Examples of Optical Observatories

Unit 2 – Optical Instruments:

INTRODUCTION

What is the only thing we can study from stars and galaxies? Light

Although Light is the only thing we can study, we can tell allot of info from light

What is light? Discussion Light is a wave

WAVES INTRO:

What is a wave? A disturbance in a medium that transmits energy from one place to another. Do a stadium wave in the room, notice hands and people do not move across the room, only energy from the people.

Demo – tuning fork and water demo – shows energy is transferred into the water

There are two ways in which waves can travel: Transverse and Compression

Transverse Wave: A wave in which the medium travels perpendicular to the direction of wave motion. Slinky demo What is a medium? The substance in which a wave travels through Ex: The medium for waves in the ocean is water. The medium for sound from me to you is the air.

Examples of transverse waves: Water, stadium wave, S-earthquake waves, electromagnetic radiation (light)

Compression Wave: A wave in which the medium travels parallel to the direction of wave motion. Slinky demo, sound box demo

Examples of compression waves: Sound, P earthquake waves

There are also two types of waves. Waves that need a medium to travel through, and waves that don’t. Mechanical wave – A wave that needs a medium to travel through Ex: almost all waves.

Electromagnetic wave – A wave that does not need a medium to travel through Ex: Electromagnetic waves – light, x-ray, microwave, UV, IR, etc…

Wave Terminology:

Wavelength, Amplitude, Frequency.

Wavelength – The length of a wave. () Often measured from crest to crest and trough to trough Draw waves, have students determine # of wavelengths (compression and transverse)

Amplitude – The height of the wave – A measurement of the amount of energy in a wave. The bigger the wave, the more energy it carries (ocean wave vs tsunami) Do examples on board

Frequency – The number of waves that pass a given point in a certain amount of time. Measured in Hertz (Hz) – cycles per second. f = # waves / time

Ex: radio stations are in megahertz. 96.1 megahertz, … Do examples of equation.

______

Properties of Light

What is the only thing we can study from stars and galaxies? Light

Although Light is the only thing we can study, we can tell allot of info from light

Light is a wave. It carries energy from one place to another and does not need a medium to travel through. Electromagnetic Radiation can be broken into a spectrum:

Different light carries different amounts of energy.

Create the following chart, having students fill it in Radio contains LOW energy / Big wavelengths Gamma Rays contain HIGH energy / SHORT wavelengths.

Ex: After Visible light, comes UV. UV is higher energy than visible. It can penetrate the skin and causes Skin damage, and skin cancer.

X-ray is higher energy than UV. It not only penetrates the skin, it can go through the body, and gets stopped by dense bone (x-ray images). It is more damaging and can cause all sorts of cancer at higher doses.

Gamma-rays are higher energy than X-rays, it even penetrates dense bone. This causes damage as it goes through the entire body. Gamma-rays are emmited during nuclear blasts.

- Waves Worksheet - Image sheets of astro objects in different wavelengths. Stresses importance of viewing objects in different types of light.

Why do we care?

Electromagnetic radiation is the only thing we can study from stars and galaxies.

Luckily, we can tell allot from light.

If we look at a simple model of the atom, electrons Are arranged in orbital. Give an electron energy, and it jumps to a higher orbital. Eventually it will fall to its normal orbital. When it falls, it looses energy. This energy gets released in the form of electromagnetic radiation (photons).

Different atoms give off different energies of light.

It’s kind of like a finger print. Each atom has its own unique fingerprint, or spectra of light.

Therefore, by looking at the spectrum given off by an object, we can tell what elements it is composed of.

Spectrograph – a device that separates light into its spectra.

Do Spectroscope demo with H, He, CO, Ar lights. - Star Spectrum handout

Optical Telescopes –Refractors & Reflectors

Why do astronomers use telescopes?? Magnification, resolution, Brightness. Actually, magnification is the least important. It ‘s mainly used to make objects brighter, visible to us.

1. Brightness – The primary purpose of a telescope is to gather light and make faint objects appear bright.

Brightness depends on the size of the lens, the Objective Lens. The bigger the lens the more light it can gather.

Our pupils have a lens just mm’s across, The Keck telescope in Hawaii has a 33 foot mirror (lens). It can see things millions of times fainter than the eye can. Nocturnal animals have larger lenses.

2. Resolution – How sharp an image looks. This isn’t magnification. Example: HDTV vs normal TV. Atari VS Old Nintendo VS Sega The larger the lens, the higher the resolution.

3. Magnification - How many times larger an object looks. We will soon learn that any telescope can have almost any magnification. That’s why it’s not that important of a factor.

The formula for magnification is M = fo / fe

fo = Focal Length of the Objective fe = Focal Length of the Eyepiece

REFRACTOR TELESCOPE:

Lets see what telescopes look like:

A Refractor telescope uses lenses to bend light.

Do lense demo – have students find focal lengths. There are two lenses, an Objective (Primary), and an Eyepiece (the one you look through)

The two lenses are placed their focal lengths apart.

Ex: fo = 1000 mm, fe = 20 mm - The length of the telescope is therefore 1020 mm. Any other distance and the image will look blurry.

Whats the magnification? Fo/fe = 1000 / 20 = 500 x

The Objective lens focal length is always bigger than the eyepiece lense. (if not, it would de-magnify the image)

NEWTONIAN REFLECTOR TELESCOPE

Instead of an objective lense to bend light, it uses A mirror to reflect and bend the light.

So your head is not in front of the mirror when looking into it, there is a Diagonal mirror placed in the tube, which bends the light 90 degrees, so your head is at the side when looking in.

Show refracting mirror

CASSEGRAIN REFLECTOR TELESCOPE

There is another type of scope, the Cassegrain telescope.

It is basically a Newtonian reflector, but with the eyepiece behind the mirror.

There is a small hole drilled into the middle of the objective mirror. The diagonal is now straightened, so the light bounces back through the hole. The eyepiece is then put behind the objective.

Why do this??? It shortens the length of the scope. Also, today large computers are attached to the eyepieces. It is difficult to put them on the side of the scope. It’s easier to mount them behind the scope.

Therefore, almost all professional telescopes today are Cassegrain Reflectors.

ADVANTAGES and DISADVANTAGES of reflectors and Refractors:

Refractors:

Advantage: Better resolution (no diagonal blocking part of the objective)

Disadvantage: Suffer from Chromatic Abberation (different colors refract differently, colors get smeared). Size limit of Objective lens. (If one makes the glass lens too big, it will start to sag, or bend. The max size is approximately 1 meter (3 feet).

Reflectors:

Advantage: No size limit, the sky is the limit. One can make the objective mirror as big as they want, since they can support the back of the mirror. Usually they make lots of small mirrors and put the mirrors together.

Disadvantage: Slightly less resolution due to the diagonal mirror.

- Do telescope lab Examples of optical observatories: Keck I and Keck II in Hawaii – worlds largest Hubble Space Telescope GEMINI – in Chili

Places to build observatories: Away from Cities - Observatories need to be placed in areas with little light pollution Mountain tops – The more atmosphere a telescope looks through, the blurrier the image – ex: heat waves on a hot road, twinkling stars.

Atmospheric Windows:

The Earth’s atmosphere allows some types of light to come in, and blocks others.

Atmospheric window handout:

Transparent: Visible, radio Semi-Transparent: Infrared Opaque: Ultraviolet, X-ray, Gamma Ray

What if Most UV light was not blocked? What happens as we deplete our ozone layer? What if gamma rays were not blocked?

Radio telescopes:

Parts: Dish – large dish that focuses the rays – Direct TV dish, etc.. Dish does not have to be very smooth, wavelengths are very large, so the radio waves will bounce off surfaces that are not very smooth - Ex: Arecibo – is a wire mesh.

Receiver – Receiver is placed where the dish focuses the rays. Receiver gathers the rays and is sent to a computer to analyze and make a False Color Image. False Color Image: Bright areas = high intensity Dark areas = Low intensity Resolution – Since radio waves are VERY large, they have a VERY low resolution. Ex: the moon looks blurry in radio waves.

Interferometer Array – A way to increase resolution. Astronomers observe the same object with different telescopes at the same time. The data from ALL of the telescopes is sent to a computer, where the data is put together (images are stacked on top of one another). This creates a higher resolution image.

Length of Array telescope = distance the telescopes are separated.

Where to place Radio Telescopes:

Valleys – There is allot of radio interference. TV stations, Cell phones, Communication Satellites, Radio Stations, Airplanes, etc… For astronomers to properly observe objects in space, they must “hide” from this radio interference. Valleys are a good place to “hide” from the interference. Mountains tend to block radio waves. Ex: one looses radio stations while traveling through mountains.

Examples of Observatories:

Arecibo – Worlds Largest single radio telescope. It is in Puerto Rico VLA – Very Large Array – in New Mexico – an array of dozens of radio telescopes on train tracks. They can move the telescopes any distance they want. VLBA – Very Large Baseline Array – An array of some of the worlds largest radio telescopes. ~ size of Earth.

Infrared Telescopes:

Can use a regular mirror for these telescopes, the focal points are just different than with visible light. Must be placed High in the atmosphere or in space. Example: SOPHIA – an aircraft with a telescope built into it. Spitzer – an infrared telescope in space – studies star and planet formation.

High Energy Telescopes:

X-ray and Gamma-Ray

The light is to energetic to be reflected off a dish or mirror. Instead astronomers channel these light rays onto a CCD (charged Coupling Device). Any digital camera has a CCD in it. Photons hit the chip, which excites electrons and allows them to transfer. This electron signal creates an image.

X-ray and gamma ray telescopes have funnels made of gold in order to channel the high energy light.

Examples of Observatories:

Chandra – X ray telescope – in space.

Watch HST/ESA movie

Review sheet