Refractors | Reflectors | Catadioptric

Telescopes: Bend, Bounce, Both

There are three types of telescopes used by the amateur astronomer: refracting, reflecting and catadioptric. Each telescope retrieves light in a different way. The first uses curved lenses to refract the light; the second uses curved mirrors to reflect the light; the third uses a combination of lenses and mirrors to refract and reflect the light—essentially, bend, bounce, both, respectively.

Refractors

Structure
Refracting telescopes use a curved lens that bends light as an objective. These telescopes are commonly used as logos to conjure the image of astronomy, and they are the type of telescope that the infamous Galileo Galilei used.

A basic refracting telescope is an OTA with a lens at the aperture that focuses light on the eyepiece at the opposite end of the tube—sometimes through a prism, which redirects the light to an eyepiece on the side.

This structure creates some image flaws, usually chromatic aberration. When white light (the combination of every color in the rainbow) passes through a medium (in this case the glass of the objective), the image is created in every color. As the image leaves the medium, it is reassembled by piling the images on top of each other. However, every color bends slightly different through the medium. This causes the image to reassemble slightly off. The slight difference makes the image appear surrounded by a corona of colors.

The problem of chromatic aberration has been compensated for using achromatic lenses and apochromatic lenses.

• Anachromatic lenses are made of two different materials, which help to negate the dispersal effects of each medium.
• Apochromatic lenses are made of three different materials, which virtually
  makes chromatic aberration unnoticeable.

Uses
A refractor with achromatic or apochromatic improvements is the best for near sky viewing because it offers a dark field of view at a high magnification. These characteristics show very crisp images. Use a reflector to study the moon’s craters, the planets and double stars.

Maintenance and Portability
A drawback of the refractor is that it is not very portable. The higher focal ratio that gives the telescope its ability to show sharp images is directly related to the focal length, in this case the length of the OTA. A great refractor will be very cumbersome.

But, the maintenance of a refractor is very low. The manufacturer collimates the refractor, with only minor, initial adjustments necessary for the observer. It is a stable system, so the motion of transporting it, although difficult, will not jostle the components.

Relative Cost
Unfortunately, the achromatic and apochromatic lenses of the refractor increase the price considerably, sometimes from hundreds to thousands of dollars. Refractors with the corrective lenses tend to be the most expensive of all apparatus. Still, many feel that the quality of what is seen makes them worthwhile.

Reflectors

Structure
Reflecting telescopes use curved mirrors as objectives. The basic reflector is the newtonian reflector, which is the most popular telescope for beginners.

Newtonian Reflector
The newtonian reflector has a curved mirror to collect light; the light is reflected to a flat secondary mirror that directs the image to an eyepiece.

This structure has some flaws. Although reflectors do not suffer from chromatic aberration (as refractors do) because the reflected light never passes through a medium, the curved mirrors can cause the edges of the image or the field of view to become distorted. Usually, the newtonian reflector suffers from either spherical aberration or coma.

• Spherical aberration occurs because the light hits the curved mirror at slightly different times after slightly different distances, causing it to be reflected at different times and distances. This difference causes the edges of the field of view to blur.
• Coma occurs because the focus of the image is askew, which causes the image to have a fanned edge. The image is off-center, therefore unfocused in part.

These problems are less apparent as the quality of the equipment increases.

Another drawback of newtonian and cassegrain reflectors is that not all of the light that enters will be focused to a point that the eye can see; much light is lost. As discussed earlier, a telescope is as good as the amount of light it collects to focus. While the lens of a refractor collects and focuses all of its light (if properly aligned), the curved mirror of a reflector bounces some of its light in a way that misses the eyepiece.

Uses
The reflector is a good deep sky telescope. Usually, it does not offer the crisp images of a refractor; however, the images are good enough for the amateur astronomer.

Maintenance and Portability
The compactness, especially of the cassegrain, makes reflectors very portable. Getting as much focal length as a refractor offers from an instrument half the size of the refractor lets the observer easily pack the telescope into a car’s trunk to escape the light pollution. Or, it allows easy storage in a garage, to be set up for quick backyard viewing.

However, reflectors must be collimated often, in part because of jostling from travel. They must be aligned when first purchased and they must be realigned each time the mirror is severely bumped, which happens when packing and transporting them. Moving to and from the garage for backyard viewing or to and from the car to escape light pollution heightens the probability that minor adjustments will be needed.

Relative Cost
In general, the money that would be invested in an adequate refractor can purchase a better reflector. The better reflector equates to the affordability of a larger aperture to collect more light and of better mirrors to process the light. As mentioned before, the newtonian is popular among beginners because of acceptable images at a low price.

High-quality mirrors are needed to minimize the spherical aberration or coma inherent in the design, which increases the price. The assembly line that creates less expensive components for less expensive telescopes does not have the precision necessary for negating aberrations. The observer often compromises, deciding that a focused image in the center is acceptable for amateur viewing at a reduced cost. Also, many different levels of quality exist; it is possible to not buy the best and not settle for the worst.

Catadioptric

Structure
Catadioptric telescopes use a combination of curved lenses and mirrors as objectives to collect light. The basic types of catadioptric are schmidt-cassegrains (SCTs) and maksutovs. Both use the same structure, but each uses different versions of the structure; for example, both use a lens but each uses a different type of lens.

In catadioptric telescopes, the light enters the lens, called a corrector plate in this model. Then, it hits a curved mirror at the back of the telescope and is reflected to the secondary flat mirror to be redirected to the eyepiece.

Uses
Catadioptric telescopes are good for all-around viewing because they have the attributes of both refractors and reflectors. In addition, the negative aspects of the refractor and the reflector often negate each other. If using one eyepiece, the catadioptric gives adequate deep sky viewing. If using another eyepiece, it gives adequate near sky viewing.

To take advantage of the wide range of viewing possible with a reflector, additional eyepieces must be purchased. However, most telescope owners invest in several eyepieces for viewing different matter under different conditions anyway. Indeed, telescopes often come with more than one.

Maintenance and Portability

The compact size created by the cassegrain-style “folded” focal length make the catadioptric the most portable of telescopes. Only binoculars are moved easier. These telescopes can offer large apertures and short OTAs—beneficial for light collection and for transportation and storage.

However, there is as much maintenance and collimation on these as there is on reflectors due to the mirrors and suspended components throughout. Adjustments must be made as often as in reflectors.

Relative Cost
Catadioptric telescopes, in general, are less expensive than refractors and more expensive than reflectors. However, the added expense is applied to the wider range of viewing. With the refractor, the observer sees near sky objects well. With the reflector, the observer sees deep sky objects well. With the catadioptric, the observer sees both objects at a median price.