Unit 1 Physics Detailed Study 3.1 Chapter 10: Astronomy

Galileo’s Telescope  The basic principles used in the telescopes used by Galileo, are still the principles used in basic refracting telescopes.  A large convex lens, with a long focal length is used to form a real image, this is known as the objective.  The image is viewed through a smaller convex lens, with a short focal length this is known as the eyepiece.  In general with the eyepiece, the shorter the focal length the greater the magnification.  The ratio of the two focal lengths can be used to determine the magnification of the telescope  Astronomically, the ratio of the angle made by the object when viewed through the telescope to the angle when viewed by the naked eye. Section 10.4The Telescope: From Galileo to Hubble

Galileo’s Telescope Section 10.4The Telescope: From Galileo to Hubble

Galileo’s Telescope  This type of telescope is actually known as the Keplerian telescope as Kepler examined the optics of the system.  The Galilean telescope works on the same principles, however, it uses a concave eyepiece, this results in an image that is upright, this is more important for terrestrial use, but not for astronomy.  Though the magnification of a telescope is important, the light-gathering power of a telescope is more important. The light-gathering power increases the detail of a star.  The light-gathering power is dependent on the diameter of the objective lens. The bigger the lens, the greater the light-gathering power.  The light gathering power of a lens is proportional to the area of the lens. So a lens twice as big will be four times more powerful. Section 10.4The Telescope: From Galileo to Hubble

Galileo’s Telescope  The light-gathering power of a telescope is effected by two main issues as the lens size increases.  Large lenses are very hard to make, as the curvature of a lens determines the focal length, the curvature needs to stay constant, this leads to the centre of the lens becoming very think which introduces distortion of the images.  The other issue is that different colours have different refractive indexes, as the light goes though a lens the blue light of an image will focus at a different point to the red light, this is known as chromatic aberration and as lens size is increased, this becomes more of an issue. Section 10.4The Telescope: From Galileo to Hubble

Newton’s reflecting Telescope – and others  Modern telescopes have replaced the objective lens with a curved (parabolic) mirror, known as either the objective or primary mirror. These mirrors are easier to make, and chromatic aberration is not an issue.  The issue with parabolic mirrors is that the focus is not in front of the mirror. So to observe the image, Sir Isaac Newton placed an angled secondary mirror at the focus of the primary mirror, this allowed him to see the image through an eyepiece.  Like the refractive telescope, the light-gathering power of a telescope is effected by the diameter of the mirror. This means that as the diameter gets bigger, the focal length gets bigger, and so to much the length of the structure.  The Cassegrain telescope has a secondary mirror placed directly in front of the primary mirror, which reflects the image down a tube placed in the middle of the primary mirror, this can half the size of a telescope for a given focal length. Section 10.4The Telescope: From Galileo to Hubble

Follow that star  As we know, when we observe the night sky, the stars overhead move at a constant rate. Though this rate is fairly slow when looking with the naked eye, it can be much quicker when looking through a telescope. If we are interested in a particular star or planet this can cause a problem.  To over come this problem two different mounting systems have been developed to help track stars. These systems are the altitude-azimuth system and the equatorial system.  The altitude-azimuth system essentially holds the telescope in a fork. The as the altitude (angle above horizon) of a star changes, the telescope pivots around the horizontal axis. As the azimuth (essentially compass bearings) of a star changes, the fork rotates around the vertical axis.  This motion is controlled by two motors in modern telescopes. Section 10.4The Telescope: From Galileo to Hubble

Follow that star  The equatorial system relies on the fact that the earth rotates, and the stars remain at the same declination.  The equatorial system is setup so that one axis is pointing at the SCP, this is known as the polar axis. This is the axis of rotation of this system, as it is rotated through this axis, it will maintain its declination.  Once focused on a particular star, we only need to rotate around the polar axis because as the earth rotates, only the right ascension of the star changes.  Provided this system is rotated at a constant rate, the telescope will follow the star’s movement across the sky. This can be achieved by a simple motor. Section 10.4The Telescope: From Galileo to Hubble

Seeing stars  The resolution of a telescope is its ability to distinguish between two different objects, that are very close to each other.  One way of increasing the resolution of a telescope, is to increase the size of the primary mirror, this allows for greater light-gathering power and results in a more defined image.  However, even with a really huge mirror, the image is still not perfect. This is due to the effects of the atmosphere on the light.  There are two ways of overcoming this issue, however both are quite expensive. Section 10.4The Telescope: From Galileo to Hubble

Seeing stars  One way of overcoming the effects of the atmosphere is to take the atmosphere out of the equation. That is, to put a telescope out in space.  One such telescope is the Hubble Space Telescope.  In 1990 the HSP was launched into space. It was expected to be able to resolve to 0.1 arcsec, however, due to a manufacturing issue, the mirror was not perfectly parabolic so produced disappointing results.  This issue was fixed by fitting a number of specially made secondary mirror. Section 10.4The Telescope: From Galileo to Hubble

Seeing stars  The other way over overcoming the effects of the atmosphere is to compensate for its effects.  This method is known as adaptive optics. In this process, the shape of the primary and sometimes secondary mirror is changed, due to either the effects caused by atmospheric turbulence, or temperature fluctuations in the mirror itself.  These changes are made and monitored by a very powerful computer, acting with very fast actuators, which deform the mirror as required. Measurements are taken up to 100 times per second. Section 10.4The Telescope: From Galileo to Hubble