At least two, but may have many. Depends on the design of the telescope.
Reflecting telescopes are far-and-away the most common. This is because the telescope can be smaller and lighter, and because lenses always absorb a little light while mirrors can be almost perfect reflectors.
A lens and a mirror both serve to bend light in a particular way. A lens bends the angle at which the light goes, while a mirror reflects the light away.
There are many uses of diverging lenses including being used as a microscope lens. The main use is to correct short sightedness.
Quality of the Lenses and/or mirrors first. Coatings put on those mirrors/lenses. Telescope design - Refractor, Reflector, Catadioptric (Combination of lenses and mirrors. What do you want to observe in the sky? - Planets? (The highest quality refractor is the best but most expensive. Stars and Nebulae? you want to get a large diameter reflector to gather as much light as possible. Do you want a compromise in design quality choose a catadioptric. You want a good and sturdy mount (one with mass). many telescopes come with computer controls to make set up and locating objects easier. Telescopes and their uses are simple and very to extremely complicated at the same time. Quality = costs money, Quality + bigger size = often exponentially more money and weight The two leading telescope brands are Meade and Celestron. Perhaps if one wants the best compromise of resolution with light gathering capacity and portability at the lowest price you would choose a catadioptric which is probably why these are the most popular selling telescopes. If someone is a beginner they might want to choose a telescope 4" or smaller. intermediate 5" to 8". advanced - 8" to 10" to 20" etc........ There is an "Astronomical" amount of things one could consider in acquiring a telescope and Astronomy is a hobby that can entertain for a lifetime. One can get a very good feel for astronomy and its potential from a good quality 4" or 5" telescope for a beginner and would be a good choice for such at a reasonable price. This would help you decide if you liked astronomy. There is a lot more to learn than is what I stated here but these are good fundamentals. It is often stated that the best telescope is the one that's used the most. Meaning that if you buy a big expensive telescope and have to bring it in and outside it may not get used often, but a smaller one that is easily brought in and out may get a lot more use and satisfaction out of. I am talking with over twenty years experience and I know I will never run out of things to do with my telescopes. Oh, and did you want to take pictures? Sorry that's for another book.
A camera lens may have a shield called a "lens hood" encircling it to prevent strong light entering from the side. Although such side light would not be focused with the image, it could reflect off the internal surfaces of the lenses to cause a "flare" of light within the image. This problem is more critical in lenses with many elements and lenses of long focal length, so we most often see lens hoods on telephoto lenses.
It depends on the type of reflecting telescope.
There are many types of telescopes you can use. Refracting telescope which receives light directly throughout a series of lenses. Newtonian Reflecting is another which receives light from a primary mirror and reflects it to a secondary mirror which sends the light to the eye piece. Newtonian Reflectors are my favorite.
Many historians explain that Galileo was the first person to use a telescope. In 1610 Galileo discovered Saturn's rings. He also observed Jupiter's four moons and viewed the different phases of Venus. This lead to the study of sunspots and various celestial activities. Even though Galileo is credited with being the first to make practical improvements and enhancements to the use of the telescope, he was not the inventor, and therefore not the first person to use a telescope. That person would be Hans Lippershey. Hans Lippershey was born 1570 in Germany, but was raised in Holland. He invented the first refracting telescope in 1608. A lens maker, he designed this telescope from two lenses and applied for a patent, intending that it would be important for the military. He successfully demonstrated the usefulness of his refracting telescope to the military.
The image in a refracting telescope appears bigger by the magnification factor provided by the telescope's eyepiece. This is typically around 50-100 times for most amateur telescopes.
2 or 3
Those terms are most commonly used for optical telescopes. However, the same design can be used for non-visible frequencies; for example, radio telescopes of the "satellite dish" variety are essentially reflecting telescopes.
It depends on the particular design.
Yes, a convex lens can disperse white light into its constituent colors by refracting different wavelengths of light by different amounts. This phenomenon is known as dispersion, and it can be observed through a prism or a convex lens.
Refractors and reflectors. One should add the largely Russian-developed Cassegrainean combines features of both systems-called also Maksutov after its Russian inventor who was awarded the Staliln Prize for this feat. They are adapted to (Mirror=Lens) telephoto camera lenses, also.
Just One A Convex one a simple microscope is a simple magnifier?
I don't know what a "4 black telescope" is. You'd have to be more specific and at least give a manufacturer and model number.
Large lenses deform under their own weight, but mirrors can be supported. Reflectors do not suffer from chromatic aberration like refractors do. Large mirrors need only one optical surface, achromats four surfaces to grind. Large, very clear lenses are harder to cast than more tolerant mirror blanks.