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. 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.
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.
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.
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.
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.
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.
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.