THE EARTH AS AN OBSERVATORY

Learning Outcomes To understand that the Earth’s atmosphere is transparent to some electromagnetic radiation and opaque to others. To distinguish between refracting and reflecting telescopes. To appreciate that most optical and infra-red observatories are located on high mountains or in space. To describe the nature of the Van Allen Belts and relate their discovery to early space missions.

What is the Electromagnetic Spectrum?

Which Waves are Absorbed by the Atmosphere? Harmful X Rays, Gamma Rays, and most UV and Infra Red waves are filtered out by our atmosphere Microwaves Visible light and Radio waves can pass through. To understand that the Earth’s atmosphere is transparent to some electromagnetic radiation and opaque to others.

Why does this happen? Ozone serves to absorb the harmful (to most living things) ultraviolet radiation from the sun. Without this protective layer in the atmosphere our skin would burn when exposed to sunlight. Carbon Dioxide absorbs in the far infrared portion of the spectrum which is related to thermal heating and results in a 'greenhouse' effect. Water Vapour absorbs energy depending upon its location and concentration, and forms a primary component of the Earth's climatic system.

Refracting Telescope A convex lens is used at the end of a tube to bring an image into focus at a point. Another convex lens called an eyepiece is used at the opposite end of the tube to magnify the image. The lens is typically made of glass coated in a film to allow less light to be reflected and more to pass through the lens. To distinguish between refracting and reflecting telescopes.

Reflecting Telescope A reflector collects light at one end of a tube and reflects it off a concave mirror. It is brought to a focus by a secondary mirror further up the tube at a 45 degree angle which is then magnified using an eyepiece. This type of reflector is called a Newtonian reflector. Another form of reflector is the Cassegrain reflector. This reflects light from its concave lens to a secondary mirror. This mirror is facing the primary mirror and reflects the light back down towards the primary but focuses through a small hole behind the primary. A reflector's mirror is usually made of glass with an aluminium coating.

Size Although neither type of telescope is perfect there are several reasons why reflectors have gained in popularity over refractors: Higher resolution can be achieved with a smaller tube Refractors suffer from chromatic aberration. This means that colour can sometimes be warped Refractors suffer from lens sagging. The weight of the lens is supported by the edges and gravity distorts this. The maximum size refractor can be up to 1 metre wide.

Mini Review 1

Mini Review Can you now write the difference between a reflecting telescope and a refracting telescope in your book?

Answer Reflecting telescope, uses a concave mirror, and uses the principles of reflection to project an image. A refracting telescope uses a convex lens to refract the image, and project it into an eye piece. Refractors suffer from chromatic aberration (where light f different wavelengths refract slightly differently). This means that colour can sometimes be warped Refractors suffer from lens sagging. The weight of the lens is supported by the edges and gravity distorts this. The maximum size refractor can be up to 1 metre wide. Reflecting telescopes are hard to maintain, (mirror recoating etc)

Optical and Infra Red Observatories From what you have learned so far discuss with your partner why most infra red and visible light observatories are located high up on mountains or space? To appreciate that most optical and infra-red observatories are located on high mountains or in space.

Answer Frequently, larger observatories are built in remote locations far removed from light pollution. They are also built on high mountains so there is less atmosphere to obscure viewing. Also they are located in areas that have good, dry weather and fewer clouds, so typically nearer the tropics or Equator. Dry weather is especially important for infrared astronomy as water vapour in the atmosphere is an interference. Less infrared light penetrates to the surface so a higher location is better for those observations.

Other Observatories X-Ray observatories are typically on-board spacecraft as are many ultraviolet and infrared observing stations. Space telescopes also have the advantage of not having to deal with twinkling and turbulence in Earth's atmosphere. Radio Telescopes are typically far away from cities and towns as microwaves from mobile phones and other transmitters interfere with signals

What are Van Allen Belts? Van Allen Radiation Belts, two zones encircling the earth in which there are relatively large numbers of high-energy (fast-moving) charged particles. The particles are mainly protons and electrons, which are trapped within the belts by the earth's magnetic field. The Van Allen radiation belts are centred along the earth's magnetic equator in a region of the upper atmosphere called the magnetosphere, or exosphere. To describe the nature of the Van Allen Belts and relate their discovery to early space missions.

How are they related to the early space missions? Use the computers to research how Van Allen Belts helped with early space missions. We will be discussing this information later.

Apply task Research and find some images of all of the different types of observatories:- Infra red Visible Light Radio wave X ray Gamma ray

Review Write how you have met each of your objectives. To understand that the Earth’s atmosphere is transparent to some electromagnetic radiation and opaque to others. To distinguish between refracting and reflecting telescopes. To appreciate that most optical and infra-red observatories are located on high mountains or in space. To describe the nature of the Van Allen Belts and relate their discovery to early space missions.