Opening Up A Refractor
ATelescope is a very high-cost and sophisticated instrument and most of us cannot afford to unassemble one. But what if we open up a small, low-cost telescope to get a practical idea of what is inside ours?
The telescope I opened is a refractor. So to understand the components and working of a refractor telescope, we first need to understand the basic principle on which it works which is refraction.
What is refraction?
Suppose you dip a pencil in a glass of water, what do you observe? You will see that the pencil appears to be bent at the liquid surface. This is because of refraction.
As light travels from one medium to another which have a density difference, it tends to bend at the common surface. So when light enters from the air(rarer medium)into water(denser medium), it tends to bend at the liquid surface. Due to this, the pencil appears to be bent.
So now that we have a basic understanding of refraction, we are ready to understand the components of the telescope. Let’s begin with the objective lens.
The Objective Lens
The Objective lens is the one through which light enters the telescope. It is a convex lens. Its diameter is the aperture of the telescope. The light-gathering power of the telescope depends directly on the diameter of the objective and so does the resolution. The resolution of a telescope is its ability to separate two-point sources into separate images. Therefore an objective lens of greater diameter is desirable.
The Eyepiece
The eyepiece in this type of telescope contains two small, convex lenses. So how did I analyze that these are convex? When I held these lenses at a distance I was able to see an inverted image in the lens, which shows that a real image is being formed, and it is a feature of a convex lens to form real images. Since these lenses have one plane and one curved surface they are plano-convex lenses.
The function of the first lens in the eyepiece is to make the inverted image formed by the objective lens erect while that of the second is to magnify this image formed.
The eyepiece lenses are comparatively much smaller than the objective. This is because magnification is equal to the focal length of the objective lens divided by the focal length of the eyepiece lenses. Therefore greater the focal length of the objective lens and the smaller the focal length of the eyepiece lenses, the greater will be the magnification.
If the magnification of a telescope is 100 then this means that if the angular separation between two stars is 1' then they would appear to be 1'*100=100' apart when seen from the telescope.
Main tube
The length of the main tube of the telescope gives us the focal length of the telescope. It is the distance between the objective and the eyepiece.
Image Formation
Now let us finally try to understand how exactly image formation takes place in a telescope. Light from a distant object enters the objective lens and a real image is formed in the tube at its second focal point. The objective lens and the first lens in the eyepiece are far enough apart that the first eyepiece lens inverts the image formed by the objective. The second eyepiece lens acts as a magnifier and keeps the image upright and in a location that is easy to view.
References-
- NCERT textbook class 12-physics
- https://www.britannica.com/
- https://www.ifa.hawaii.edu/
- https://opentextbc.ca/
- https://astronomy.swin.edu.au/
