A telescope is to first attend a star party or an event or Join Astronomica where there are a few telescopes showcased. That will give you an idea of what a telescopes capabilities are and how binoculars compare with telescopes, or which is the best telescope to buy. In this article, we have tried to present some theoretical parameters that characterize a telescope, and a good understanding of these parameters will help you choose the right telescope for you.
There are quite a few important factors that determine the capabilities of a telescope:
Telescope Parameters
Telescope Parameters
- Telescope type
- Aperture
- Focal Length
- Type of mount
- Motorization / Computer Control or GOTO
- Compatibility with standard eyepieces
- Cost
We shall now see how each parameter affects the capabilities of a telescope.
Telescope Type
There are many types of telescope which are classified as per the optics uesd. Telescope can be broadly classified as:
Telescope Type
There are many types of telescope which are classified as per the optics uesd. Telescope can be broadly classified as:
- Refracting telescopes or Refractors
- Reflecting telescopes or Reflectors
- Catadioptric telescopes
Refractors use lenses to create an image of the faint / distant object. Reflectors use mirrors to focus an image of the object. Catadioptric telescopes use a combination of mirrors and lenses to create an image.
What are the pros and cons of each type? We will discuss refracting and reflecting telescopes as they are popular.
Refracting Telescopes
Refractor Telescopes use lenses to have image of the Object. Combination of lenses gives you required mangnification (hyp). Refractors are usually expensive because a well-engineered combination of several lenses is required to create a good image. Creation of Lenses are expensive because it consumes time and also requires a lot of engineering.
Refractors suffer from a defect called “Chromatic Abbaration”. This defect can be minimised by using special techniques to manufacture lenses and this results in “Achromatic Lenses” which again makes it expensive. But to remove chromatic abbaration so that the image is more crisp , Other techniques are used to create more error free lenses. Thus, there are two types of popular refractors:
What are the pros and cons of each type? We will discuss refracting and reflecting telescopes as they are popular.
Refracting Telescopes
Refractor Telescopes use lenses to have image of the Object. Combination of lenses gives you required mangnification (hyp). Refractors are usually expensive because a well-engineered combination of several lenses is required to create a good image. Creation of Lenses are expensive because it consumes time and also requires a lot of engineering.
Refractors suffer from a defect called “Chromatic Abbaration”. This defect can be minimised by using special techniques to manufacture lenses and this results in “Achromatic Lenses” which again makes it expensive. But to remove chromatic abbaration so that the image is more crisp , Other techniques are used to create more error free lenses. Thus, there are two types of popular refractors:
- Achromatic Refractors
- Apochromatic Refractors or ED Scopes
An achromatic refractor will gives you a quality image. Apochromatic lenses produce the most crisp images, and are ideal for photography of celestial objects. Apochromats are more expensive than Achromats.
Refractors are not preferred for visual observations because as the aperture increases the cost increases exponentially, so only small aperture telescopes are used for visual observation to view planetary objects and close by objects.
Pros:
Refractors are not preferred for visual observations because as the aperture increases the cost increases exponentially, so only small aperture telescopes are used for visual observation to view planetary objects and close by objects.
Pros:
- Very crisp image
- Are usually portable
- Ideal for astrophotography because the light gathered is more as compared to Reflectors.
Cons:
- Good apochromatic refractors are of huge cost.
- Small aperture Refractors can view only planets and bright celestial objects only.
- They are expensive
Reflecting Telescopes
Reflecting telescopes produce an image by using combination of mirrors.
Reflecting telescopes produce an image by using combination of mirrors.
- Newtonian reflector
Newtonian reflectors are the most popular for visual observing of distant celestial objects.
Newtonian Reflectors
This is the most popular design for visual observing. Newtonian reflectors have two mirrors - a "primary" concave (actually parabolic) mirror and a "secondary" flat mirror. The primary mirror gathers the light from the celestial object and focusses it. The secondary mirror only changes the direction of the light, so that it is suitable for viewing. Big Aperture Mirrors are economical as compared to big aperture Lenses and the cost reduces heavily. Newtonian reflectors are the most suitable for visual observing and can also be used for Photography.
Pros:
This is the most popular design for visual observing. Newtonian reflectors have two mirrors - a "primary" concave (actually parabolic) mirror and a "secondary" flat mirror. The primary mirror gathers the light from the celestial object and focusses it. The secondary mirror only changes the direction of the light, so that it is suitable for viewing. Big Aperture Mirrors are economical as compared to big aperture Lenses and the cost reduces heavily. Newtonian reflectors are the most suitable for visual observing and can also be used for Photography.
Pros:
- They are cheaper
- They come in a large range of apertures, starting from small 3-inch apertures to more than 30-inch aperture.
- They are ideal for visual observing
- They can also be used for Astro Photography.
- Desired Magnifications can be obtained by using various focal lengths.
Cons:
- Image quality usually not as good as apochromatic reflectors
- Are usually designed to produce inverted images
- Collimation is not easy, which requires skill and technique to learn, but it is not that problametic, it can be learned easily. Various collimating tools are available to help in collimation.
Catadioptric Telescopes
This design is usually preferred for astrophotography of small celestial objects. This design accomodates long focal lengths in a shorter tube length, making it less bulky. A favourite amongst astrophotographers.
To summarize:Schmidt Cassegrains are suitable when you want large magnifications, especially for photography.
Pros:
This design is usually preferred for astrophotography of small celestial objects. This design accomodates long focal lengths in a shorter tube length, making it less bulky. A favourite amongst astrophotographers.
To summarize:Schmidt Cassegrains are suitable when you want large magnifications, especially for photography.
Pros:
- Large magnifications with small sizes
- Ideal for photographing small objects.
Cons:
- Expensive than Reflectors
Aperture
The clear aperture of a telescope is the diameter of the objective lens or primary mirror specified in either inches or millimeters (mm). The larger the aperture, the more light it collects and the brighter (and better) the image will be. Greater detail and image clarity will be apparent as aperture increases. In astronomical telescopes as Telescopes Mumbai, Binoculars Mumbai India Telescope in MumbaiThe light-gathering power is more important than magnification.
The light-gathering ability of a telescope varies as the square of the diameter of its aperture. Which means that a 8" telescope is 4 times more powerful than a 4" telescope, which in turn is 4 times more powerful in Telescopes Mumbai, Telescopes India Telescope in Mumbai (Maharashtra)
To summarize: Larger aperture helps you view objects more clearer by gathering more light from teh object.
Pros:
- Larger aperture show more detail
- Larger aperture increases contrast
- Larger aperture shows fainter objects, more distant galaxies
- Large aperture is ideal for deep-sky observing (seeing galaxies, nebulae, star clusters etc.)
Cons:
- Larger aperture costs more
- Larger aperture makes telescopes bulky
Focal length, usually measured in mm, refers to the focal length of the primary optical element (objective lens in refractors, primary mirror in reflectors). Larger focal length gives more magnification with the same eyepiece. Smaller focal length gives more field-of-view with the same eyepiece, but less magnification.
To observe planets in great detail, an objective with a long focal length and high optical quality is preferred. To observe deep-sky objects, an objective with a short focal length is preferred.
