TRG – Telescope Review Guide

Catadioptric telescope

Catadioptric telescopes use a combination of mirrors and lenses to fold the optics and form an image.  There are two popular designs, the Schmidt-Cassegrain and the Maksutov-Cassegrain.

In the Schmidt-Cassegrain, light enters through a thin aspheric Schmidt correcting lens, then strikes the spherical primary mirror and is reflected back up the tube to be intercepted by a small secondary mirror. The mirror then reflects the light out the back of the instrument where the image is formed at the eyepiece.

Advantages

ﾷ Most versatile type of telescope
ﾷ Best near focus capability of any type telescope
ﾷ First-rate for deep sky observing or astrophotography with fast film cameras or CCD’s
ﾷ Excellent for lunar, planetary and binary star observing plus terrestrial viewing and photography
ﾷ Closed tube design reduces image degrading air currents
ﾷ Compact and durable

Disadvantages

ﾷ More expensive than reflectors of equal aperture
ﾷ Slight light loss due to secondary mirror obstruction compared to refractors
The Maksutov-Cassegrain telescope design has basically the same advantages and disadvantages as the Schmidt. It uses a thick meniscus-correcting lens with a strong curvature and a secondary mirror that is usually an aluminized spot on the corrector. The Maksutov secondary mirror is typically smaller than the Schmidt’s giving it slightly better resolution for planetary observing.

However, the Maksutov is heavier than the Schmidt and because of the thick correcting lens, it takes a long time to reach thermal stability at night in larger apertures. The Maksutov optical design typically is easier to make but requires more material for the corrector lens than the Schmidt Cassegrain.

Welcome back to part 3 of my series on how to buy a telescope.

The reflecting telescope

Reflecting telescopes use a parabolic mirror at the back of the telescope to gather and focus the light to a flat secondary mirror that the image out of an opening at the side of the main tube. You observe through an eyepiece on the side of the tube up near the top.

ﾷ Easy to use
ﾷ Excellent for faint deep sky objects such as remote galaxies, nebulae and star clusters because their larger apertures collects more light
ﾷ Low in optical irregularities and deliver very bright images
ﾷ Reasonably compact and portable
ﾷ A reflector costs the least per inch of aperture compared to refractors and catadioptrics

Disadvantages

ﾷ Not suited for terrestrial applications
ﾷ Slight light loss due to secondary obstruction when compared with refractors
ﾷ The tube is open to the air, which means dust on the optics even if the tube is kept under wraps

Check back often for the continuation of my series on how to buy a telescope. Part 4 will discuss catadioptric telescopes.

Buying a new telescope can be confusing, especially for the first timer. Many are left wondering which telescope to get. Understanding telescopes and their various accessories can go a long way in choosing the best one for you. Whether you are a beginner or old pro, in this first of a six part series, I will help you better identify the different types of telescopes and help you find the best telescope for you.

Aperture of a telescope

A telescope’s singular most important attribute is its aperture. Aperture is a measure of a telescope’s light gathering ability and determines the brightness and sharpness of everything viewed through your scope. Aperture is the diameter of the main lens or mirror and as the aperture increases so does the details of the image you see. A good 10″ aperture scope shows sharper images than even a well-made 6″ aperture telescope.

Since aperture is so important, many astronomers are misled into believing that getting the biggest aperture you can afford is the simplest way to choosing the best telescope for you. This isn’t always the case. If you make your decision based solely on the size of aperture, you might end up owning a big telescope that sits unused.

Where will you use your new telescope? If  nearby in the backyard then having a large telescope will be of advantage to you. If you need to carry the telescope to to an observing site away from city lights, you will definitely need something compact, yet powerful. Don’t forget that you may have to assemble and set up your telescope in the dark. A scope that is too big to carry outdoors and too difficult to set up just won’t get used.

Telescope Power

Power and design, like aperture, are other important considerations when deciding which telescope to purchase. Power is a critical factor since it determines how close a view you can get of space objects such as planets but it is not a decision making factor. You can control the  power by using different eyepieces.

An eyepiece is the small removable lens assembly you look into. Most telescopes come with several eyepieces and you can buy more separately. Remember to match the power of your eyepiece lens with the aperture of your scope for clear images. The lowest powers are the easiest to use, especially for beginners, and provide the most pleasant views.

Telescope Designs

Even among telescopes with the same aperture, some designs are more portable, others give sharper images while still others are more economical. There are three basic kinds of telescope to choose from depending on your specific requirements.

ﾷ Reflecting telescope,

ﾷ Refracting telescope, and

ﾷ Catadioptric telescope.
Despite their differences in size these 3 telescope types have the same light-gathering properties. They also have a similar purpose, to collect light and bring it to a point so it can be magnified and examined with an eyepiece, but each does it differently. Therefore, each type of telescope has its pros and cons, which you can match with your observing needs.

Come back soon for part 2 of this article when I will discuss refracting telescopes!