Telescopes - Reviews, Prices

Telescopes are instruments which are specifically designed to observe any remote objects by way of the collection of the electromagnetic radiation. Telescope is derived from the Greek language; tele denotes far and skopein means to look or see. This word was coined in the year 1611 by Giovanni Demisiane who is a Greek mathematician, who upon seeing the instrument that was exhibited by Galileo during a banquet that took place in the Accademia dei Lincei was inspired to name it.

Initially the telescope was first observed to have functioned in Netherlands in the early 17 th century. Hans Lippershey, Zacharias Janssen and Jacob Meitus of Alkmaar were the 3 persons to whom the credit of using a telescope in working condition goes to. They were refracting telescopes that were first operated in Netherlands in the year 1608. Galileo Galilei who is an Italian inventor started to personalize his telescopes and when he used the instrument and found what could be done with it made a great revolution in the history of astronomy. The study of celestial objects took a new turn from then.

The telescopes of this early period were found to have extremely less optical quality and there were aberrations found in the lens used. One such defect called as the chromatic aberration was quite common. The white light which comprises all colours does not focus in the one particular point and found to refract blue light more when it is passed through a simple lens.

Chester Moore Hall in the 18 th century found a solution to this problem with his crown and flint glasses. This is an Achromat lens which produces a totally colour-free image. Later on in the twentieth century, there was a marked improvement in the achromatic refractors. The oil-spaced objectives helped in clearing problems like internal reflections that were found in the lens-system and in an air to glass surfaces where the reflections caused immense light loss. The oil immersed lens proved to be a relief for all the inaccuracies because of the irregularities found in the surface of the lenses. The 1950s saw the telescope to be coated with magnesium fluoride which replaced the oil and also a glass which was composed of calcium fluorite was introduced. Takahashi, Ltd a Japanese company manufactured the first telescope with fluorite objective in the year 1977. Later on apochromatic lenses were introduced by Astro Physics in 1981 and they were color free, triple lens system. These were the new age of telescopes which had travelled a few centuries.

TYPES OF TELESCOPE

Telescope is a special term which includes a multiple array of instruments and basically they work under the same principle of collecting or grouping electromagnetic radiation for analysis or study. The optical telescope is the most commonly and widely used type and there are a few types under this category. There are the refractor telescope, the reflecting telescope, catadioptric telescope and many others.

Optical telescopes collects and focuses mainly on the light which has its source from the electromagnetic spectrum. These telescopes use curved optical elements like glass or mirror to bring the electromagnetic radiation or light to a particular point of focus. They increase the brightness and size of the distant objects with lot of clarity.

Refracting Telescope

Refractor telescopes mainly make use of the lens for image formation. The refractor telescope is a sealed tube with an objective lens at one side and an eyepiece at the other. The lens concentrates the light to the eyepiece. For a good starter, one should use a 60 mm refractor telescope which gives a better image of the moon and other celestial objects. In a refracting telescope light is usually bent as it passes from air medium to a glass medium. The curved surfaces of the lens allow the light to travel through the bent path and reach the optical axis of the lens. The light is focused to form an image if the surface of the lens is properly shaped.

It is better if the young and new observers use a small refractor as it is easy to carry, produces sharp images and has a multi-purpose flexibility.

Positive attributes of a Refractor

The advantages from either achromatic or an apochromatic refractor mainly depends on its quality. This presides over almost all the other telescopic designs. Since refractor telescopes are foremost in the use of observing both planetary and double stars, they are said to be the best in the field. There are two major advantages in the use of a refractor telescope.

First being the aperture clarity that is innate in the construction of a refractor. There is no obstruction found which causes the light to disperse all over into darker spaces and the said contrast in excellent in these types of telescopes.

Secondly, these refractors demands very low maintenance. Lens is the key factor and it does not require any recoating and it generally is fixed in the tube in such a way that misalignment is never possible unless put in the path of some major and unexpected calamity.

Negative factors of a Refractor

Initially the cooling aspect of the refractor was found to be a major inconvenience due to its long and closed tube. Lately, the aluminum tubes which are thin-walled are used in modern refractors which have reduced the cooling period quite considerably but still the time taken to cool in refractors is found to be a major negative aspect.

Also there is the issue of chromatic aberration of the bright images that seem to be more common in achromatic refractors. It is exposed to be like a shaky and pale fringe of colour that can be seen around celestial objects like Jupiter or the Moon.

Finally the rate of a refractor is initially found to be quite high as both the achromatic and the apochromatic lens are quite costly. The cost ratio found between a lens and a mirror is nearly 10:1 and the amateur astronomers are greatly deflated in their interest to buy a refractor telescope.

Reflector Telescope

A reflector telescope basically functions on the principle by using a set of mirrors placed appropriately to form an image. There is one large primary mirror followed by another smaller secondary mirror. It allows the incoming light or image to bounce through to a corresponding lens. Dim and faint objects like a nebulae or a distant galaxy is easily picked up with a reflecting telescope.

This is an instrument which has an all-purpose use and is flexible in viewing deep sky images and even distant planets.

Middle of the 17th century found James Gregory, a Scottish mathematician to have invented the first reflecting telescope. He gave an absolute and good description of it in the book called Optica Promota. Though he is credited with the invention, it is found that he never really made it. His description of the telescope was supposed to have used an ellipsoidal and a parabolic mirror.

Sir Isaac Newton of England was the one who designed and constructed the first working telescope with reflecting mirrors. He used a spherical mirror which had an aperture of one inch with a tube length of six inches. This model did not satisfy Newton as it had a few initial errors and he set out to make an improved version with a 2 inch aperture and a larger reflector. This was the very first Newtonian Reflecting Telescope which was presented to the world and kept in the Royal Society of England.

All the Newtonian reflectors included two mirrors in it, with a large primary mirror found at the bottom and a secondary mirror which is flat and sits nearly on top. Light is first reflected on the primary mirror which is once again reflected on the secondary mirror and the image is reflected to the eyepiece. There is also the Cassegrain reflector with a parabolic initial mirror and a hyperboloidal secondary mirror in which the light is allowed to pass through a centre hole of the initial mirror. This was designed by a French man Sieur Guillame Cassegrain who was a sculptor.

The mirrors used in the reflector telescopes were made of speculum. This was an alloy with a combination of eighty percent copper and twenty percent of tin. This needed frequent polishing as it would easily corrode when exposed to weather. Later on Justus Leibig, a German chemist painted the glass with thick coating of silver which was much easier to remove and recoat. This also did not disturb the curvature of the mirror. Only in the year 1932, a young physicist from the California University of Technology experimented with an aluminum coating which was found to reflect fifty percent more light than its silver counterpart and much more cheaper. Modern reflector telescopes now permanently use aluminum coating.

Positive Attributes of a Reflector

There are two major advantages in using a reflector telescope. First being that there is absolutely no chromatic aberration and secondly mirrors usually are found to have only a single optical surface. In an apochromatic lens, one can easily find a minimum of four to eight surfaces. Pertaining to the cost factor, mirrors are much cheaper than a lens.

Negative Attributes of a Reflector

The central obstruction caused by the secondary mirror effects to more scattered light and thus resulting in contrast loss of the image viewed. Modern manufacturers counter act this by mirrors which resulting small central obstruction.

Another negative aspect is the “coma” effect which is nothing but a defect which makes the stars to appear like a comet near the fringes of the viewing field. So a “coma corrector” has become a necessity for any Newtonian type of reflector telescope. One can find this effect in scopes with a smaller focal ratio of 5 and less than it. The corrector helps in improving the viewing field significantly.

As far as maintenance goes, mirrors do not require constant coating but on the whole reflectors are highly sensitive while moving and should be very careful when transported. If it is not a permanent fixation, the collimation of the telescope should be done before each fresh observation. Accurate collimation is of utmost importance to achieve a good result with a reflecting telescope.

A very large reflector which has a very thick primary mirror takes a long and difficult time to cool to the ambient temperature. Sometimes fans are used to speed up the cooling process. Also these large telescopes frequently require a ladder to see through the eyepiece.

Catadioptric Telescope

The combination of using both the lens and mirror to form an image is the integral part in the catadioptric telescope. The word “catadioptric” means “pertaining to” or “due to both refraction and reflection of light”. They are also known as the hyprid telescopes or compound telescopes.

Bernhard Schmidt, a German astronomer was the first to design a compound telescope using a spherical primary mirror at the back and corrector plate of glass in the front of a telescope in order to avoid or remove any spherical aberration. It is mainly used to take photographs by placing a photographic film at its prime focal area.

Another popular type of a compound telescope was designed by Dmitri Maksutov who was a Russian astronomer, in the year 1944, in which he used a corrector lens whose shape was spherical. This resulted in a highly compact catadioptric telescope. This corrector combined with a primary mirror with a centre hole was popularly called as the Maksutov-Cassegrain or the Mak-Cass. Questar Telescopes commercialized the Mak-Cass in the 1950s with their images almost to the correction of the most refined apochromatic refractors with the same aperture. On the other hand their length was just one third of a normal refractor.

Even though the Schmidt camera appears to precede the Schmidt-Cassegrain of the modern cameras, combining of the Schmidt corrector plate with the Cassegrain telescope was done only in the 1960s. With this particular reflector, light is bounced by a secondary mirror from the primary mirror with a hole in it and the image is corresponded to the image seen through the eyepiece.

The first Maksutov-Newtonian was created in the early 1990s when the Newtonian reflector was coupled to a Maksutov corrector. A famous Canadian company from Ottawa ,Ceravolo Optical Systems, was the first to deliver the Mak-Newt to the world. The optic paths are absolutely not folded and the secondary mirror do not impart any loss of mirror with a flatness to it. Both the overall physical length and the effective focal length are of the same. And in the end there was no hole in a Mak-Newt primary mirror and was positioned likewise that of a traditional Newtonian telescope.

Ritchey- Chretien Telescope

This telescope was found in the first and early decades of the twentieth century by an American optician George Willis Ritchey and Henri Chrétien, a French optical designer.

It was found by these two designers that when the amplification factor of the secondary mirror is lower then they flatter the field of image. Their telescopes had a lesser magnification with 2.7 x whereas the Schmidt-Cass had a 5 x magnification. Ritchey- Chrétien Telescope was totally coma free and have hyperbolic mirrors for both the primary and secondary mirrors which automatically correct coma but a Schmidt-Cass uses a spherical mirror which do not correct coma. Ritchey- Chrétien Telescope have two optical surface wherein the Schmidt-Cass has four optical surfaces, but still they are widely popular and used. The real reason behind this is that Ritchey- Chrétien Telescope is highly expensive to manufacture and so its purchase cost is dynamically quite extravagant.

To conclude, a person who wishes to buy a telescope who may be an amateur or a trained person, should be well aware about the basic history of a telescope and the different variations in it. In course of time and history each type of telescope has seen upgradation by different inventors and designers which has led to the various modern telescopes that are available in market today. All the different types of telescopes are individual in nature and have their own advantages and disadvantages. They have their own specific factors and the buyer should know his or her own needs in buying a telescope.