Category : Cambridge Astronomy Association

Federation of Astronomical Societies Convention 2008

I decided to attend my first Federation of Astronomical Societies Convention which provided me with my first whole day of astronomy talks, held at the Cambridge Institute of Astronomy on Saturday 21st September 2008.

The first talk was by Andy Fabian entitled “The Power of Black Holes”. Andy talked about the possibility of their being a black hole in the middle of our galaxy which can be viewed on ly in infrared as there is a bulge in the middle of our galaxy. He went through the history of who discovered black holes, such as John Michel who in 1784 was the first to work on the possibility of a black hole. He also talked about astrophysical black holes which have olny mass and spin and about gravitational red shift.

The second talk was by Nik Szymanek entitled “Photographing the Night Sky”, which was really why I attended the whole day, and his talk did not disappoint, with lots of amazing astrophotography images in the talk and his talk also contained an 8 minute video. I think in the future I would like Nik to get a bit more technical and actually tell us exactly how to take his type of magical photographs.

Another talk was by Barrie Jones entitled “The Seach for Extra-Terrestrial Life”. This talk covered the search for Exoplanets, which are mainly found by radial velocity. He also told us about what needs to exist on the planet to allow us to find a habitable planet, such as temperature, carbon and water together or ozone and oxygen photosynthesis or oxygen and methane.  For us to find technically intelligent aliens they need to have binocular vision, hands and brains.

The final talk was by Somak Raychaudhury entitled “Einstein’s Outrageous Legacy – Black Holes, Cosmic Illusions and Dark Energy”, but I did not stay for this, as I think I had already seen the speaker at a Cambridge Astronomy Association night.

There were lots of stalls in the main foyer, including stalls from Astronomica, Green Witch, Cambridge University Press, Nik Szymanek and many other book stalls as well as stalls from the national and other local astronomy clubs.

The Sun through a Coronado PST TelescopeGreen Witch set up a Coronado PST telescope outside which many people used to get their first view of the Sun, this is the image I managed to get by putting my Canon 400d up to the eyepiece of the Coronado PST.  There was a really good solar flare coming from the Sun but this does not seem to have come out on the photograph.

Tea, Coffee, soft drinks and biscuits were served all day long, never have I drunk so much tea in one day! The weather was perfect and during lunch there was a tour around the on-site telescopes. Here are some photographs I took of the various domes, namely the Northumberland, 32 inch, Schmidt camera telescopes as well as the Institute of Astronomy library building.

Schmidt Camera Dome Northumberland Telescope Dome Institute of Astronomy Library Building 36 Inch Telescope Dome

There was also an imaging competition, as well as a raffle. Overall a good day out and I am glad I attended, especially as the location of the day was local for me.

The Magnetic Sun

“The Magnetic Sun.” Dr Alice Courvoisier was the title of the August 2008 Cambridge Astronomy Association talk. 

A diagram of the sun explained each named part of the Sun including the photosphere, chromosphere, filament, convective zone, radiative zone and prominence.

It was also explained to us that the solar material in the Sun is a plasma and so conducts electricity.

The lecture included of course sunspots, and it was interesting to find out that the actual recording of sunspots did not begin until 1610. William Herschel thought that sunspots were openings in the Suns atmosphere.

The centre of a sunspot is called an Umbra whilst the outer part is called the Penumbra. The Umbra appears darker as it’s cooler than the other parts. We were also told that sunspots appear in pairs which have opposite polarity. There is an 11 year sunspot cycle and the polarity of the sunspots also reverses after 11 years.

The talk then asked the question can we predict solar activity? This section ended by quoting “At the moment there appear to be too many uncertainties in the current solar cycle models to allow for sound predictions”, which was taken from a Nature article.

We were told that solar winds are when the Sun expels a million ton of particles which are threaded by magnetic fields.

Solar activity can also cause problems for us on the Earth, as geomagnetic storms induced by solar events can interrupt radio, satellites, cause power cuts and damage pipelines. . In March 1989, 6 million Canadians lost power for 9 hours because of the Sun’s magnetic activity.

Overall this was an interesting talk that taught me more about the Sun than I knew.

Alice first came to the UK to follow a masters course at Cambridge University in 1999. Then, after a two year break teaching physics in Madagascar, she came back to do my PhD in the Department of Applied Mathematics at Leeds University, which she completed in 2006. Alice is currently working as a post-doctoral research associate in Leeds; where her main interest is trying to understand the origin on large-scale magnetism in astrophysical bodies.

Stars Dancing with Black Holes

Stars Dancing with Black Holes was the title of Marc Freitag’s talk at this months meeting of the Cambridge Astronomy Association.

Marc began by explaining what actually is defined as a black hole; known to have a most extreme case of gravitation, where the mass is concentrated in one point and it is surrounded by a spherical horizon.

Marc also had an interesting slide detailing the flow and gravity pull of a black hole which included details on subsonic flow, supersonic flow and about the sonic horizon.

There is believed to be a black hole at the centre of our Milky Way.

We were told that the effects of Massive Black Holes (MBH) on stars can be used to reveal the presence of Massive Black Holes and determine their properties. We also learnt that stars around Massive Black Holes in a spherical nucleus produce a rosette orbit and that as a star is close to an MBH you get a Keplerian ellipse.

Stellar Massive Black Holes are the end product of evolution of stars greater than 20 to 30 suns, and they are only 1-2 stars in 1,000.  This version accumulates around the Massive Black Holes.

One interesting future piece of research is to be the LISA spacecraft which is being designed to measure the mass and rotation of Massive Black Holes. IT will measure Gravitational Waves (GW) which are emitted when a black hole passes a massive black hole.

A Gravitational Wave is a contraction of space in one direction and an expansion in the other direction. Gravitational Waves also produce contraction and expansion of space itself.

The LISA spacecraft consist of three spacecraft that will form the shape of a triangle in space in an orbit similar to that of the earth.  The distance between the spacecraft will be 5 million km apart.

LISA will detect gravitation waves by comparing distances between “test masses” in different directions using lasers. A similar research project on the ground has been executed called LIGO in the US and VIRGO in Italy.

Overall the talk was very in depth and I got a bit lost half way through with all of the formulae and physics, but the talk was littered with interesting images, simulations and videos.

I look forward to see the launch of LISA in the future, and to see if it will be possible to detect gravitational waves as well as if it will be possible to keep the three spacecraft evenly spaced across space in order to constantly create a perfect triangle.

Introduction to Astronomy – Week 6

This was the final Wednesday night on the Introduction to Astronomy course and a bit of a shame it was all over.

This week we had a talk by Roderick Willstrop entitled “History of the Telescope”. Roderick took us through 400 years of the main developments of Telescopes and the major telescopes that have been built, including Space Telescopes.

Roderick went through a long list and dates of the major advances over time, these included:

Name Date Comments
Ham Lipperschey 1608  
Galilei Galileo 1609  
Johannes Hevelius 1647 Convex and Concave lens at eyepiece end
Isaac Newton 1668 Reflector
James Hadley 1721 1st successful 6″ reflector with 5ft tube
Chester Moor Hall 1733  
John Dolland 1759  
William Herschel 1776 7ft reflector, Uranus discovered on 13th March 1781, 40ft Reflector in Slough, 20ft Reflector in Bath
Pierre Louis Guimand 1799 Flint Glass, 9.6″ Refractor 1824 Dorpat Observatory Russia
Joseph von Fraunhofer 1824 9.6″ Dorpat refractor designed by Fraunhofer in Berlin, discovered Neptune 1846
Edward Cooper 1834 Built 13.3″ refractor built at Markree Castle, Ireland
Duke of Northumberland 1838  
Rev. John Craig 1852 25″ reflector
William Parsons 1845 6″ reflector at Birr Castle
William Lassell 1861 48″ reflector in Malta
Andrew Common 1879 36″ reflector
James Lick 1888 36″ telescope with 58ft focal length on Mount Hamilton
George Ellery Hale 1897 40″ telescope with 63ft focal length
Paris Exposition 1900 49″ refractor
Hale Telescope 1948 200″ telescope
BTA Russian 1975 6m and 40″ thick mirror
Multiple Mirror Telescope 1987 6 x 1.8m mirrors (72″) which was re-built in 2000 with a single 6.5m mirror
Keck I 1993 2 x 10m mirrors in 36 segments
Keck II 1996  
South African Large Telescope 2005 11m telescope
Large Binocular Telescope 2005 8.4m mirrors

Hopefully I wrote all these notes and dates correctly, although there may be some errors, if anyone wants to correct me, feel free to do so.

After the talk, we went out and visited the 3 mirror telescope (3MT) which Roderick actually designed.

The Three Mirror Telescope (3MT)

Three Mirror Telescope at CambridgeThe Three Mirror Telescope (3MT) has been developed at the Institute of Astronomy by Dr. Roderick Willstrop. Its optical design is unique because it is the only form of telescope which combines the three advantages of a wide field of view, very small sharp images, and all-reflection optics.

Reflecting telescopes have been built with apertures up to 10 metres, but they have fields of view limited to 40 arc minutes (Ritchey- Chretien two-mirror design) or 1 or 2 degrees with a 3- or 4-lens Wynne corrector near to the focus of the main mirror. Schmidt cameras can give fields of view of 7 or 8 degrees, but their apertures are limited to about 1.3 metres because they use a thin glass lens large enough to cover the whole aperture.

The 3MT has a field of view 5 degrees in diameter, and the ray- theoretical image size is less than 0.33 arc seconds everywhere, and less than 0.1 arc seconds over the central 1 degree of the field. Because no lenses are needed the images are perfectly achromatic, and furthermore it is possible, in principle, to build this telescope with a larger aperture than any Schmidt camera.

The design is based on a simpler one discovered in 1935 by the French optician Maurice Paul and rediscovered independently in 1945 by the American James Baker. The original Paul-Baker design had a paraboloidal primary mirror, a convex spherical secondary mirror and a concave spherical third mirror. If the aperture of the primary mirror was f/4, it would have given acceptable images, and a field of view of about a degree.

The good performance can be explained as follows: if the second mirror were a convex paraboloid (instead of spherical) the light from a distant star would be made parallel again after the second reflection. (This arrangement of two coaxial and confocal paraboloidal mirrors was described by Mersenne in 1636.) The third mirror would then also have to be paraboloidal to focus the light, and the field of view of the whole system would be no larger than that of a single paraboloidal mirror. The essential feature of the Paul-Baker design is that the second mirror is spherical, so the light is not exactly parallel after the first two reflections, but is deviated in just the same way as by the corrector lens of a Schmidt camera. Then the third mirror must also be spherical to focus the light, and a large field of view with sharp images is obtained. This telescope has also been called the Mersenne-Schmidt.

Baker suggested that the Hale 200-inch (5.08 metre) telescope might be given a wider field by using two auxiliary mirrors. It would not have been acceptable to enlarge the central hole in the irreplaceable 200-inch primary mirror. The third mirror, 2.1 metres (84 inches) in diameter and weighing one or two tons, would therefore have been mounted directly above it, so the system was never built. The field of view could have been just over 1 degree. In the f/3.3 Hale telescope, it would have been necessary to modify the shape of the secondary mirror to retain small, sharp images.

Introduction to Astronomy – Week 5

This week we had a talk by Lindsay King entitled “Galaxies – The Universes Building Blocks”. This week Lindsay went beyond the local group of galaxies and looked at different types of galaxies and clusters of galaxies. She then also looked back in time to the youngest galaxies at the limits of observation and showed how these most distant objects are detected.

The talk began with a look at Hubble’s galaxy classifactions as well as a mention to Fritz Zwicky who studied the Coma Galaxy Cluster in 1933 which was 300 million lightyears from Earth. At this time Fritz put forward the theory that unseen matter must exist, which we now call Dark Matter.

Lindsay also told us about the effect ‘RedShift’ which is when the speed at which a galaxy moves away changes the colour of the light. A red galaxy is moving further away, whilst a blue one is closer.

We were also introduced to the furthest galaxy away from us, which was discoverd by the Subaru telescope which is a Japanese telescope located in Hawaii. This furthest galaxy is said to be 13,000 million lightyears away.

We also got an explanation as to what “Gravitational Lensing” is. Mainly that mass distorts space-time and that galaxies form  where dark matter exists in order to trap gas and make stars. It was also noted that galaxy clusters are so large that they warp space-time around them.

The Schmidt Camera Telescope

Cambridge Schmidt Camera TelescopeAfter the talk we went out to see the now unused Schmidt Camera telescope. It has not apparently been used since taking images of Halley’s Comet in 1986, and may be moved to Wales, to be used in the project for imaging potentially fatal asteroids coming towards the Earth.

The instrument itself was built in 1952 by Grubb-Parsons of Newcastle-upon-Tyne and replaced an older telescope in the existing dome, which had been made by T. Cooke & Sons Ltd. of London & York at the time of the move of the Solar Physics Observatory from South Kensington to Cambridge.

It is a `Classical Schmidt’ – the simplest and most efficient form of the ingenious wide-field camera invented in 1930 by Bernhard Schmidt of Hamburg Observatory. Light from the sky falls upon a 61 cm (24-inch) mirror with a spherical reflecting surface, at the bottom of the tube. It is reflected to a focus in the centre of the tube and half-way up it, 163 cm (64 inches) from the primary mirror. At the focus a photographic plate P 15 cm (6 inches) in diameter, which must be bent to fit a curved surface, records the star images in an area of sky 5 degrees in diameter. (The full Moon is half a degree in diameter.)

Without any further optical element the star images would be of poor quality owing to “spherical aberration”: light falling near the edge of the mirror would come to a focus too close to it, and light falling near the centre of the mirror would be focused a little too far away.

Schmidt’s invention was to place at the centre of curvature of the primary mirror, near the top of the tube, a weak meniscus lens (in this case 43 cm (17 inches) in diameter) with one aspheric optical surface: this makes the light which passes through it near the edge diverge slightly, lengthening the focus of the outer parts of the mirror, and makes the light passing through near the centre converge, shortening the focus of the centre of the mirror.

This optical combination of lens and mirror forms a fast, efficient camera giving sharp star images of uniform quality over the full 5 degrees field. It is an ideal sky-surveying instrument; by contrast the 36-inch (91.4 cm) telescope, with its paraboloidal mirror of 4.1 metres (162 inches) focal length (f/4.5), has a field of view only 7.2 arc minutes in diameter with images smaller than 2 arc seconds.

The auxiliary 15 cm (6 inch) telescope is for guiding. The exposure time is usually of the order of 10 minutes, and during this time the image can wander about on the photographic plate mainly because of irregularities in the refraction in the earth’s atmosphere. These are corrected by maintaining a star image at the intersection of the cross-wires in the guiding telescope.

The Palomar and U.K. 48-inch (1.22 metres) Schmidt cameras which were used to make the all-sky surveys (now kept in the Cambridge Astronomical Survey Unit) have apertures nearly three times as large as our telescope, but focal lengths (and tubes) only twice as long. Only one Schmidt camera (the 53-inch (1.35 metres) at Tautenburg, Germany) has ever been built larger than these two.

The reason is that, if a Schmidt camera is simply scaled up, its image size is also scaled up, and as Bernhard Schmidt himself predicted, the 48-inch Schmidts are close to the practical limit. The main image defect arises because the thin lens can correct the “spherical aberration” of the mirror exactly in only one colour of light, (usually blueish green), red light is under-corrected, and blue or ultra-violet light is over-corrected.

To minimise the length of the tube, and so the size and cost of the dome, the 48-inch Schmidts have been made with an aperture of f/2.5 (this one is f/3.7) and the “spherical aberration” of the mirror is then 3.2 times as large as in the camera.

Three of the largest Schmidt cameras have been fitted with “achromatic” lenses which reduce the residual colour errors, but astronomers now use very fine-grain emulsions, and wish to observe a wide range of colours of light, so these large Schmidts are still at the practical limit of size.

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