Magnifying glasses, spectacles and lenses all are a part of the optical field and evolved step by step. Their invention is co-related as the convex and concave lenses used in each of them are used with specific precision and adjustments but all are basically belonging to the same field. Telescope is one such invention which is based on the use of lenses and is very significant as it acquainted us with the world of stars and solar systems. Although, the invention of Telescope is credited to one man, its invention was due to the contribution and effort of many other people who reached very near to it during that time.
Roger Bacon is known to have used magnifying glass for reading during the 13th century. It was considered as a boon for many scholars who had to retire unwillingly due to poor eyesight although they were considerably younger in age. Around the same period, Italian craftsman started making glasses for people with poor eye sight. Around 1608 in Holland, for the first time two types of lenses-concave and convex were used together to form an optical instrument which resembled telescope. This early opera –glass resembling telescope was invented by Lipperhey. Although he had applied for the patent, his invention could not be reproduced and hence he could not get it. Also for magnification the concave lens should be stronger compared to the convex lens. But it was vice-versa in the telescope invented by Lipperhey.
Galileo when found about the invention of Lipperhey in the spring during 1609, he decided to make the necessary improvements in it and started right away. Around that time, he was working as Professor of Mathematics in Padua University, Venetian Republic. Since Galileo was already renowned for being a superb experimentalist, he immediately could arrange the spyglass along with the magnification of the three. But this was already made by other physicists of that time. But he found out one of major features of magnification while working on different lenses- magnification is proportional to the ratio of the power of the concave lens to the convex lens. This gave him an understanding that so as to obtain greater magnification, it was necessary to use weak convex lens and a stronger concave lens. By the month of August Galileo himself accomplished linear magnification of about ninefold. This was leaps ahead to what was available then in the market. Galilio demonstrated his invention before the Senate of Venice and it was even tested for eyeing enemies for military use on ships.
There are two types of telescopes with different principles: Refractors and Reflectors. The type of telescope is based upon the kind of primary employed in it. The main focusing element present in the telescope is called primary. Refractor type use glass lens as primary element and are termed as objective lens whereas the reflector type uses mirror as the primary. Glass lens have limitations as the gravity distorts them after a time period and its upper limit is 1 meter. That is the reason why most of the modern professional instruments are reflector type but even today the amateur instruments use refracting lenses. These telescopes are categorized on the basis of diameter of their primary.
If the telescope has larger diameter, the fundamental qualities are enhanced much to the desire of the astronomers. The two fundamental qualities are: Light gathering ability and resolution.
Light gathering :It works on simple theory. The larger the telescope, more light it gathers and more clarity in observing fainter objects. Since most of the telescopes use circular mirrors, the light collected is also proportional to the area of circle which is expressed as the pi times the sq. of radius or half the diameter. So the light collection ability of the telescope is considered as proportional to the diameter sq. Photographically it is expressed that 10 second exposure on a telescope of 10 meter will need 1000 seconds on 1mt. telescope to notice similar faint object. Hence diameter size is considered very important in professional telescopes.
Resolution:There are many stars in the sky very closely placed. And when the astronomers wish to view these two separately, resolution becomes the key factor. Similarly when the study has to be perceived in detail, resolution again becomes essential. Resolution is quantified in context with the angles.