Telescope Optics Tutorial

Telescopes and Telescope Optics Introduction

This webpage gives a brief discussion of the major types of telescope optics. The goal is to introduce the three most popular types of telescopes (refractors, reflectors, and catadioptic telescopes) and describe their construction and features.

Popular Telescope Types

The three most popular telescope types are refractors, Newtonian reflectors, and catadioptic telescopes  (top to bottom in the below photo, respectively).  The refractor uses lenses to gather and magnify the image, and this is the type commonly found in binoculars and children's toys. The Newtonian reflector uses a parabolic primary mirror to gather light and a small secondary mirror to reflect the light out of the telescope and into an eyepiece; these are the open ended types of telescopes where you look into the front-side of the tube. The last major type of telescope is a catadioptic telescope, which uses both lenses and mirrors. Catadioptic telescopes use a  spherical mirror to gather light, a corrector plate (lens) to correct for the effects of focusing light with a spherical mirror, and a smaller secondary mirror to reflect the light into the eyepiece.




Aperature and Magnification

Before discussing the major telescope types, it is important to understand the concepts of aperture and magnification. Aperture refers to the area (size) of the telescope lens or mirror. Telescopes with larger lenses or mirrors collect more light and give a brighter image.  Larger apertures also give better resolution, meaning that they can better resolve closely placed objects into separate objects.

The 
focal length is the distance from the lens or mirror center to where the collected light is focused. With astronomical optics, long telescope focal length is associated with high magnification. This is because magnification is equal to the telescope focal length divided by the eyepiece lens focal length.

So whats important to understand here is that aperture (lens or mirror area) determines how much light is collected and how bright the object will appear. Magnification is dependent on the telescope primary mirror (or lens) and eyepiece focal lengths.  So let's compare two telescopes with the same focal lengths, the same eyepieces, but different sized apertures. Both telescopes will give the same magnification, but the image in the larger aperture telescope will be brighter, show more detail, and have better resolution.

Different objects typically require different levels of magnification. Planetary observers use longer focal length telescopes and short focal length eyepieces to give very high magnifications. Deep sky observers typically use less magnification but larger apertures to give brighter images with more resolving power, and astrophotographers will use special lenses to actually reduce the focal length to allow faster gathering of more light. The basic point of all this is that magnification is only one of many considerations in choosing a telescope.

Refractors

Refractors are the types of telescopes that most of us recognize. The refractor uses an objective lens (the large front lens) to gather more light than the human eye. Light waves entering parallel into the telescope are converged at the focal point (f in the below diagram) and light waves entering at an angle are converged onto the focal plane (dashed yellow line in the below diagram). The light converged at the focal point and focal plane combine to give the image (inverted, purple arrow in below diagram). The eyepiece consists of one or more small lenses and magnifies the image.  The below ray diagram shows why refractors give inverted (upside down) images; this is generally not a problem for astronomical images, but an additional lens (porro prism) is required to reinvert terrestrial images.





A major problem with refractors is a defect called chromatic aberration, causing colored halos. Chromatic aberration is when different frequencies (colors) of light converge at different focal lengths (below top diagram). This is because the glass refractive index decreases as the light wavelength increases, or alternatively the focal length increases as the light wavelength increases.  One solution is to combine different types of glass into a single achromatic lens. The achromatic lens (below bottom diagram) consists of two different lenses (crown glass and flint glass).  The achromatic lens will converge red and blue light to the same focal point, but other wavelengths (green light) will still give some degree of chromatic aberration. The apochromatic lens adds another lens or optical material to also bring green light to the red-blue focal point, however note that these telescopes are very expensive and can cost in excess of several thousand dollars.





Refractors are very popular for a variety of reasons: they are inexpensive (at small aperture), very rugged, and are a sealed unit requiring little maintenance.  Refractor aperture is limited to less than 1 meter. This is because large glass lenses become very expensive and because they will bend, due to their weight, distorting the image. Even though refractors are limited in size, they are still very popular. A large advantage is that refractors are sealed units, where the optics are not exposed to the outside air. The sealed tube gives less turbulence and air distortion than open ended reflectors, resulting in sharper images, which are excellet for planetery viewing.

Newtonian Reflectors

The Newtonian reflector was invented by Sir Isaac Newton in the 17th century and uses a parabolic primary mirror  to gather and focus light.  Parallel entering light waves converge at the focal point (f in the below diagram) and light waves entering at an angle through the focal point exit parallel to the mirror (light blue ray in the below diagram). In a manner similar to the lens, the light converged at the focal point and focal plane (dashed yellow line) combine to give the image (inverted, purple arrow in below diagram).





The Newtonian telescope uses a  parabolic primary mirror (purple in below diagram) to collect and focus light, and a smaller flat secondary mirror (green in below diagram) to reflect the image out of the telescope tube and into the eyepiece. The secondary mirror is supported on an assembly called a spider (blue in below diagram). Both the primary mirror cell and spider have adjustment screws (collimating screws) to change the orientation of the two mirrors; this allows adjustment or collimation of the optical axis to give the optimal image. Despite the spider and secondary mirror blocking a portion of the light, they do not cast a circular shadow onto the image.  The main effect from the spider sitting in the optical path is to reduce contrast and cause light diffraction around the spider vanes (causing stars to appear with diffraction spikes).





Newtonians are very popular because they are less expensive (per inch of aperature) than other types of telescopes, are simple and very easy to build, and do not exhibit chromatic aberration.  Major disadvantages are: an open tube exposing the optics to dirt and turbulence, sometimes awkward viewing positions due to a front mounted eyepiece,  requirement for periodic collimation, and comatic aberration.

Newtonians do not exhibit chromatic aberration (as with refractors), however they are prone to  comatic aberration (coma).  Coma causes light to be focused differently depending on its distance from the mirror's central axis (center). A point source of light (star) in the center of the field of view focuses properly to a point. Light from stars in the edge of the field of view (off axis) focus to different points in the focal plane. The below diagram illustrates coma from an off axis star in a  parabolic mirror. Each annulus on the parabolic mirror (the colored rings) forms a separate image
(comatic circle) onto the focal plane. Smaller annuli will form smaller comatic circles. Ovelapping comatic circles form a cone or comet like image. The coma effect increases directly with distance from the center of the field of view.






Catadioptic Optical Systems 

Catadioptic telescopes use a combination of lenses (dioptrics) and mirrors (catoptrics) to collect and focus light. Catadioptic telescopes reflect the light path several times inside the telescope tube. By reflecting the light path internally, the catadioptic telescope allows the focal length to be contained in a tube that is shorter than the focal length. The most noticeable advantage of a catadioptic telescope is that the "folded" optical path results in a shorter, more portable telescope, which can be supported on a lighter weight mount. The below diagram shows a simple comparison of the light paths in a reflector (top) and catadioptic telescope (bottom).  The reflector is divided into three colored sections (red, green, and blue). The catadioptic telescope diagram shows how the same red, green, and blue distances can be contained in a tube that's roughly 33% of the reflector's length.





Catadioptic telescopes use spherical mirrors instead of  parabolic mirrors.  Spherical mirrors are simpler and less expensive to fabricate than parabolic mirrors, but also reflect light differently. Comatic aberration can be significantly reduced by switching from parabolic mirrors to spherical mirrors, however this will require a lens to correct for how spherical mirrors reflect light.  The below photo shows the difference in how light is reflected from a parabolic mirror and spherical mirror (left and right, respectively). The parabolic mirror
reflects the light to a single focal point, however the spherical mirror reflects the light to different focal points. Light reflecting from the outer parts of the spherical mirror converges closer to the mirror than light reflecting from the center; this is called spherical aberration. Spherical aberration can be corrected by installing a correcting lens (corrector plate) on the end where light enters the telescope.  






The below diagram shows the optical tube for a typical catadioptic telescope (in this case a
Celestron C8 Schmidt-Cassegrain telescope). Light enters through the corrector plate (pale blue) reflects off of the primary (dark blue) and secondary (light blue) mirrors and exits the optical tube through the center light baffel to the diagonal and eyepiece. The telescope is focused by turning the black rear knob, which slides the primary mirror forward or backwards on the light baffel. 



Schmidt-Cassegrains

The Schmidt corrector plate (found in Schmidt-Cassegrain telescopes) is thicker at the edges and center, and flat on the side facing the spherical primary mirror. The Schmidt corrector plate (yellow in below diagram) corrects the incoming light so that light striking the outer portion of the spherical primary mirror focuses onto the same spot as light striking the inner portion. Light entering through the corrector plate is reflected from the concave spherical primary mirror onto a convex spherical secondary mirror (green in below diagram).  The secondary mirror is held onto the Schmidt corrector plate by collimating screws, so it can be adjusted (collimated). Light reflecting off of the secondary mirror exits through a hole in the center of the primary mirror and into the eyepiece. Since the secondary mirror is attached directly to the Schmidt corrector plate (without a spider assembly), this eliminates diffraction spikes. The telescope is focused by turning an adjustment knob, which moves the entire primary mirror forwards or backwards.





Major advantages of Schmidt-Cassegrain telescopes are: compact sealed optical tube, lightweight and portable, a long focal length contained in a short telescope that is suitable for both planetary and deep sky viewing, and significant reduction of comatic aberration
compared to reflector type telescopes. Disadvantages are optical aberrrations such as coma and astigmatism.


Maksutov-Cassegrains

Another popular type of Cassegrain telescope is the Maksutov (MAK)-Cassegrain. A Maksutov optical system (below diagram)  uses a concave spherical meniscus corrector plate as opposed to the more complicated Schmidt corrector plate. The meniscus corrector plate is simpler to manufacture than the Schmidt corrector plate. In the MAK design, the convex spherical secondary mirror is called a mirror spot (green in the below diagram), and attaches directly to the meniscus corrector plate (without collimating screws).  The fixed mirror spot requires no alignment, meaning that the MAK does not require periodic collimation.





The MAK has many of the same advantages as the Schmidt-Cassegrain. Additional advantages of MAK optics are that the spherical meniscus corrector plate is less complicated to fabricate and can be polished to a very high accuracy, giving very high quality images.  MAKs give a wide field of view with better image contrast than Schmidt-Cassegrains. Disadvantages are that that large meniscus corrector plates become expensive and heavy, limiting MAK aperture size to generally less than 7 inches.

There are many other variations of catadioptic telescopes. The following link gives a concise chronological overview of catadioptic telescope development: The 300 Year Evolution of the Maksutov-Cassegrain Telescope.
                                                                                                                                                                   


All original graphics, photographs, content, and writings are copywrited © 2007-2014 by the author and all rights are reserved. Do not copy or reproduce in any form without prior written consent.