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Q1. What is a telescope?

Ans. The simplest definition is; it is an instrument that gathers light and focuses that light into an image. Afterwards that image can be magnified. This instrument is mounted in a manner so that it allows you to hop from one object to another.

Q2. How many types of telescopes are there and how do they work?

Ans. There are basically three types of telescopes:

i) Refractor type ii) Newtonian Reflector type iii) Catadioptric type

Refractor type:
To refract means to bend. Light enters the lens at the front of the tube. This lens is called the objective. The light is refracted down the length of the tube where it eventually reaches its focal plane (where the light becomes focused at a specific point).There, an eyepiece mounted in a focuser, allows that light to be magnified into an observable image. The better the quality and alignment of the optics, the better the image produced by a refractor will be.

This statement holds true for all the telescopes.
Unfortunately, high quality refractors tend to be very costly due to the cost associated with producing high quality optics. One of the chief disadvantages of simple and chief refractors is that for astronomical viewing, they have tendency to add false colors to images. This is called chromatic aberration. These colors usually take the form of a pale violet halo around the observed object. But don't be scared off, there are many excellent entry level refractors that will perform excellently.

Newtonian Reflector type:
Designed by Sir Isaac Newton, rather than lenses, the reflector uses two mirrors to bring light to the eyepieces. Light travels down the tube to the primary mirror, which is the larger of the two mirrors. The primary mirror is generally a parabolic (concave) mirror. It reflects the light back up the tube to the secondary mirror. The secondary mirror is an ellipse with a flat surface which is mounted at a 45 degree angle on a device usually called a spider. The light is reflected from the secondary mirror to the eyepiece where it can be magnified into an observable image. Cost wise, reflectors offer the most aperture. They produce soft images that are free of any added color.

Catadioptric Telescopes:
Catadioptric telescopes are essentially a combination of a refractor and a reflector. There are two very popular flavors widely available in today's telescope marketplace. One is Schmidt-Cassegrain and the other is the Maskutov-Cassegrain. These telescopes fold the light three times allowing for a much shorter tube. Because of their clever use of corrective lenses and lack of spider to hold the secondary mirror, they are free of many of the optical defects present in reflectors and refractors.

Light enters the front of the tube through a corrective lens. The light then travels down the tube to primary mirror. From there it is reflected up the tube to a secondary mirror which in turn reflects the light back down the tube to the focal plane. The focusing mechanisms of Catadioptric telescopes are different than in reflectors and refractors. Instead of moving the eyepiece in and out of the focal plane, the whole primary mirror is moved in and out.

Catadioptric telescopes are expensive telescopes.

A telescope's ability to gather light depends on:

Aperture - the larger the primary optics (the largest lens or mirror) the more light is captured.

Optical quality - the more reflective a mirror and the more a lens allows light to pass through it, the more light is transmitted to your eye.

Contrast - the more absorptive the inside surface of the tube is, the less light will bounce around in that tube, which leads to improved contrast (darker backgrounds).

Collimation- Even if you have best optics, if they are not properly aligned, your light gathering capacity will be diminished.

A telescope's ability to focus properly is dependent on:

Collimation: how well the optics are aligned.

Optical Quality of the eyepiece.

Focusing mechanism:
a focuser that is smooth and allows for minute adjustments is necessary. The focuser must be aligned properly to perform at its best.

A telescope's mount needs at a minimum the following characteristics:

Stability:
the telescope should not sway or bounce on the mount.

Rigidity:
when you move and focus the telescope, the stand should be solid enough to bear out the vibrations, otherwise the vibrations would be seen in the eyepiece.

Smooth motions:
the mount should move smoothly from object to object .

Some useful formulas for telescopes:

1) f-ratio

f = focal length of objective
     --------------------------
        Diameter objective

2) Magnification

Magnification = focal length of objective
                        -------------------------------
                         focal length of eyepiece

3) Exit pupil

Exit pupil = focal length of eyepiece
------------------------------
f-ratio

Exit pupil = diameter objective
----- ----------------------------
Magnification with eyepiece

4) Minimum and maximum magnification

Minimum magnification = diameter of objective
------------------------------
Maximum pupil opening (about 6mm on average)

Maximum magnification = 2*diameter objective in millimeters.
Maximum magnification = 50*diameter objective in inches.

5) Longest and shortest focal length eyepiece

Longest focal length eyepiece = focal length objective
                                               ----------------------------------
                                                   Minimum magnification

Shortest focal length eyepiece = focal length objective
                                               ----------------------------------
                                                   Maximum magnification