too bright
as poor sky transparency amplified the effect of background light pollution, but it did highlight the potential for the telescope in dark sky areas.
Our 10mm-eyepiece with its 72° field of view giving
30x magnification, was more rewarding: the star field contrasted nicely against the background.
Bright stars such as Regulus and Arcturus demonstrated their distinctive blue and red hues respectively, and there were no unwanted colour aberrations visible around them.
Moving the brighter stars towards the edge of the eyepiece started to reveal the effects of coma, which was quite pronounced at the perimeter.
In the 10mm eyepiece the Great Hercules Cluster, M13, was a bright, but fairly indistinct object, but swapping to a 4.5mm eyepiece revealed more individual stars.
At 66x the view was darkened, the brighter stars were tight and round with text-book airy discs, and we felt that we had probably reached the maximum useful magnification.
Many telescopes contain objective lenses made of glass.
In contrast to this, the FL55SS uses fluorite, which is a crystal. Fluorite is expensive to produce, being fragile and difficult to work on, but fluorite has optical properties that make it very desirable as light can pass through it with minimal dispersion, unlike glass.
In practice this means that the overall view is sharper, as all the colours of light entering the telescope are focused to a single point, rather than being dispersed into a fuzzier appearance.
Fluorite lenses reduce chromatic aberration, an effect usually seen as unwanted coloured rings around brighter objects.
Using parfocal colour filters and a CCD camera on the OTA alone, we checked the focus point of red, blue, and green light in comparison to each other – and as a whole through a luminance filter – to see just how good the fluorite lens is.
The results were encouraging; red and green light focused together, with just a tiny deviation towards blue. This result is typical of high-end apochromatic telescopes.
Built-in dew shield and tube internals
A generous built-in dew shield helps to prevent problems with dew and restricts stray light from entering the scope and interfering with the view.
Internal surfaces of the dew shield and scope are treated with a flat black coating, helping to eliminate reflections that reduce contrast at the eyepiece or camera.
Lightweight and compact
The fit and finish of the FL55SS is high quality and the Vixen-style dovetail bar, although removable if required, forms an integral part of the telescope assembly.
The tube itself measures just 282mm and weighs 1.5kg. Combined with the Reducer HD Kit the total weight is below 2kg.
Dual-speed rack and pinion focuser
Smooth and precise focusing is a prerequisite for fast astrographs with a shallow depth of focus.
The no-nonsense, dual-speed, rack and pinion focuser offers just that, with a solid, sturdy-feeling adjustment mechanism, no slop in the drawtube and no change in focus when locking it in place.
This review originally appeared in the June 2019 issue of BBC Sky at Night Magazine