telescope advice
Portable and devoid of colour-fringing when observing bright targets, this doublet refractor is any astronomer's dream
We test the Altair Astro 60 EDF on a summer evening
Thinking of booking a late holiday this year to somewhere abroad with dark, clear skies? Then as the dedicated astronomer you are, you’ll want to take a ‘scope with you. It’ll need to be small and light so as not to exceed your baggage allowance, while at the same time have great optics so that you can make the most of those dark skies.
Altair Astro have the perfect ’scope for you: the Altair 60 EDF, which they are calling their lightest, most compact telescope yet. The telescope tube weighs just 1.5 kilograms and is 23cm in length – your socks will take up more room in your luggage! As such, the 60 EDF makes for a spectacular grab’n’-go telescope for day- or night-time observing.
It is, of course, the night-time observing that we’re most interested in. What’s really interesting about the 60 EDF is the coating on its lenses. Refracting telescopes use lenses to focus the light to a point, but this results in a phenomenon known as chromatic aberration or ‘colour fringing’. Lenses focus different colours (wavelengths) of light slightly differently so that they don’t all arrive at exactly the same focal point. This can result in fringes of colour around the celestial object that you are observing. To minimise the chromatic aberration two lenses are used, one convex, the other concave, together forming a doublet lens element that can modify how different colours of light are focused so they arrive at the same focal point, counteracting the chromatic aberration.
The vast majority of refractors these days are doublets – some are even triplets – but what makes the 60 EDF interesting is the coatings on its lens element. The 60 EDF uses coatings of synthetic fluorite, known as S-FPL53. These coatings give the
lens element extra-low dispersion properties (hence ‘ED’ in the ‘scopes name), reducing chromatic aberration even further. S-FPL53 is used in Altair Astro’s celebrated 125 EDF APO refractor, but you can experience it here in the 60 EDF for a much lower price. And boy, does it work well!
Testing was done during the summer months here in the UK, so sadly we didn’t have the perfectly dark skies that you may be craving when travelling abroad. Nevertheless, there was still plenty to observe. Venus, a shining evening star, showed its gibbous phase clearly and obviously, with barely a hint of colour fringing thanks to the 60 EDF’s lens element. Switching eyepieces to view a first quarter Moon, the dark plains of Mare Tranquillitatis (the Sea of Tranquillity, Apollo 11’s landing site) and
Mare Serenitatis (Sea of Serenity, Apollo 17’s landing site) contrasted nicely with the older, more cratered highlands, with the vast blast mark of the crater Copernicus deep in early morning shadow.
Turning towards the stars later in the evening, the ‘scope’s wide field of view became obvious. Slewing through the Milky Way as it ran through the Summer Triangle was a joy to behold. Ramping up the magnification to 120x, and with a bit of patience in the not-completely-dark sky, all four members of the ‘Double Double – Epsilon Lyrae – were visible, brought into focus by the excellent dual-speed 1:10 rack and pinion focuser. The focuser is nothing fancy, but it works wonderfully smoothly, with 75mm of drawtube travel.
There are of course limitations to a telescope just 60mm in aperture. We weren't able to resolve the stars of M13, the Great Globular Cluster in Hercules, while the Ring Nebula was barely a smudge of light, its doughnut-shape not evident.
Of course, affix the 60 EDF to a good tracking mount such as Vixen’s Polarie or the AstroTrac, and imagers will have no problem getting great pictures of these deep-sky objects,
particularly the larger