getting started martin crow crayford manor house astronomical society

BASICS ASTRONOMY 1.1

Getting started

Martin Crow Crayford Manor House Astronomical Society

Crayford Manor House Astronomical Society

www.cmhas.wikispaces.com


Aims and goals of the course

An introduction to the basics of how to observe the night sky.

How to use binoculars and telescopes.

How to find things in the sky.

The essentials of how to begin the study of astronomy.


Definition of Astronomy

Astronomy is pretty much the study of everything physical in the universe.


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BASIC GLOSSARY OF TERMSStars: Shines by its own light. Our Sun is a star.


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Planets: An object that is only seen through reflected light.

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Moons: orbit planets and are also objects that are also only seen through reflected light.

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Asteroids: Rocky objects - orbiting debris.

If you can see one like this you’re in trouble!!!

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Comets: Icy / rocky objects – develop “tails” when in the inner Solar system.

Wikipedia

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Meteors: Dust and debris vaporising as they enter the atmosphere and commonly known as “shooting stars”.

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Solar Systems: A star and its retinue of planets.

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Nebula: objects that are within our own galaxy – the milky way.

Julian Tworek


Planetary Nebula: objects that are within our own galaxy – the milky way.

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Open clusters: the are loose clusters of stars located within our galaxy.

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Globular clusters: the are dense clusters of stars located around our galaxy and others.

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Galaxies: A system of stars. The Milky Way is our Galaxy.

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Universe: Everything that we can see.

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Basic glossary contd.

Eclipse / Occultation: When one body passes in front of another and obscures it.

Parsec: Is 3.262 light years or approx 206,000 A.U. You also get kilaparsec , megaparsec and gigaparsec

Ecliptic: The line of apparent motion taken by the Sun across the sky.

Astronomical Unit (A.U.): The mean distance from the Sun to the Earth approx. 150 000 000 km.

Light Year: The distance light travels in one year approx. 9 500 000 000 000 km or approx. 63,000 A.U.


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Basic glossary contd.

Orbit: The path followed by an object due to gravitational attraction.

Universal Time (U.T.): Time at the Greenwich Meridian


The Seeing: The steadiness of the atmosphere

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Getting started The Mark 1 eye ball


The eye acts like a camera and can be described in similar terms.

The lens in the eye has a focal length of around 16 mm and the aperture of the pupilvaries between 7 mm to 1.5 mm. This gives focal ratios from f2.3 to f11.

The retina is the light detecting structure and consists of some 100,000,000 lightsensitive cells of two types : rods and cones.

Rod cells cover the entire retina and work well at low light levels.

Cones are clustered near the optical axis of the lens and operate best at high light levels.Individual cones are optimised to responded to wavelengths of light and give us colourvision.

The cones are about 2 microns in diameter and are packed to a density of about 1000 per square millimetre near the optical axis.

The cones match the diffraction limit of the lens providing an angular resolution ofabout 80 seconds of arc at best. Away from the optical axis resolution is much lower.

The peak efficiency of the rods is at 505 nm wave-length and is about 15%. The peak sensitivity of the cones is about 10% at 570 nm this corresponds, not surprisingly, to thepeak intensity of the solar spectrum.


The eye detects light by the chemical breakdown of the chemical rhodpsin. This triggers signals to be sent along nerves to the brain for it to interpret. A lot of pre-processing is done at the eye.

In addition to filtering noise, the network of nerve cells pre-processes the signalsgenerated by the light- sensing cells so as to detect edges, lines and small differences in colour. So that the signals that travel to the brain are not raw brightness data but partially processed information on the shape , size and colour of objects in the visual field.


Dark adaption

Dark adaption is the eyes natural response to very low light conditions.

Full dark adaption takes between 20 – 30 minutes to attain. However, even brief exposure to white light will reset the adaption and you will need to start again.

To aid seeing in the dark red light can be used as this does not destroy dark adaption.

When seeing your surroundings through dark adapted eyes no colour can be perceived.

Averted vision

When viewing faint objects through a telescope or binoculars turning your gaze slightly to one side can help see detail that cannot be seen by looking straightahead. This is because when dark adapted the edges of vision are more sensitivethan the centre.


Binoculars

The best instrument to start doing astronomy is a pair of binoculars.

They are relatively inexpensive and very easy to use.

Demonstration – how to set up and use.

What binoculars to buy.

10 x 50 are ideal.

Magnification Object lens

The exit pupil is the diameter of the image at the eye piece of the binocular.This can be calculated by dividing the diameter of the object glass by the magnification.

For example: 50 ÷ 10 = 5 mm

For the most efficient use of binoculars it helps if the exit pupil match the irisDiameter of the dark adapted eye.

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Getting to know the sky and find your way around.

Plough


Polaris


Cassiopeia


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getting started martin crow crayford manor house astronomical society

Documents

astronomical unit

started martin

wikipedia slide

julian tworek slide

reflected light

system of stars

light years

loose clusters of stars