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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