Did You Know?
The telescope was invented by a Dutch eyeglass maker named Hans Lippershey, who patented it in 1608. He used it to observe only terrestrial entities, while famed Italian astronomer Galileo Galilei, after making some changes to the existing telescope, used it to observe physical features of celestial bodies like the Moon and satellites of Jupiter (Io, Ganymede, Callisto, and Europa).
Type of Telescope
If you are a beginner or wish to start exploring the night sky, inexpensive refractor (objective is a lens) or reflector (objective is a mirror) telescopes are good to go. They are available in the aperture range of 1.2 inches to 6 inches. Telescopes that have a combination of lens and mirror both, termed as compound telescopes (Schmidt-Cassegrains, Maksutov-Cassegrains, Gregorian), are high-end with multiple features. And naturally, they are on the expensive side of the price spectrum.
Aperture and Magnification
The quality of a telescope depends on its aperture or diameter. Greater the aperture, more the amount of light collected, which enables it to show a larger number of objects with greater detail. The downside being, a larger aperture increases the size of the telescope, which hampers portability. 4 to 8 inches aperture is best for a beginner's telescope. Generally, a telescope's maximum useful magnification is 50 times its aperture in inches, so avoid ones with implausible magnification.
Focal length is the distance between the lens/mirror and the point at which all the incoming rays converge to form a sharp image. The longer the focal length, the greater is the magnification. Please do not presume that the length of the tube of the telescope is its focal length. In compound telescopes, even though the tube is short, the image formed is sharp and clear. This means, even longer focal length can be achieved by using the right combination of lenses and mirrors.
When lenses or mirrors are made, their clarity depends upon how well polished they are. A wave error is the extent of smoothness or roughness of the surface of lens or mirror. it is measured in fractions of wavelength. The more the surface is polished, the clearer and sharper the image. Carefully read the product specification and try to find the wave error, mentioned in fractions of wave length, preferably ⅛ or less is good for a telescope.
This factor is generally applicable to high-end telescopes. While observing a distant object, if you move away from the eyepiece and happen to observe a spectrum of colors, then the telescope has aberration. Any kind of aberration leads to blurring of the image. Before buying, ensure that all the lenses do not have any chromatic aberration, and all mirrors used in the telescope do not have spherical aberration.
Resolution depends on all the factors like aperture, magnification, focal lengths of lenses or mirrors, wave error, and also varies according to certain conditions (e.g. light or dark or polluted or dry or humid atmosphere). All these factors effect the sharpness and clarity of the image.
As a beginner or amateur, you would naturally want to carry the scope with you to the best skygazing spot. But, if the instrument is too large, you may have to lug it around and so never use it. Eventually, the telescope that will be used the most is the one that is easiest to set up. So opt for a light and portable model.
Mounts and Handling
An often overlooked aspect is the mount, which is every bit as important as the telescope's optical tube. A mount holds a telescope and also defines how it moves. There are basically 2 types of mounts; altazimuth and equatorial. Altazimuth allows the telescope to move up and down and back and forth. The equatorial mount is designed to follow the movement of objects in the sky. An easy to handle mount is vital for a pleasurable stargazing experience.
Please ask the vendor for ways to maintain the optical instrument in a proper manner. For example, some mirrors are delicate and deform on even the slightest pressure exerted by our bare hands. So opt for a low maintenance piece or seek professional help for maintenance and care.
There are billions of stars so far that their light is yet to reach our planet, and other billions of stars which are dead millions of years ago, but their light is still visible to us. The universe is humongous and incredibly spectacular.