The anti-reflection coatings are so good that it requires a bright source to render them visible to the camera. The 60mm, f/6 air-spaced doublet is fully multi-coated and composed of highly desirable low-dispersion FPL53 glass and a matching lanthanum element for excellent colour correction for both visual and photographic applications. Such exquisitely crafted instruments from high-end manufacturers such as Takahashi and Astro-Physics use exotic glasses, ground and polished to complex geometries, to create compact refractors that provide essentially perfect imagery for visual and photographic use, but they usually command an eye-watering price. In the intervening years, opticians have developed so-called ‘apochromatic’ (or ‘apo’) refractors, typically composed of a three-glass element sandwich capable of bringing three colours of the visible spectrum (typically red, green and blue) to a common focus.
However, for an achromat of typically encountered sizes to have an acceptable level of colour correction, an optician needs to make the instrument’s focal length at least twelve times the diameter (or aperture) of the lens, which immediately explains the long tubes of classical refractors. In fact, the use of crown and flint types of glass to fashion a so-called ‘achromatic’ lens capable of bringing two wavelengths of the colour spectrum (usually red and blue) to focus dates back to the British inventor Chester Moore Hall, in around 1730. I quickly learned that a simple compound lens composed of a sandwich of two types of glass with opposing dispersive (prismatic colour-making) qualities only goes a limited way to cancelling out this annoying inherent defect of refractors. Thus I had my introduction to the bane of simple lensbased instruments: chromatic aberration. While I was immensely proud of my four-inch, f/5 refractor, its images of the Moon and bright planets were surrounded by a vivid purple halo. I built my first telescope out of a 100mm diameter, two-element government surplus lens of 500mm focal length in the early 1970s. For casual terrestrial observations and quick looks at the Moon the TS PhotoLine 60mm, f/6 is quite at home on a sturdy photo tripod such as the author’s Manfrotto 055 shown here.
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