Refracting Telescope Sources:
Refracting telescopes are used to view images of the moon, planets, star clusters and general sky gazing. However, they tend to be smaller in aperture than other types so they are not as good for viewing fainter sky objects such as galaxies and nebulae.
Hollow Tube – Provides the structure of the telescope
Convex Objective Lens – Bends light source into the tube, and then magnifies
and focuses it at one point.
Eyepiece – The light focused then disperses through this lens
Focuser – Adjusts the focus of the image at eyepiece
- Great for viewing plants, the moon, star clusters, and splitting binary and multiple star systems.
- They have classic telescope appearance
- They are smaller than their reflective counterparts
- They require little maintenance
- A good refracting telescope is very expensive
- The size and refracting power of these telescopes are limited to the diameter of the lenses
- The larger the objective lens, the more likely for it to sag
- They have small aperture and thus are less well adapted for viewing very faint objects such as galaxies and nebulae.
These telescopes rely on the property of refraction to function. The primary lens in a refracting telescope is the convex objective lens which converges light to a focal point based on its refractive index. The focused light then becomes magnified when a properly placed eyepiece allows all the focused light to pass through. Changing the eyepiece allows different magnification because the magnification is determined by the ratio of the focal lengths of the objective lens and the eyepiece. The following diagram shows that the image scale in the focal plane is determined by F (focal length of the objective) the distance between the lens and the focused image. Also, the telescopes sensitivity is determined by the collecting area of the objective lens (or primary mirror) which is proportional to the square of its diameter.
In the last few decades, Takahashi Seisakusho Ltd. of Japan has modernized refracting telescopes. In 1977, they introduced the world’s first astronomical telescope with a fluorite objective as large as 150mm (6″) in diameter. The remarkable performance of the fluorite element allowed the production of f/8 telescopes with only two elements in the objective. Coating technology had also improved during this period so that the glass elements could be fully multi-coated to prevent light loss and internal reflection. In subsequent years, the refracting telescope with fluorite objective was refined and most recently Takahashi has developed a 4 lens (including 2 fluorite elements) telescoped named the f/5.