Are you getting confused by the many different types of glass used in refractor telescopes that are available today? Well, I was too so I did some research. The biggest challenge when building refractor telescopes is to reduce or completely eliminate chromatic aberration which creates ‘false’ colors when observing or photographing the night sky. In order to do so, good quality refractors use extra low (ED) or super-low dispersion (SD) glass types. Low-dispersion glass is mated with two or more other glass elements to reduce or eliminate chromatic aberration. The number of glass elements, as well as the type of spacing used, and the build quality of a refractor telescope, ultimately determine whether the telescope is truly free of false color (apochromatic).
The first refractor telescope of Galileo Galilei had some issues…
When Galileo Galilei pointed his telescope toward the Moon and Jupiter in 1609, he used a refractor telescope. The design Galileo Galilei used in 1609, commonly called a Galilean telescope, used a single convergent (plano-convex) objective lens, and a divergent (plano-concave) eyepiece lens. Unfortunately, lens technology in the 17th century was poor, and Galileo had to work with aperture stops to reduce the diameter of the objective lens (increase its focal ratio) to limit chromatic aberrations. As a consequence, his telescope produced blurry and distorted images with a narrow field of view. But it was still good enough to discover the craters on our Moon, the four biggest (Galilean) Moons revolving around Jupiter, and the different phases of planet Venus.
Isaac Newton wanted to avoid the issues related to lens-based, refracting telescopes, which suffered from this dispersion of light into colors (chromatic aberration). So he constructed his first mirror-based telescope in 1668 that successfully bypassed that problem. There’s much more to be said about the pros and cons of mirror-based telescopes, but in this article, we keep the focus on refractor (lens-based) telescopes.
Improved lens quality thanks to Ernst Karl Abbe and Zeiss Optical Systems in the 19th century
It was not until the 19th century that a German optical scientist named Ernst Karl Abbe started to work as a partner at Zeiss optical systems, in Jena Germany, where he tested and improved the quality of lenses. Ernst Abbe came from a humble home – his father was a foreman in a spinnery. However, his father continued to financially support him throughout his career and Ernst eventually got a position as a university lecturer at the University of Jena in 1863. He became an associate professor of experimental physics, mechanics, and mathematics in 1870, and a full professor in 1879. In 1871, he married Else Snell, daughter of the mathematician and physicist Karl Snell, one of Abbe’s teachers, with whom he had two daughters. In 1866, he also was appointed as a research director at the Zeiss Optical Works, where in 1886 he invented the first apochromatic lens, a microscope lens that eliminates both the primary and secondary color distortions.
Among his many discoveries is the Abbe number. The Abbe number, also known as the V-number is an approximate measure of the material’s dispersion, with high values of V indicating low dispersion. The Abbe number, Vd, of a material is defined as Vd = nD – 1 / nF – nC, where nC, nD, and nF are the refractive indices of the material at the wavelengths of the Fraunhofer C, d, and F spectral lines (656.3 nm; 587.56; and 486.1 nm, respectively). This formulation only applies to the visible spectrum. Based on this formula, he created the Abbe diagram, also known as ‘the glass veil’, which plots the Abbe number against the refractive index for a range of different glasses (red dots). Glasses are classified using the Schott Glass letter-number code to reflect their composition and position on the diagram. Fluorite and phosphate glass turned out to have very low dispersion, making them very useful to counteract chromatic aberration in refractor telescopes.
How to compare glass quality in today’s refractor telescopes (FPL-51, FPL-53, FPL-55, FCD-100, etc.)
Since the 19th century, the lens quality further improved, and besides Schott – a German international glass company where Ernst Abbe got his test material – most modern-day refractor telescopes include optical glass from OHARA (Japan), HOYA (Japan), and other companies. The most popular and well-known glass type used in refractor telescopes today is probably OHARA’s FPL-53 glass, a fluorophosphate type of glass with a high Abbe number of 94.95. However, this is not the only ultra-low dispersion type that is available. Well-known companies like Stellarvue and Takahashi – which have been in the business of producing hand-made, high-end refractor telescopes – have recently switched to FCD-100, a dense fluor crown type of glass, with an Abbe number of 95.10. Other low-dispersion glass types include CAF2 glass made of Calcium fluorite, with an ABBE number of 94.99, and FPL-55 glass with an ABBE number of 94.6. According to Stellarvue, FPL-55 may be especially useful to produce apochromatic (color-free) refractors beyond the 130mm aperture range. Of course, the Abbe number is only one of the quality measures of optical glass. If you want a full overview of optical glass and its quality indicators, you can visit this website (refrativeindex.info).
Stellarvue president Vic Maris made this overview of the most common types of glass used in refractor telescopes on Stellarvue’s website, with their Abbe (Vd) number. He identifies the four glass types mentioned above as super-low dispersion (SD) glass. Ohara’s FPL-52 glass is discontinued, but FPL-51 and FK-61, are identified as extra-low dispersion (ED) glass, with Vd’s of 81.54 and 81.69. Schott Borosilicate Crown (BK7) at 64, is indicated as standard glass.
Achromatic vs Apochromatic vs Superachromatic Refractor Telescopes
The quality of a refractor telescope does not only depend on the quality of the glass used. The number of glass elements, as well as the build quality of the telescope, also matter a lot. To be truly free of (false) colors or apochromatic, a refractor telescope objective must focus three different colors in the same spot and certain optical defects like spherical aberration and coma must be corrected in two different places on the visual spectrum -as defined by….Ernst Abbe! As already mentioned, the telescope used by Galileo Galilei simply had one low-quality lens. Modern lens-based telescopes use at least two glass elements and are called achromatic doublets. These achromats bring two wavelengths into focus in the same plane – typically red (~0.590 µm) and blue (~0.495 µm). Apochromatic lenses are designed to bring three colors into focus in the same plane – typically red (~0.620 µm), green (~0.530 µm), and blue (~0.465 µm). Most apochromatic telescopes are made of three lens elements (often called triplets), but some very well-constructed doublets perform very close to apochromatic standards. Finally, we have super achromats, meaning that in addition to red, green, and blue, the near-infrared can also be focussed on the same plane. The pictures below show the variations in focus in inches, by (optical) wavelengths with color indication in microns.
The most affordable Achromat telescopes are often made of two glass elements; Flint glass and a Schott Borosilicum Crown. This is fine for visual use but not for astrophotography. However, when extra-low (e.g. FPL-51, FK-61) or super-low dispersion glass (e.g. FPL-53, FCD-100) is paired with good quality mating glass like a borosilicate crown element in the front and/or a lanthanum element in the back, such refractor telescopes are often great for astrophotography. There are also different choices to make regarding the spacing between the glass elements. Vic Maris mentioned that air-spaced telescopes performed slightly better in terms of color correction than oil-spaced telescopes when pairing them with FCD-100 in the 80mm to 130mm aperture range. The build quality also matters. If you compare a well-built doublet to a substandard-built triplet with the same type of glass, the doublet will outperform the triplet…