The telescope is an instrument that, in principle, enables the observer to see the stars and planets with a much larger eye.
Telescopes use either a lens or a mirror.
The larger a telescope‟s lens or mirror, the greater its ability
to collect light, and hence the greater the distance it can see. One of
the most famous telescopes, the „Hubble‟ was developed by the
National Aeronautics and Space Administration (NASA). Its mirror
has a diameter of 2.5 meters (250 cm) directed towards the stars to
collect their light. The surface area of this mirror is large – about
The following is generally how scientists think about the function of a telescope to see stars:
“The telescope with its reflecting mirror two hundred inches in diameter, serves as a sort of bucket to catch as much light as possible from a star and concentrate it on
film: it could pick up the light of a ten watt bulb a million miles away. The purpose of the telescope is not to magnify, for no matter how great the magnification,
no star would ever show up more than a point of light.”
When the human eye is compared to a telescope, it is evident
that the eye has its own objective lens that collects light. In a dark
place, the pupil of the eye becomes wide open, so that it can collect
maximum light. The circle of an open pupil in the average human
eye has a diameter of about 0.7 cm and a surface area of 0.38465
cm2. In comparison with the lens of the eye, the light-collecting
surface of the Hubble telescope‟s objective has a diameter 357.14
times larger than that of the eye. Its surface area is about 127,551
times larger than that of the eye. This means that the Hubble
telescope collects 127,551 times more light than the human eye. If all
this light were used to create brighter rather than larger images than
the naked eye sees, then the light of those images must be 127,551
times brighter than the images the naked eye sees. For this reason,
the telescope can make a star appear 127,551 times brighter.
Assuming that a star is so far away that it is barely visible to
the naked eye, we know that the Hubble telescope can make the star
appear 127,551 times brighter. Does this mean that the Hubble
telescope enables observer to see the star if it were 127,551 times
farther away? The answer is no. The Inverse Square Law3 says that
the light that we receive from a star is inversely proportional to the
square of its distance. According to this law, at that distance, the light of the star becomes 1275512 or 16,269,262,700, times dimmer, far too dim for us to see with the telescope.
This raises the question: What is the maximum distance an
object can be seen through the Hubble telescope? The answer is
357.14 times the distance that the naked eye can see. The reason is
that an object 357.14 times farther away, its light becomes 127,551
times dimmer. Since the Hubble telescope can make a star appear
1,270,551 times brighter, then looking through the telescope the star
would be barely visible.
Figure 1 shows an object located at A, that is barely visible to
an observer. Therefore, the distance d shown in the figure is the
maximum distance that the object is visible to the naked eye.
If the object were at location B, a distance 357.14 times
greater than d (Figure 2), then its light, according to the Inverse
Square Law of light, would be 357.142, or 127,551 times dimmer.
Although the object at B is invisible to the naked eye, it would be
barely visible through the Hubble telescope because the telescope
makes the light of the object appear 127,551 times stronger. Here we
see that a very large telescope, such as the Hubble, enables us to see
only 357.14 times farther than the naked eye can see.
The calculation does not account for the magnification of the size of
the object by the telescope, for any magnification causes the object to
appear dimmer because the light must be dispersed over a wider area.
(Magnification does not help to increase the viewing range of the
telescope beyond 357.14 times the range of the human eye.) Further,
the calculation assumes that the apparent size of the object remains
the same, but in reality, when an object is farther away, its apparent
size becomes smaller. However, this does not reduce the range of the
telescope because what we are primarily concerned with is the
amount of light that reaches the eye. Finally, the calculation assumes
that no light from the object has been blocked by dust or dissipated in
its passage through space. Otherwise, the telescope‟s range of view
would be further reduced.
The calculation did not include photography or digital cameras or
concentrating the light of stars on film, for a long period of time. In
that case, the viewing range of the telescope would be several times
greater. Recently, by using digital cameras, astronomers are able to
further increase the viewing range of a telescope.
In this chapter, we investigated some of the facts about telescopes
and their capacities. It was demonstrated that the capacity of a large
telescope to see the distant stars has been overestimated.
The result of this calculation indicates that a very large
telescope (such as the Hubble) enable observers to see only 357.14
times farther than the naked eye, pointing to wide-ranging
implications regarding many theories related to size and distances of
stars and galaxies.
This is the First Chapter of “Revolution in Astronomy by Bahram Katirai” free PDF Download at Keychests This book comes highly recommended by the legendary Tom brown.