400 years of the Telescope
Prof. John Penny describes himself as a mechanical engineer who works as a Senior Lecturer at Aston University in their School of Engineering. However, he is also a committee member of Birmingham Astronomical Society and also co-edits their newsletter. Pulling these two threads together meant that he was able to offer a fresh perspective on four centuries of telescope design and evolution.
Due to time constraints he focused his narrative on observations using visible light. It would not be until after the Second World War that astronomers could investigate the skies using other wavelengths of light such as radio waves or infrared.
He began with the Galilean refractor in the seventeenth century. Galileo may have received all the credit for the invention of the telescope but it is more likely that it was conceived in Holland around 1608. It was used mainly to study the Sun and the planets, as technically it was rather poor. The earliest designs consisted of a long tube with a glass lens at each end. The lens at the far end of the tube was called the objective lens and the one nearest the observer the eyepiece. Unfortunately the lenses were very poor quality and, being lenses, they suffered from a problem called "chromatic aberration". This means that different wavelengths of light are focused at different points resulting in a blurry image surrounded by a coloured halo. Having a longer tube reduced this effect but it made the telescope heavy and difficult to aim but this did not stop some astronomers from building versions hundreds of feet long.
The next improvement came in 1668 with the Newtonian reflector; named after its inventor Sir Isaac Newton. This telescope design has no lenses as the magic is all performed with mirrors (well, discounting the eyepiece lens). One large main mirror collected the light and another steered it into the eyepiece. So, no fuzzy blurs of colour! However, in the early versions of this design, only 16% of the collected light ever reached the observer's eye and the primitive mirrors did not reflect the colours of light evenly, making the images appear orange.
The next step forward came with the achromatic refractor in 1733 invented by an optician named Hall. Sadly, his efforts to keep the design a secret failed and another optician called Dolland (of Dolland and Aitchison fame) ended up with the patent. This design improved upon previous refractors by reducing chromatic aberration. It did this by mating two lenses together to make one objective lens at the far end of the telescope tube. Initially it was only used for terrestrial telescopes as it had a very small field of view and the objective lens was difficult to produce. Today the largest refractor in the world resides at the Yerkes Observatory at the University of Chicago and it has a whopping 40-inch lens.
Looking to the future, the next generation of telescopes for the 21st Century is promising to be an exciting time for telescope design. The use of segmented mirrors allows for huge light collection areas. The OWL (Overwhelmingly Large) telescope when built will have a main mirror diameter of 100 metres and will take 15 years to build at a cost of a billion Euro. It will be able to see stars at the edge of the Universe in unprecedented detail and examine planetary atmospheres for the signs of life. It seems that astronomy is still looking up for some time yet.