Telescope Lenses: Clearly an Interesting Subject

This article will attempt to shed some light on the complicated subject of optical lenses, particularly, those used in astronomical instruments. I will discuss the science involved and then describe application for telescopes used in “backyard” astronomy.

Lenses are designed to take advantage of what happens to light when it hits curved surfaces. Because light consists of electromagnetic waves (tiny electric and magnetic fields), it interacts with the tiny fields that are also found in matter. When light, traveling in the vacuum of space in a straight line, hits something else (like earth’s atmosphere), it slows down as it interacts with the tiny fields composing the material it hits. This process results in bending of the light, or refraction (that’s why the sky is blue). If you recall the first two articles in this series, one of the three telescope types I discussed, namely the refractor, utilizes this property of light exclusively. The reflector and catadioptric scopes depend on mirrors and lenses (in their eyepieces) as well, to create a magnified image for our eyes to transmit to the optical lobes of our brains, where the “seeing” will actually occur.

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A lens is a transparent object (a disc of glass) that makes an image it takes from an object that is a source of light rays. It has curved surfaces that take advantage of refraction, to the extent that all the light rays entering the lens can all be brought to the same place, called the focal point, or focus. When the image is focused for our eyes, it can be magnified by a second lens (known as the eyepiece). The amount of magnification can be controlled by the focal length of the eyepieces. The focal length is the distance between the lens and the focal point, usually designated in millimeters (mm.) Because of the physics of light and lenses, the shorter (lower millimeter number) the focal length of the eyepiece, the higher the magnification.

In practice, astronomical lenses are never quite this simple; as multiple lenses are often combined in eyepieces to improve the quality of the observed images. Refractor scopes depend entirely on lenses in the main body of the telescope as well as in the eyepieces. The cost of this type of equipment to the consumer increases as the scope’s aperture increases, due to costs of manufacturing larger and larger lenses. With refractors, the cost is also related to correcting errors (or, aberrations) in the lenses that are natural, or inherent properties of glass. Those aberrations can interfere with the sharpness of the image (especially at the edge of the image).

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Another practical point regarding magnification and eyepiece focal lengths is this: as the magnification increases by using lower (shorter) focal length eyepieces, a point is reached at which the focus (sharpness) of the image begins to diminish, because you are magnifying everything between the end of the telescope and the sky object you are observing (turbulence in the atmosphere, dust, light pollution, etc.,). In addition, our eyes also have limits to their ability to resolve smaller and smaller details.

Next time, Orion, the Hunter, a beautiful Winter “object”.

May you have clear skies!

George Drake, M.D.
ETHOS Volunteer
Michiana Astronomical Society Member
References:
Physics II for Dummies Holzner, Steven Wiley Publishing, Inc. 2010
astrofarmfrance.com ( Searched 11-11-20)