When it comes to buying a telescope there are some terms that will confuse the average buyer. Terms such as aperture, focal ratio, magnification, refractor, reflector, and many more will be explained here.
Those terms will be explained here in plain simple language that will help you determine the most important factors to you when it comes to buying the telescope that fits what you will use it for the best.
Altazimuth and Equatorial mounting
Altazimuth mounting is a specific type of a telescope mount that has two axes that the telescope turns on. One is pointed vertically (back and forth) the other is pointed horizontally (up and down; altitude).The former is the azimuth axis while the latter is the elevation axis.
Telescopes that utilize the Altazimuth mount usually have to rotate around both of its axes at the same time in order to follow celestial objects that appear across the sky while the Earth rotates.
Smaller telescopes that use the Altazimuth mount can be operated by a human being and there are many commercial types available on the market that is operated that way.
Equatorial mounting is a telescope mounting where one of the axes about which the telescope rotates is turned parallel to the Earth’s axis of rotation.
Equatorial mounting can therefore follow celestial objects across the sky while the Earth rotates, the axes rotates at a constant rate of one revolution per day. Equatorial mounting can be used with geared-down motor.
There is however also many types which are motor-driven and computer controlled which allows for the operation of larger telescopes.
Aperture – What is The “Best” Aperture for A Telescope?
When talking about Aperture people often refer to the aperture of the telescope as the “power” of the telescope. That the more apertures the telescope has the more power it will bring its user. This is true to an extend but be aware that there are other important factor that determine the usefulness of the telescope.
Aperture means simply the working diameter of a lens or mirror, the diameter being the size of the largest beam of light that can get through it. This is the ability if the lens or mirror to gather light. The higher aperture the more light the telescope can gather which will result in a better image.
When talking about aperture the measure is either talked about in inches or millimeters.
Barrel distortion
This is one form of aberration of the optical system. Barrel distortion is when magnification is decreased with distance away from the optical axis. The result is that squares are shown to have their sides “bulged out”.
Chromatic Aberration
When we look through a telescope we are looking at light that has travelled a long way from distanced stars and planets far away. The problem that we face is that different optical systems handle different colors of light differently which causes the colors of the image we recive in our telescope to be incorrect.
A “catch-all” term for this problem is Chromatic aberration but the problem can be narrowed down into different kinds of problems:
- chromatic difference of magnification,
- chromatic variation of spherical aberration,
- lateral color,
- longitudinal chromatic aberration,
- longitudinal color,
- and spherochromatism.
Chromatic Difference of Magnification (also known as Lateral Color)
The aberration of the optical system when the magnification differs for different colors of light. The result of this when looking at an image that is composed of multiple colors, such as green, red, and blue, is that the size of the object is disorder for the size of different colors. This can often be noted at the edge of objects of different colors or brightness by colored fringes.
Chromatic Cariation of Spherical Aberration (also known as Spherochromatism)
Again, this type is an aberration of the optical system when spherical aberration is different in different colors of light.
Longitudinal Chromatic Aberration (Longitudinal Color)
Again, this type is an aberration of the optical system when the focal length of a system differs for different colors of light.
To take an example, if you where to look at a white object through a telescope with longitudinal chromatic aberration you would (we as human being are sensitive to green light) focus on the green light making the red and blue colors of light out of focus and would form an out of focus violet halo around the object.
Focal Length
The focal length of a telescope is the measure from the primary lens or mirror to where the light converges to focus. In other words, the focal length is a measure of a telescope how effective it is at converging light. A high focal length has less optical power than a low focal length. The reason is the low focal length brings light faster to focus because the light has to travel a shorter distance.
High focal length has on the other hand higher magnification but creates a narrower angle of view compared to the less magnification and more power of a low focal ration which results in wider angle of view. Focal length is measured on both millimeters and inches.
Focal Ratio: What is important here?
The focal ratio another important factor that you will need to be aware for the specific uses you intent for your telescope.
The focal ratio is calculated by taking the aperture size and by dividing it with the focal length. The focal length is in return the measure from the primary lens or mirror to where the light converges to focus.
A telescope with the aperture of 4.5 inches and the focal length of 45 inches will have a focal ratio of f/10.
Magnification
The result when a eyepiece is used with a telescope. Magnification is calculated by dividing the focal length of the telescope with the focal length of the eyepiece. See telescope accessories for more on magnification and eyepieces.
Types of telescopes ( See Types of Telescopes for detailed information)
- The Refracting Telescopes
- Reflector Telescopes (Newtonian or Dobsonian Telescope )
- Catadioptric Telescopes or Cassegrain Telescopes (Cassegrain Telescopes)
