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Refractors

Refractors

The word “refractor” means a telescope whose principal focusing element is a lens.  A refractor telescope is a type of optical telescope which is also referred to as a refracting telescope.  Its curved primary (or largest) lens gathers light, bends it, and sends it back to a focal point where it is further modified by the use of another set of lenses called the eyepiece.  The curvature and size of the primary lens dictates the amount of distance needed to achieve the focal point.  Refractors can claim the fame of being the first telescope design, and they have remained a popular choice for imagers and planetary observing enthusiasts alike.  The crisp images they reveal, along with their versatility as a visual and imaging instrument for objects, makes them ideal for using the sky as well as on land.  

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Refractor Telescope History 

 

The refracting design was first used in spyglasses, then later in astronomical telescopes and telephoto camera lenses.  Of course, this type of technology had great military implications and it wasn’t long before several countries were putting their best opticians to work designing refractors for themselves.  Thankfully, Galileo Galilei heard about this new combination of lenses and what they could do – and independently designed his own refractor.  While it only magnified the image about 30 times, rather than turn it towards incoming enemies, Galileo turned it towards the night sky and created the first astronomical telescope.


Evolution of the Refractor Telescope 

 

Unfortunately, design flaws quickly plagued the beginning refractors.  By changing lens configurations to make shorter focal lengths, chromatic and spherical aberration was introduced.  Since each color in the spectrum has its own wavelength, it gets focused in a different position when it is refracted and focusing all optical wavelengths of light into a single point by refraction is impossible. However, the longer the telescope, the less the chromatic aberration and unwanted color smears.

Another initial problem with early refractor telescopes is called spherical aberration.  Lens grinding methods were crude compared to modern standards, and the edges suffered the most.  Not only would the light be bent too much or not enough for good focus, but the act of holding the objective lens by the edges causes the glass itself to sag.  So many things could go wrong as the lens size and grinding proportions changed.  Even the glass itself was extremely difficult to get pure enough for high quality lenses.  Close to a century passed before a man named Chester Moore Hall came up with a solution: use two objective lenses that had different dispersion properties!  “Crown” glass made from alkali-lime silicates has low refractive index and low dispersion factor.  “Flint” glass is high in lead, but it's extremely pure with high refractive index and low dispersion.  By polishing the surface and joining them together, the red and blue spectrum could now come to focus on the same plane.  As time went on, astronomical refractor telescope designs changed as additives were combined with the lens elements to create ever better and more pure lenses.  Opticians learned to combine lenses to reduce the focal length and compensate for optical errors.


How a Refractor Telescope Works 

 

A simple refractor, or refracting telescope is a hollow tube which uses a primary lens at its opening to diffract, or change the path of incoming light waves.  This primary lens is called the “objective lens” and is used to collect more light than the human eye.  When light passes through the objective lens, it is bent – or refracted.  Light waves that enter on a parallel path converge, or meet together at a focal point.  Light waves which enter at an angle converge on the focal plane.  It is the combination of both which form an image that is further refracted and magnified by a secondary lens called the eyepiece.


Types of Refractor Telescopes 

 

The process of combining two objective lens styles together in the crown and flint fashion was later independently discovered and patented by John Dollond.  This design quickly improved and the “achromatic refractor” telescope was born.  The design became immediately popular, and the joined two objective lenses were referred to as a doublet.  Over the course of the years, many doublets were created and known by such names as Littrow, Fraunhoffer, and Clark after their engineers.

Design improvements have changed very little since that time, but the quality of the glass has dramatically increased.  Today’s “apochromatic refractors” have objectives built with special, extra-low dispersion materials referred to as ED glass.  The introduction of flourite has also helped to overcome chromatic aberration as well.  However, lens sag may never be overcome and giant refractors will never happen because of gravity.


Looking Through A Refractor Telescope 

 

As with any telescope, the size of the primary lens, or objective light gathering source is key to telescope’s optical performance.  The larger the primary, the more light can be gathered, revealing ever fainter objects in greater detail.  But since a refractor telescope lens can never be large, why has the design endured?  Unlike a reflector telescope, the refractor tube is sealed.  Like looking through glass, looking through air also causes a certain amount of diffraction, as well as turbulence and heat wave issues.  Because the interior of a refractor is never exposed to the outside air, the view is considered to be far more crisp and steady.  This makes the refractor ideal for studying planetary details or resolving close double stars.  Since both the primary and secondary light gathering source are locked into place, this also means the refractor telescope requires less maintenance to keep its optical parts aligned.

Another consideration on behalf of the refractor telescope is its ability to be used for both terrestrial and celestial applications.  Special additions to the viewing end of the telescope called a “star diagonal” will invert the image again so that it is correctly oriented and give the viewer a more comfortable position when aimed at the zenith.  The small size and portability of the refractor also makes it an excellent choice for travellers, and modern binoculars are nothing more than a pair of twin refractors!

Small refractor telescopes also make great starter telescopes because they are rugged, maintenance free, easy to aim and offer sharp images.  The refractor telescope can also serve double duty for terrestrial subjects, such as wildlife studies and birding.  However, a large aperture refractor telescope is also very expensive - up to five times more than a comparative mirror.  But their high optical quality makes them the choice of astrophotographers and optical connoisseurs.

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