PHYSICS || ASTRONOMICAL TELESCOPE

Astronomical Telescope

The Structure of an Astronomical Telescope

Describe the structure of an astronomical telescope

An Astronomical Telescope is used for observing heavenly bodies like stars and planets (generally bodies which are very far away from normal vision of human eyes ). Like compound microscope, it consists of two convex lenses, objective lens and the eyepiece.

The focal length Fb of the objective lens is longer than the focal length Fe of the eye piece lens.Rays of light from a distant object are nearly parallel when they strike the objective lens of the Telescope.The objective lens forms a real image, inverted and diminished image IQ of a distant object is in the focal plane.The eye piece forms the final magnified image at infinity

When the telescope is adjusted in such a way that the final image is at infinity it is said to be in normal adjustment.In this case the distance between objective lens and eyepiece is (Fb + Fe) This is the maximum separation between the objective lens and the eyepiece lens.

The Mode of Action of an Astronomical Telescope

Describe the mode of action of an astronomical telescope

The main reason for a distant object to be smaller is that the two objects subtend different angles at the eye. In other words, we can say that different angles substended by the eye causes a distant object to appear smaller.

The object AB and CD are of the Same height.The object CD is closer to the eye than AB.

The object CD appears to be taller than AB because angle B that CD subtends at the eye is greater than the angle x subtended by AB at the eye. Images there can be made to appear large by bringing them closer to the eye.

In a telescope the final image is magnified because it subtends a much greater angle at the eye than does a distant object observed without a telescope. B is the angle subtended by the final image at the eye and X is the angle subtended by a distant object.

The Magnification of an Astronomical Telescope

Determine the magnification of an astronomical telescope

The magnification of a telescope is defined as the ratio of the angel B (in radians) subtended by the final image at the eye to the angle X subtended by a distant object at the eye.

Thus, for telescope the magnification is given by:

M = B/x ………………………………….i

From figure B= IQ/ID ……………………..ii

X = IQ/IA ………………………………………..iii

But Insert eqn (ii) and (iii) into eqn (i)

M = (12/ID)

(IQ/IA)

M = (IA/ID)

But IA = fo and IF =fe

M = fo/fe……………………………….(x)

Where

Example 3

fois the focal length of two thin converging lenses of focal lengths 25cm and 4cm respectively. It is focused on the moon which subtends an angle of 0.6° at the objective lens. The final image is formed at the observers least distance of distinct vision (25cm in front of the eyepiece). Find the diameter of this image.

In the previous figure:

X = h/fo

Where fo is the focal length of the objective lens

X = h/25

Where X is the angle in radians subtended at the objective lens by the moon.

H = 25x

H = 25 (6/10 x 11/180)

H = 25 (6/10 x 22/7 x 1/80)

H = 0.2619m

The height of the image, h = 0.2619m

The distance of this image from the eyepiece is obtained from the relation:

• I/U + I/V = I/fe = 4cm
• V= -25cmV = -25cm
• I/U – I/25 = ¼
• I/U = (1/4 + 1/25)
• (I/U)-1 = (25 +4) -1/100
• 100U= (100/29)

The magnification, m of the lens:

• M = V/u
• M = (25CM/100/29)
• M = 29/4

Let the height of the final image of the moon be h:

• M = Hi/h
• hI = mh
• HI = (29/4) (0.2619)
• HI= 1.90cm

The Height of image Hi = 1.9cm

Hence

The diameter of the final image of the moon will be 1.90cm

Observation of the universe today are best made from the Hubble Telescope. Outside the Earth’s atmosphere, this telescope suffer from less interference.

Uses of an Astronomical Telescope

Mention uses of an astronomical telescope

Astronomers use telescopes because they're much better than our eyes. Here are a few reasons:

1. Telescopes see lots of colours - telescopes can collect light that our eyes are unable to: radio, microwave, infrared, ultraviolet, x-rays and gamma rays.
2. Telescopes collect lots of light - our pupils are only a few millimeters across, so we can only collect photons over a tiny area whereas telescopes can collect photons of huge areas (e.g. a football fields worth for radio telescopes).
3. Telescopes see fine details because of the wave nature of light and the nerves in our eyes, we can only see details about the same angular size as Jupiter's width. Telescopes can allow us to resolve fine details - like Jupiter's Great Red Spot.
4. Telescopes can record observations with cameras - You can see things with your eye and draw them, but telescopes can share observations with the world! This is especially important for convincing skeptics that what you saw was real!

A Simple Astronomical Telescope

Construct a simple astronomical telescope

A simple telescope