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9.7 Option - Astrophysics: 1. Earth-based observations

Syllabus reference (October 2002 version)
1. Our understanding of celestial objects depends upon observations made from Earth or from space near the Earth	Students learn to: discuss Galileo’s use of the telescope to identify features of the Moon discuss why some wavebands can be more easily detected from space define the terms ‘resolution’ and ‘sensitivity’ of telescopes discuss the problems associated with ground-based astronomy in terms of resolution and absorption of radiation and atmospheric distortion outline methods by which the resolution and/or sensitivity of ground-based systems can be improved, including: adaptive optics interferometry active optics	Students: identify data sources, plan, choose equipment or resources for, and perform an investigation to demonstrate why it is desirable for telescopes to have a large diameter objective lens or mirror in terms of both sensitivity and resolution

Extract from Physics Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.

Prior learning: Preliminary module 8.2

discuss Galileo’s use of the telescope to identify features of the Moon

Galileo did not invent the telescope, but he did refine its design. He built refracting telescopes which produced an upright image, and masked out the edge of the front lens of his telescope to overcome spherical aberration. Galileo was the first to use a telescope to make systematic astronomical observations of the features of the Moon as well as observations of the phases of Venus, the moons of Jupiter, the rings of Saturn and sunspots.
In particular Galileo made both qualitative and quantitative observations of the Moon. He observed that the Moon was rough, like the Earth, and drew sketches of vast plains (Mare) and mountains. He also made quantitative measurements of the lengths of the long shadows cast by the mountains of the Moon when they were near the edge of the shadow (called the “limb”) and directly facing Earth (first or last quarter). From this he was able to estimate that these mountains were at least several kilometres high.
Galileo’s telescope was described as an “instrument of the devil” as it allowed him to observe properties of the moon and Jupiter which challenged the prevailing Aristotelian view, endorsed by the Church of Rome, that the heavenly bodies were perfect and unchanging and that the Earth was at the centre of the Universe.

Telescopes The Rice University, USA.

Moon The Rice University, USA.

discuss why some wavebands can be more easily detected from space

Background

Almost all our information on the cosmos comes to us in the form of electromagnetic radiation. Because the range of wavelengths is so vast, the electromagnetic spectrum is loosely divided into bands, based on wavelength and on how the radiation can be produced and detected. These bands include very short wavelength gamma rays, x-rays, ultra violet, visible, infrared through to very long wavelength radio waves.

The wavelength where one band ends and another starts is to some extent blurred; for example very short wavelength ultra violet light may sometimes be considered as a “soft” or long wavelength x-ray. Also you may read where some bands are further divided; for example very short wavelength radio waves are commonly referred to as microwaves, whilst other subdivisions of the radio band include HF, VHF and UHF.

A waveband is a part of the electromagnetic spectrum covering a specific range of wavelengths.
The Earth’s atmosphere absorbs, scatters or reflects some wavelength bands more than others. Some bands are strongly absorbed and do not reach the ground, and are best observed from outside the atmosphere. Other wavebands are absorbed little and penetrate easily to the ground where they can be observed.
The highly energetic gamma rays and x-rays ionise molecules making up the atmosphere and are therefore strongly absorbed in the upper atmosphere, with very little reaching the ground. These wavebands are more easily detected by telescopes placed in orbit outside Earth’s atmosphere, such as COBE and Chandra X-ray telescope, than by ground-based telescopes.
Some bands of ultraviolet radiation are strongly absorbed by the ozone layer of the atmosphere, while others penetrate to the ground.
The atmosphere does not scatter or absorb the visible waveband very much. Consequently these wavelengths pass through the atmosphere relatively unhindered and reach the ground. This is why we have sunlight and starlight, and optical telescopes can be used effectively at ground level.
Infrared wavelengths are only partially absorbed. As an alternative to the expensive procedure of placing them into space, infrared sensitive telescopes may be placed on mountain tops above the densest regions of the atmosphere.
Some parts of the radio band are also absorbed to various degrees while others pass easily through the atmosphere. Radio telescopes such as the one at Parkes, NSW, were built to take advantage of this. Very long wavelength radio waves are reflected by the ionosphere.

identify data sources, plan, choose equipment or resources for, and perform an investigation to demonstrate why it is desirable for telescopes to have a large diameter objective lens or mirror in terms of both sensitivity and resolution

Identify data sources by looking in physics practical books or astronomy books for investigations that allow you to investigate sensitivity and/or resolution of telescopes.
Plan your investigations by considering the validity of the results you obtain. They must genuinely link the diameter of the objective to the sensitivity or the resolution, without any other factors, such as magnification, being involved. Consider using the same lens or mirror with different diameter diaphragms to change the effective area used.
Choose equipment by selecting and carefully setting up the most appropriate equipment available for this investigation. You could consider using a datalogger with a light probe to record appropriate data.
Perform the investigation you have planned. Alternatively, you could use a procedure such as that shown below.
As a result of your investigation, you should be able to show examples of how using a larger diameter objective lens or mirror will give you both a brighter image (telescope is therefore more sensitive) and a sharper image (telescope therefore has a higher resolution).

Sample procedure

Choose two instruments of the same magnification but different objective diameters, such as: two different astronomical telescopes if available; a single telescope with the correct eyepiece to match the magnification of its finder; or two different pairs of binoculars. (Binoculars consist of two telescopes mounted side-by-side, with prisms in the light path to shorten the length of the instrument. The magnification and objective diameter (in mm) are shown as, for example, 10 x 50.)

Choose an astronomical object with features that are easy to see in a small telescope, such as the Moon or Saturn. If you are working during the day, choose the most distant object you can see, such as a tree-covered hill.

Make notes on the relative brightness and the relative clarity of the same object through the two instruments. If equipment is available, try to photograph the same object through the two instruments and compare the developed images. Relate any differences in brightness and clarity to the diameter of the objective lens or mirror.

Alternative procedure using the Internet

Alternatively you might like to search the Internet for photographs of the same object taken by two different sized telescopes but under the same conditions. Try a search using some or all of these terms: “aperture diameter”, “objective diameter”, “resolution”, “photograph”, “telescope”. In this case, your data sources would be relevant Internet sites and you would choose your resources from amongst images found at these sites, together with information about the size of the telescope with which they were made. Compare the photographs for brightness and clarity, and relate any differences to the size of the telescope objective lens or mirror.

define the terms ‘resolution’ and ‘sensitivity’ of telescopes

The resolution (or resolving power) of a telescope is a measure of the ability of the telescope to reveal fine detail. Resolution is usually described in terms of the smallest angle of separation between two points of light, such as two stars close together in the sky, that can be seen as two distinct images. Resolution depends on the diameter of the objective lens or mirror and on the wavelength of the light.

A telescope with low resolution will see closely positioned stars as fuzzy and blurred together, while a telescope with high resolution will produce sharp, distinct images. Resolution may be limited by atmospheric conditions and the quality of the optics. For most large astronomical telescopes, resolution is stated in arc second (1 arc second (1”) = 1/3600 degree).
The sensitivity of a telescope is a measure of the minimum intensity of light that needs to fall on the telescope to form a suitable image. Sensitivity is proportional to the area of the light collecting surface, so doubling the diameter of a telescope’s objective should result in a four fold increase in sensitivity. Thus the sensitivity of a telescope is often referred to as its light gathering ability.

In practice, the sensitivity of any particular instrument can also depend on other things such as the quality and age of the optics. The relative sensitivity of two telescopes may easily be compared qualitatively by comparing the brightness of the image produced by the same object in each telescope. A telescope with greater sensitivity produces brighter images and can detect fainter stars than a lower sensitivity telescope.

discuss the problems associated with ground-based astronomy in terms of resolution and absorption of radiation and atmospheric distortion

Ground-based detectors of electromagnetic radiation from space, including telescopes, sit beneath a constantly changing sea of air, water vapour, other gases and dust. Variations in temperature and pressure, and corresponding changes in refractive index, cause stars to ‘twinkle’, exhibiting rapid variations in colour and intensity. From an astronomical point of view, this atmospheric distortion causes images to shimmer and go in and out of focus, thus lowering the overall theoretical resolution of the telescope.
Gamma rays, x-rays, ultraviolet, infrared and parts of the radio region of the electromagnetic spectrum are absorbed and scattered to different extents by the atmosphere. Ground-based astronomy in these wavebands is very difficult because of the low intensity of radiation reaching the ground. So much light from the violet end of the visible waveband is scattered, making the daytime sky bright blue, that optical astronomy is virtually impossible other than at night.
The true colour of images is altered because at ground level the variations in absorption with wavelength mean that we are not seeing an accurate reproduction of the intensity of the spectrum. All these affects are accentuated if the object being observed is lower in the sky because the path length the light has to travel through the atmosphere is greater.
The atmosphere also scatters extraneous light into the telescope from unwanted sources such as nearby houses, cars and towns. This light pollution is increasingly becoming a major problem for astronomers.

outline methods by which the resolution and/or sensitivity of ground-based systems can be improved, including:

adaptive optics

interferometry

active optics

One straight-forward way to reduce the atmospheric distortion and allow a telescope to operate near its theoretical resolution is to place the telescope as high in the atmosphere as possible. This means placing ground-based telescopes on high mountaintops, above the densest part of the atmosphere, above most of the moisture in the air (which blocks microwave radiation), and above most of the air currents arising from weather patterns and convection. High mountaintops also have the advantage of remoteness from human activity so light pollution is less serious. Astronomical instruments are sometimes carried aloft in high-altitude balloons or aircraft.
Adaptive optics: This uses a system of electronically controlled thrusters or supports which adjust the shape or the angle of the telescope mirror to compensate for image distortion caused by the atmosphere. Sensors quickly detect atmospheric distortion which is then analysed by a computer. Thrusters are then used to bend the flexible mirror or adjust multiple mirrors into the shape to produce the best possible image. The success of adaptive optics relies on the detection and correction taking place very quickly compared to the length of time over which the distortion lasts.
Interferometry: The reason that a large diameter mirror gives a sharper image is because the reflections of the wavefront at various points across the diameter add via the law of superposition in such a way as to produce a sharper image. This suggests that it may be possible to produce a sharp image by carefully adding the wavefronts of two or more smaller telescopes separated by a large distance to give an image sharpness that is theoretically equivalent to one single larger telescope. This procedure, known as interferometry, is very effectively used with radio telescopes, where two or more telescopes in different parts of the world are linked into an array.
Active optics: To increase the sensitivity of a telescope, the area of the primary mirror needs to be large. Unfortunately the larger area of a mirror, the more susceptible it is to becoming distorted and the thicker it needs to be to retain an accurate shape with changes in temperature and telescope orientation. For many years this meant that there was a fundamental limit to the largest sized telescope that could be built. However by using a thin, sometimes segmented, primary mirror and slowly monitoring the reflection of the wavefront off it, it is possible to apply pressure to various parts of the primary and correct for the deforming effects. The monitoring must be done slowly enough so that changes observed in the wavefront are indeed due to correctable changes in the optics and not to rapid random changes caused by uncontrollable atmospheric effects.