About Telescopes and Astronomy
A telescope is a piece of equipment that allows individuals to observe objects at great range by collecting and converging visible light waves, which are electromagnetic radiation, into a point of focus. This process is achieved in a number of different ways that are determined by the specific design and model of telescope used, of which there are several. The earliest recorded use of telescopes pertained to refracting models, which are explained in detail below.
The Most Common Types of Telescope
This buying guide references optical telescopes in its guidance, as these are generally the only kind of telescopes purchased for use in the home or on a personal or hobby level. However, there are other kinds of telescope, which are used scientifically, or are mounted on a large scale for use in applications such as research or defense. Though there are many variations, the two most common types that will be familiar to members of the public are optical and radio telescopes.
These types of telescope collect and focus light waves to magnify images at great and varying lengths. They are commonly used for astronomy and come in three main types.
Radio telescopes are designed for use in radio astronomy, which is astronomy studying celestial objects on radio frequencies. The discovery of radio waves emitted from celestial objects was made in the early 20th century. These are the large satellite arrays seen on mountain tops and at science installations.
Different Types of Optical Telescope
There are three basic kinds of optical telescope available: refracting telescopes, reflecting telescopes, and catadioptric telescopes.
The term refraction refers to the process of light waves bending when passing from one medium of density to another, such as from air into glass. It is this process that refracting telescopes take advantage of, by using a specially designed lens to bring the reflected light into a point of focus, thus magnifying the image.
Reflecting telescopes contain two mirrors; a larger primary mirror is positioned at the bottom of the telescope's central tube, with a secondary mirror, which is smaller and flat, near the top of the tube. When light waves enter the top of the tube, it impacts the primary mirror, then being reflected into the secondary mirror. From here it is reflected into the eyepiece, producing a magnified image.
The term catadioptric essentially refers to both properties of the aforementioned telescopes, using a combination of refractor and reflector aspects in their designs.
Advantages and Disadvantages of Refracting Telescopes
The following are the advantages and disadvantages of refracting telescope designs.
Refracting telescopes of a good-quality build bring some benefits over the other types of telescope. One such benefit is that refractors possess a completely clear aperture, the opening through which the light waves travel; this means that there is no object causing the light waves to be dissipated into darker areas within the telescope, increasing the clarity of the contract. In other telescopes, the light waves do not always travel in such a straight manner and bounce off of a number of mirrors before entering the eyepiece.
Furthermore, these telescopes require relatively low maintenance. The lenses contained within a refracting telescope do not have to be recoated, whereas mirrors do. Also, they do not usually need collimation, which is the aligning of the instruments within for maintenance, as the contained lens is fixed into place and will not move over time unless damaged.
Due to refracting telescopes possessing a closed tube design, it takes a greater amount of time for them to cool down to the ambient air temperature. The problem here is that rises in internal temperature within a telescope can cause image degradation.
Another disadvantage is that some refracting telescopes can have problems with chromatic aberration, which is the appearance of distortion around images. This manifests as bleeding colours, and is caused by lenses having different refractive indexes for various wavelengths of light, as not all colours are focused on the same point of convergence.
A final drawback to such telescopes is the high cost involved in producing a large lens as opposed to similarly sized mirrors. This results in refracting telescopes being generally more expensive.
Advantages and Disadvantages of Reflecting Telescopes
The following are the advantages and disadvantages of reflecting telescope designs.
One of the main advantages of reflecting telescopes is that they do not suffer the same issue of chromatic aberration that affects refracting telescopes. This results in more consistently high-quality image. Furthermore, as mentioned above, mirrors are less expensive than lenses to produce, having only one optical surface as opposed to between four and eight like refracting telescopes.
The clarity of image produced in refracting telescopes is unaffected by obstructions, giving it a completely clear aperture. In reflecting telescopes, however, the secondary mirror acts as a central obstruction, which causes light waves to scatter and thus reducing the contrast of the image.
A further problem that affects all reflectors is something called 'coma'; this is a problem that makes stars towards to edge of the viewing field to appear as a comet.
Two final points of mention here are that mirrors can require a recoating after extended use over a number of years. These telescopes are also quite affected by minor bumps or when being transported. Therefore, it should be collimated before each use, unlike the refractive telescopes that do not require such calibration. Also, these telescopes can suffer the same internal overheating problem that refractive telescopes suffer from, though a cooling fan may be installed.
Advantages and Disadvantages of Catadioptric Telescopes
The following are the advantages and disadvantages of catadioptric telescope designs.
The majority of catadioptric telescopes are very easy to move and transport because of their compact size and portability. Furthermore, the aforementioned advantages of both mirrors and lenses are combined into one unit, bringing the benefits of both previously discussed models.
These models can experiences a small loss of light due to the positions of the secondary mirror. They may also suffer something called image shift, a jump in focus, due to the primary mirror position being adjusted when focusing.
Finding Telescopes on eBay
A wide range of telescopes can be found listed on the eBay marketplace, which allows sellers to list a range of items under easy to browse categories. These categories are all accessible from the homepage of the eBay website.
From the homepage, buyers seeking a telescope can click through the Electronics category and into the Cameras & Photography area of the website. From here, a wide range of subcategories are visible relating to items that fall under this broad product range, including Camera & Photo Accessories and Telescopes & Binoculars, which is where the aforementioned telescope models and more can be found.
To filter the large range of seller listings that will be visible under this category, there are filters found on the left side of the webpage, which narrow results by a number of parameters, including the condition of the item, the price – which can be specified as an exact range – of the item on sale, and delivery methods; there are other filters too.
Telescopes are instruments that allow the remote viewing of objects at great distances away from the equipment. Each of the aforementioned basic types of optical telescope has its own advantages and disadvantages, with catadioptric possessing characteristics from both refracting and reflecting models.
Optical telescopes of all types are commonly listed on the eBay marketplace and can be found under the Telescopes & Binoculars section of the website, which is a subcategory of the Camera & Photo Accessories section. By using a number of different filters, buyers can narrow the results of sale listings by price, item condition, and other parameters.