Ray Diagrams and Telescopes

The rays parallel to the principal axis are converged onto the principal focus. 

The focal length is the distance between the lens axis and the principal focus. 

Thicker lenses bend light more, and are therefore described as more powerful. 

Powerful lenses have short focal lengths.

Rules for Ray Diagrams

1) Any incident ray travelling parallel to the principal axis of a converging lens will refract through the lens and travel through the focal point on the opposite side of the lens.

2) Any incident ray travelling through the focal point on the way to the lens will refract through the lens and travel parallel to the principal axis.

3) Any incident ray which passes through the centre of the lens will in effect continue in the same direction that it had when it entered the lens.

Positive Image Distance	Negative Image Distance
Real	Virtual
Inverted	Upright
Erect (More Positive than Focal Length) Diminished (Less Positive than Focal Length)	Erect (More Negative than Focal Length) Diminished (Less Negative than Focal Length)

Refracting Telescope

A simple telescope is made of 2 converging lenses.

One is the objective lens of long focal length

One is the eyepiece of short focal length

Concave Reflecting Telescope

In a reflecting telescope, a large concave mirror is used as the objective, instead of a lens.

               

Parabolic Reflecting Telescope

Parabolic reflectors are used to collect energy from a distant source and bring it to a common focal point, thus correcting spherical aberration found in simpler reflectors.

Cassegrain Reflecting Telescope

The Cassegrain reflector is a combination of a primary concave mirror and a secondary convex mirror, often used in optical telescopes and radio antennas.

In a symmetrical Cassegrain, both mirrors are aligned about the optical axis, and the primary mirror usually contains a hole in the centre thus permitting the light to reach an eyepiece. 

©2011 Grant Dwyer

The Plough (Ursa Major)

Compilation of the stars in 'The Plough' that were 

taken and put back in their original position.

©2011 Grant Dwyer

Phad (Ursa Major)

Meade Telescope with SPC900NC 

Camera on 14/8/11

©2011 Grant Dwyer

Merak (Ursa Major)

Meade Telescope with SPC900NC 

Camera on 14/8/11

©2011 Grant Dwyer

Megrez (Ursa Major)

Meade Telescope with SPC900NC 

Camera on 14/8/11

©2011 Grant Dwyer

Alcor (Left) and Mizar (Right) (Ursa Major)

Meade Telescope with SPC900NC 

Camera on 14/8/11

©2011 Grant Dwyer

Alioth (Ursa Major)

Meade Telescope with SPC900NC 

Camera on 14/8/11

©2011 Grant Dwyer

Arcturus (Bootes)

Meade Telescope with SPC900NC 

Camera on 14/8/11

©2011 Grant Dwyer

Stellar Evolution

Stars are in a cloud of dust and gas, most of which was left when previous stars blew themselves apart in supernovae. The denser clumps of the cloud contract (very slowly) under the force of gravity.

When these clumps get dense enough, the cloud fragments into regions called protostars, that continue to contract and heat up.

Eventually the temperature at the centre of the protostar reaches a few million degrees, and hydrogen nuclei start to fuse together to form helium.

This releases an enormous amount of energy and creates enough pressure to stop the gravitational collapse.

The star has now reached the main sequence and will stay there, relatively unchanged, while it fuses hydrogen into helium.

Stars spend most of their lives as main sequence stars. The pressure produced from hydrogen fusion in their core balances the gravitational force trying to compress them. This stage is called core hydrogen burning.

When hydrogen in the core runs out nuclear fusion stops, and with it the outward pressure stops. The core contracts and heats up under the weight of the star.

The material surrounding the core still has plenty of hydrogen. The heat from the contracting core raises the temperature of this material enough for the hydrogen to fuse. This is called shell hydrogen burning.

The core continues to contract until, eventually, it gets hot enough and dense enough for helium to fuse into carbon and oxygen. This is called core helium burning. This releases a huge amount of energy, which pushes the outer layers of the star outwards. These outer layers cool, and the star becomes a red giant.

When the helium runs out, the carbon-oxygen core contracts again and heats a shell around it so that helium can fuse in this region. This is called shell helium burning.

In low-mass stars, the carbon-oxygen core isn’t hot enough for any further fusion and so it continues to contract under its own weight. Once the core has shrunk to about earth-size, electrons exert enough pressure to stop it collapsing any more.

The helium shell becomes more and more unstable as the core contracts. The star pulsates and ejects its outer layers into space as a planetary nebula, leaving behind the dense core.

The star is now a very hot, dense solid called a white dwarf, which will simply cool down and fade away.  

Perseids Meteor Shower

Every year from mid-July to late August.

Peaks between 9-14 August.

Observed for more than 2000 years.

Their radiant in the constellation of Perseus.

Primarily visible in the northern hemisphere.

In 2011, the peak meteor shower will be coinciding with the full moon so fainter meteors will be washed out.

Pictures Courtesy of Mila Zinkova

©2011 Grant Dwyer

Vega (Lyra)

Meade Telescope with SPC900NC 

Camera on 4/8/11

©2011 Grant Dwyer

Alkaid (Ursa Major)

Meade Telescope with SPC900NC 

Camera on 4/8/11

©2011 Grant Dwyer

Advantages and Disadvantages of Different Types of Telescopes

Telescopes	Advantages	Disadvantages
Refracting	Closed tube so very little maintenance and images are more steadier and sharper Hard to disrupt alignment	Production is expensive of large lenses and they can sag as there is no support in telescope Works only at night Prone to spherical and chromatic aberration Thick lenses absorb more light
Reflecting	Large mirrors are cheaper to make than large lens Mirror only needs to be polished on one side Only parabolic removes spherical aberration Not prone to chromatic aberration Primary mirror is supported so they can be big	Works only at night Open tube so needs maintenance Easy to disrupt alignment Secondary mirror can produce diffraction effects on image
Radio	Use a wire mesh as the long wavelength radio waves do not notice the gaps, construction is easier and cheaper It does not need to be as precise as a polished mirror Radio telescopes can work in all weather conditions Radio telescopes can work during the day and night Instead of one big dish, a number of dishes can be linked together and when the distance between them is increased, the image becomes sharper	The radio telescope dish has to have a precision of λ/20 to avoid spherical aberration Radio telescopes have to scan across a radio source to build up an image which is time consuming
IR	Detects anything that gives off heat	Detector needs to be cooled to low temperatures as it gives off heat as well Can only view near IR so high, dry altitudes or orbit to view higher wavelengths
UV	Can detect objects not seen in other wavelengths	Can only view near UV so orbit to view lower wavelengths
X-ray	Can detect objects not seen in other wavelengths	Mirrors need to be at very low angles in order to work Can only view x-ray in orbit to view it’s wavelengths

©2011 Grant Dwyer