Collecting Light | Focal Length | Focal Ratio | Aperture | Light Pollution

Collecting Light

The purpose of a telescope and of astronomical binoculars (binoculars) is to collect and focus light. Although magnification is important, it is not the primary goal of astronomy equipment, as many sales advertisements claim. Deep sky objects—stars alone or grouped in nebulas or galaxies—emit light. And, “near sky objects”—planets, the moon—reflect light. So, the quality of astronomical equipment depends on its ability to gather this light and turn it into an image that the eye can use.

The primary components of astronomical equipment are:

• an objective
• an eyepiece
• an optical tube assembly (OTA).

The Objective
The objective is a curved lens or mirror upon which light from celestial objects shines. It collects light and, to a certain extent, processes it to create an image that the eye can use. For example, the light from Saturn enters a telescope, hits a mirror at the back of the telescope and is reflected toward the eyepiece. The more light a telescope catches, the better the telescope is.

The quality of the objective is one of the most important factors of determining the quality of the equipment—both in binoculars and telescopes. This is one of the components that should be invested in the most. A good objective will give a better image; consequently, a good objective will cost more. As with most features of astronomical equipment, a balance will have to be reached between:

• what the observer wants to see
• how well the observer wants to see
• how much the observer wants to pay.

Excellent images can be viewed without spending millions of dollars; however, a sufficient amount should be invested to retrieve an image good enough to make the experience worthwhile to the observer. An observer does not need the top-of-the-line model. But, avoid too-good-to-be-true sales as well.

The Eyepiece
The eyepiece is a group of lenses that further processes the light. This is where magnification occurs. Although magnification is not the most important feature, it is the most tangible effect for the beginning astronomer. Light that is collected by the objective is focused on and sent through the eyepiece to the eye. Therefore, the eyepiece helps to determine the quality of the image. Furthermore, the beginner should have at least two eyepieces to choose between. It allows the viewer to change between a larger field of view or a detailed image, depending on what the subject matter calls for.

The Optical Tube Assembly
The OTA is the tube of a telescope and, technically, the tubes of binoculars. The OTA serves two primary functions. First, it holds the components of the equipment together. This containment (besides preventing equipment loss and tired arms) keeps the components in proper placement and alignment, or collimation. The distance and angles between components within a telescope affects the image; the OTA stabilizes these variables. Second, the OTA allows the observer to “select” light. Pointing a telescope at a particular celestial object directs the observer’s sight and chooses the image. Without it, light coming from every direction (left, right, up, down, no matter that it is dark outside) is able to hit the objective. The OTA helps to isolate the light coming from the chosen object.

Focal Length

These three basic components come together to make binoculars or a telescope. Within this system, there is a measurement called a focal length. The focal length is the distance the light transverses to reach the eyepiece; the focal length is often (not always, like many reflectors, such as cassegrains) the length of the telescope itself—how far the light must travel after entering the lens at the front before it hits the eyepiece.

Focal Ratio

The focal ratio is the quotient obtained by dividing the focal length by the diameter of the objective. For example, a telescope with a focal length of 30 inches and with a lens of 5 inches has a focal ratio of 6, designated by f/6 (30/5 = f/6).

The focal ratio is important when choosing a telescope because the focal ratio determines what the equipment will view best. Although the focal ratio is present in binoculars, it is not used much when choosing them.

Equipment with low focal ratios transmits light quickly, which increases the field of view and the brightness. Such a situation is conducive to deep sky viewing. For example, a nebula millions of miles away appears much dimmer than the moon. Therefore, equipment that helps to produce a brighter image is better to see a nebula rather than the moon, which can become too bright to see any detail.

Equipment with high focal ratios transmits light slowly, which increases the contrast of the image. Such a situation is conducive to near sky viewing. For instance, equipment that helps to produce a detailed image is better to see the craters and peaks of the moon than it is to see a far-away nebula, where there is little hope of any distinction.

Aperture

Another “component” of a telescope is the aperture—essentially, the hole the light goes into. Like focal ratio, aperture size influences binoculars’ ability; however, the influence is on a much smaller scale than a telescope and not used much to categorize binoculars.

A bigger aperture collects more light. The more light that is collected, the better the telescope can “see” dimmer images. The dimmer an image a telescope can retrieve, the more the telescope will probably cost. Like when choosing an objective, a balance must be found between field of view, image quality and cost.

Light Pollution

Light pollution is the concept that electric lights are bleaching out the night sky, an occurrence that is prevalent in urban areas that have a high density of electric lights, such as streetlights, illuminated signs and more homes that let light out of their windows. The eye perceives fewer stars from within the glow of a city for the same reason that it perceives no stars during the day—the prevalent light (streetlight or sunlight) overpowers the starlight.

Also, the iris of the human eye will only let a pupil grow large enough to allow a safe amount of light to hit the retina—the part of the eye that interprets light into a type of data that is relevant to the brain. The more ambient light there is, the more the pupil closes, preventing the observer from seeing as much. For this reason, astronomers often use red light when viewing; such light is gentler to the eye and allows the pupil to dilate more. It takes less time for a pupil to grow large enough for optimal viewing if recovering from red light than it does from white light; this is the same reason an automobile’s dashboard lights are often red.

Light pollution will affect nighttime viewing by diminishing the amount of celestial objects seen. So, it may affect the choice of astronomical equipment to use. To escape light pollution, many astronomers are willing to travel away from its source, and their equipment must go with them. Portability is a factor in the choice of apparatus chosen for observation.