Back to the Future:
Refractors have a well deserved reputation for high image quality, particularly with regards to image sharpness, contrast and definition.
Newcomers to astronomy might be surprised to learn this reputation was earned not by todays modern short focal length ED and APO telescopes but by traditional long focal length Fraunhofer design Achromatic Refractors. For the latter half of the 20th century they were mostly manufactured in Japan and sold under a variety of different brands.
With their long focal ratios, typically F12-F15, they provided very high performance for the visual astronomer and, in smaller apertures, were almost entirely free of false colour. But in the late 1980s, their long slow focal ratios fell out of fashion and were replaced by compact apochromatic and fast Chinese-made achromatic refractors.
Unfortunately achromatic refractors of f10 or less are prone to chromatic aberrations and only work well at low to medium magnifications. Those wanting higher performance from a modern refractor will usually consider a medium to slow focal ratio ED doublet or faster triplet.
Fortunately, for discerning visual astronomers, there is an alternative.
About this product
Part Number: STL80AMAXI
The Scopetech STL-80A Maxi is a modern long focal length Achromatic refractor telescope manufactured in Tokyo, Japan.
Some time ago the people at Scopetech found and restored an original 80mm F15 Fraunhofer refractor telescope. They were surprised to see how favourably the design compared to modern fast apochromatic refractors. Fortunately they also found and restored an original polishing tool so were able to manufacture and reintroduce this classic telescope to the modern market.
Though traditional in appearance, the STL-80A Maxi is not an antique! It combines classic design and polishing techniques with modern Japanese manufacturing so is light (2.1kg) and portable. It also features modern lens coatings and a decent 1.25" rack and pinion focuser.
Re-Introducing such a telescope has several advantages.
- A long Focal Ratio Achromatic refractor exhibits much less chromatic aberration than their faster Achromatic counterparts and, most importantly, has less spherical aberration.
- At F15 the optical collimation is stable and, unlike some faster Apochromatic designs, it retains its collimation over time and in cold conditions.
- It is cheaper to manufacture than a similar quality Apochromatic instrument.
- The Glass types used are also stable and easier to polish.
During manufacture the inner surfaces of the telescope tube and lens edges are blackened to increase contrast and reduce ghosting. The main tube is also baffled and all air-to-glass surfaces feature HTM Multi Anti Reflection Coatings to enhance light transmission.
Under the night sky this traditional telescope delivers outstanding performance. In good seeing conditions it will handle surprisingly high magnifications, up to and beyond 3x the aperture in mm. I.e. 240x.
It is an excellent choice for relaxed grab-and-go astronomy and works well with a variety of mounts, including the Skywatcher AZ5, EQ3, EQ5, Televue Gibraltar, Rowan AZ100, etc.
In todays world of CNC mass production it is refreshing to see a traditional Japanese-made instrument manufactured to a high standard.
Please note these F15 instruments are manufactured and imported in small numbers so if it isn't in stock please be prepared to wait a few weeks.
What's in the box
- Telescope OTA
- 6x30 Finderscope & Bracket
- 1.25" Rack & Pinion Focuser with focus lock
- Vixen-style dovetail Plate
- Tube Rings
- 80mm F15 Optical Tube Assembly
- 1200mm focal length
- High quality Fraunhofer design
- BK7/F2 air-spaced doublet optics
- Multiple anti-reflection optical coatings.
- Optics with maximum transmittance 99.9% or more (one surface / actual measurement / d line 587nm average transmittance 400nm-700nm 99.8% or more)
- OTA Weight 2.1kg (6x30 Finder and Tube Rings additional)
- OTA length 125 cm
|Average Rating (2 Reviews): |
Thursday, 25 November 2021 | Neil
I've had the 80mm f15 for a couple weeks now and I've had two good nights to try it out. The short version of the review is just that it's a great scope. I've always been fascinated by long/slow achromats but have never owned one and I bought this one without realising just how good it would be. The closest scope I already had is my Skywatcher ED80 f7.5 and I've been able to swap them over on my mount and compare them view for view. The star points are crisper on the new scope and on the moon the contrast is clearly higher. The surprise was the flat field. I use fast Newtonian a lot and I'd always thought of the ED80 as being pretty flat field but just how out of focus it's edges were after using the f15 came as a shock. At magnifications up to around 150 there is almost no false colour visible on the moon, perhaps a hint of hint of blue on contrasty areas at 120x or more. On Jupiter I get the faintest of yellow halo but in either case it's doesn't affect the pleasure of the view.
Of the caveats about the scope I'd mention only a couple are to do with the optics. Neither are deal killers. Using it for observing I think it's virtually flawless but imaging the moon with a 120MC camera I did get a yellow halo on off axis parts of the moon after sharpening the image. I've heard before that many cameras are more sensitive to false colour than the eye so I assume that's the issue. The other issue is trivial but odd on such a good achromatic. The finder scope has the worst off axis false colour I've ever seen! Not a problem for me as I have a goto mount so only use it once or twice a session. Other than that it's issues are just it's size. Looking at Saturn for example it's clear that it could take 200x or more without loss of sharpness but it's just an 80mm scope so the image is getting dim by that power. It's other size issue is that it's a really long tube so be prepared to use an extending pillar even if you have a biggish mount like the HEQ5 I use. Those issues aside I love it.
A worthy revival of a classic format: a modern classic
Sunday, 15 August 2021 | Daniel
The f/15 achromat is a classic format for moon/planetary/double stars enthusiasts; the long focal length and slow focal ratio means excellent optical correction is possible even with classic optical glass types.
The ScopeTech 80mm f/15 certainly delivers on this promise. Even at magnifications at 300x (and above) the image shows no trace of astigmatism and spherical aberration correction is essentially textbook-perfect for a slow achromat. The perfect, concentric and circular airy discs is a testiment to very high quality lens figure and polish and perfect collimation.
Does it show chromatic aberration? Yes - it's an achromat. Chromatic aberration is present but is very, very well controlled; a slight amount of blue halo is seen around bright stars and on Jupiter, and of course bright objects are cast with the "romantic" faint yellowish color that stems from the fact that the purple end of the spectrum is out of focus in a classic achromat like this.
Mechanical quality is quite good, although simplistic; the photos first fooled me to believe that most of the black components were plastic while in fact they are mostly made of carefully painted light-weight metal. The light weight is a very welcome feature as it makes mounting the scope easy.
Achromats have been given a bad rap in recent years, mainly because many of them are cheaply made and show lots of CA and SA due to very fast focal ratios. Make no mistake, this scope isn’t your department store’s cheap achromat; the Scopetech f/15 is a high quality instrument. If you can mount its 1200 mm focal length tube and you are looking for a limited-aperture but very competent moon/planetary/double star scope (which is very typical for urban stargazers), look no further.
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