Types of Telescopes

There are 3 basic types of telescopes categorized by the way light enters the objective and is focused at the eyepiece…the refractor, reflector and catadioptric.

The Refractor

: Explore Scientific Refractor

The refractor uses a series of lenses to refract or bend the light coming through the objective so that it converges at the other end where the eyepiece is located. In order to achieve more comfortable viewing a star diagonal is used.

Consider the following:

The tube is closed to the air currents and this protects the lenses and helps cut down air currents inside the tube which degrade the viewing or image
Collimation is rarely required…lenses are permanently aligned
Excellent for planetary, binary stars viewing
Degradation does not occur appreciably with age unlike telescopes that have mirrors
The apochromatic designs include an additional lens system which serves to alleviate chromatic aberration but is more costly
Transmission of light (related to detail observed) is around 90%
A diagonal mirror is usually used with this design of telescope and can serve to further diminish light transmission, although still significantly better than all the other telescope designs
Excellent for planetary, binary stars viewing and out performs the other designs on a “hazy” night

However…

Due to its long and narrow design the long thin tube of the refractor is more susceptible to movement caused by the wind
The most expensive type of telescope
Requires a longer cooling period that reflectors due to closed tube system
Less suited for faint DSO (Deep Sky Objects)
Usually heavier than the other types of telescopes
The long focal length models make it more difficult to place the observer in a comfortable position for viewing
Suffers from chromatic aberration in the achromatic designs
The apochromatic designs include an additional lens system which serves to alleviate chromatic aberration but is more costly

The Newtonian Reflector

: Skywatcher Reflector

This type of telescope has no lenses. In this design a parabolic primary mirror reflects the incoming light coming through the objective onto a secondary mirror which in turn reflects and focuses the light onto the eyepiece.

Consider the following:

Excellent for viewing DSO such as Nebula, galaxies and star clusters
Good for astrophotography of DSO but more difficult than catadioptric designs in terms of focusing
Reasonably good for viewing planets and Moon
Transmission of light (related to detail observed) is around 77 – 80%
Portability is favorable up to focal lengths of 1000mm
Due to not having lenses there is no chromatic aberration

However…

Requires regular collimation
Suffers from optical aberration or coma but corrector lenses can be used to alleviate this problem
The tube is open and exposed to air currents which make it more susceptible to image degradation or “shaky viewing” due to uneven changes in temperature
The quality of the coating on the mirrors is very important to the quality of the image and will help maintain the mirrors for many years without the need for re-coating
The quality of the coating on the mirrors is very important to the quality of the image and will help maintain the mirrors for many years without the need for re-coating
As much a 1-1.5% reflectivity loss per year can occur due to oxidation of the aluminum coating of the mirrors.
Light loss (relates to quality of image and viewing) is greater than in refractors due to a mirror instead of a lens design and the central obstruction. Depending on the specific design and materials used light loss can be as much as 30%
Eyepiece location is in the front of the tube and may be in an awkward position (depending on location of object viewed in the sky). This would require loosening the rings holding the telescope tube and rotating it to a more comfortable level for the observer

The Catadioptric

: Meade Lx200, 10 inch

This telescope design is a hybrid of the refractor and reflector. Lenses and mirrors are uses to bend, reflect and focus the light onto the eyepiece. This design allows the actual length of the telescope to be shorter than either the refractor or reflector while at the same time providing increased magnification. In other words a much longer equivalent focal length is obtained in a shorter tube.

Consider the following:

Excellent for DSO viewing and imaging
Good for planetary and lunar viewing and imaging
Transmission of light (related to detail observed) is around 70 -75%
More portable due to shorter tube
Tube is sealed protecting the optics
Generally more comfortable eyepiece position
Reduces optical aberrations to minimal
Essentially no chromatic aberration
Excellent for astrophotography as it has the best focusing capability of any other design and because of the fact that higher magnifications are possible (comparing the equivalent length and size to refractors and reflectors)
Costs more than a reflector but get more aperture than a refractor for same price

However…

More expensive than reflectors
Requires collimation
Suffers from a secondary mirror obstruction similar to reflectors with light loss
A sealed tube means that it has to be “cooled” longer than a reflector
Some curvature of field of view which is eliminated in some of the more expensive Schmidt-Cassegrain models
The quality of the coating on the mirrors is very important to the quality of the image and will help maintain the mirrors for many years without the need for re-coating
As much a 1-1.5% reflectivity loss per year can occur due to oxidation of the aluminum coating of the mirrors

Web Site Map:
Astronomy Notebook
The Moon
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Equipment and Setup
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Astrophotography
An Overview Of The Methods In Astrophotography
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Contact

There are 3 basic types of telescopes categorized by the way light enters the objective and is focused at the eyepiece…the refractor, reflector and catadioptric.

The Refractor

Explore Scientific Refractor

The refractor uses a series of lenses to refract or bend the light coming through the objective so that it converges at the other end where the eyepiece is located. In order to achieve more comfortable viewing a star diagonal is used.

Consider the following:

The tube is closed to the air currents and this protects the lenses and helps cut down air currents inside the tube which degrade the viewing or image

Collimation is rarely required…lenses are permanently aligned

Excellent for planetary, binary stars viewing

Degradation does not occur appreciably with age unlike telescopes that have mirrors

The apochromatic designs include an additional lens system which serves to alleviate chromatic aberration but is more costly

Transmission of light (related to detail observed) is around 90%

A diagonal mirror is usually used with this design of telescope and can serve to further diminish light transmission, although still significantly better than all the other telescope designs

Excellent for planetary, binary stars viewing and out performs the other designs on a “hazy” night

However…

Due to its long and narrow design the long thin tube of the refractor is more susceptible to movement caused by the wind

The most expensive type of telescope

Requires a longer cooling period that reflectors due to closed tube system

Less suited for faint DSO (Deep Sky Objects)

Usually heavier than the other types of telescopes

The long focal length models make it more difficult to place the observer in a comfortable position for viewing

Suffers from chromatic aberration in the achromatic designs

The apochromatic designs include an additional lens system which serves to alleviate chromatic aberration but is more costly

The Newtonian Reflector

Skywatcher Reflector

This type of telescope has no lenses. In this design a parabolic primary mirror reflects the incoming light coming through the objective onto a secondary mirror which in turn reflects and focuses the light onto the eyepiece.

Consider the following:

Excellent for viewing DSO such as Nebula, galaxies and star clusters

Good for astrophotography of DSO but more difficult than catadioptric designs in terms of focusing

Reasonably good for viewing planets and Moon

Transmission of light (related to detail observed) is around 77 – 80%

Portability is favorable up to focal lengths of 1000mm

Due to not having lenses there is no chromatic aberration

However…

Requires regular collimation

Suffers from optical aberration or coma but corrector lenses can be used to alleviate this problem

The tube is open and exposed to air currents which make it more susceptible to image degradation or “shaky viewing” due to uneven changes in temperature

The quality of the coating on the mirrors is very important to the quality of the image and will help maintain the mirrors for many years without the need for re-coating

The quality of the coating on the mirrors is very important to the quality of the image and will help maintain the mirrors for many years without the need for re-coating

As much a 1-1.5% reflectivity loss per year can occur due to oxidation of the aluminum coating of the mirrors.

Light loss (relates to quality of image and viewing) is greater than in refractors due to a mirror instead of a lens design and the central obstruction. Depending on the specific design and materials used light loss can be as much as 30%

Eyepiece location is in the front of the tube and may be in an awkward position (depending on location of object viewed in the sky). This would require loosening the rings holding the telescope tube and rotating it to a more comfortable level for the observer

The Catadioptric

Meade Lx200, 10 inch

see also: Types of Catadioptric Telescopes

Consider the following:

Excellent for DSO viewing and imaging

Good for planetary and lunar viewing and imaging

Transmission of light (related to detail observed) is around 70 -75%

More portable due to shorter tube

Tube is sealed protecting the optics

Generally more comfortable eyepiece position

Reduces optical aberrations to minimal

Essentially no chromatic aberration

Excellent for astrophotography as it has the best focusing capability of any other design and because of the fact that higher magnifications are possible (comparing the equivalent length and size to refractors and reflectors)

Costs more than a reflector but get more aperture than a refractor for same price

However…

More expensive than reflectors

Requires collimation

Suffers from a secondary mirror obstruction similar to reflectors with light loss

A sealed tube means that it has to be “cooled” longer than a reflector

Some curvature of field of view which is eliminated in some of the more expensive Schmidt-Cassegrain models

The quality of the coating on the mirrors is very important to the quality of the image and will help maintain the mirrors for many years without the need for re-coating

As much a 1-1.5% reflectivity loss per year can occur due to oxidation of the aluminum coating of the mirrors

Web Site Map: Astronomy Notebook The Moon Images And Videos Equipment and Setup Notes on Equipment Astrophotography An Overview Of The Methods In Astrophotography Links Contact

Thanks to: La Maison de l’Astronomie and Lire La Nature Astonomie Plus, for allowing images to be taken of the telescopes on this page.

Web Site Map:
Astronomy Notebook
The Moon
Images And Videos
Equipment and Setup
Notes on Equipment
Astrophotography
An Overview Of The Methods In Astrophotography
Links
Contact