TRG – Telescope Review Guide

Yes, it’s only July but it’s not too early to start thinking about what to buy for that astronomer on your Christmas list! But what would be the best telescope to get? The choices are almost endless! Here then is a short list. You can’t go wrong with any of these:

Orion Skyquest XT4.5

Celestron CGEM 800

Orion Skyview Pro 127

Orion SpaceProbe 130

Orion SkyQuest XT8

Keep checking back as I’ll be continuing my research into great Christmas telescopes and publishing my findings here!

Last of a series

Your telescope

No matter what telescope you select, choose one that will meet your precise needs and interests. The planets, the Moon, and close stars require high power, good contrast, and sharp resolution, and if these are the objects of your attention, a refractor or reflector would be a good choice. Because very faint objects like galaxies and nebulae need a large aperture, you should invest in a big reflector telescope to view these. An all-purpose midrange telescope should serve best, for example a 6″ to 8″ reflector or an 8″ Schmidt-Cassegrain.

Keep checking back. Future articles will delve into filters used to cope with light polluted skies, astrophotography, astronomy book and software reviews, astronomical binoculars, and reviews of specific telescope models.

Next to last in my 6 part series on choosing your first telescope.

Telescope mountings

In the first four parts, I’ve discuse aperture, power and the different types of telescopes. Now I’d like to talk about an often overlooked but very important aspect of using a telescope – the mounting. A shaky view is all it takes to a fanatastic viewing session! A good mount can enhance your views. There are two basic telescope mountings:
ﾷ The equatorial and
ﾷ The altazimuth.
An equatorial mount is designed so you can easily track the motion of the sky as the Earth turns and its motions indicate celestial directions in the eyepiece. This helps when you’re trying to find your way among the stars with a map. The altazimuth mounts are simpler and just swing up, down, left and right. You have to move the scope along every so often to follow the stars, moons and planets. An altazimuth mount is both cheaper and lighter for the same degree of stability.

Come back for my concluding article when I’ll wrap everything up.

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.

Part two in my series on buying a telescope

In part one, I discussed why aperture is important in choosing a good telescope and also about telescope power. As I continue my series, I will discuss the refracting telescope. Be sure to check back for part 3 on reflecting telescopes!

The refracting telescope

Refracting telescope are the most common form of the telescope – a long tube where light passes directly from the front objective lens directly to the eyepiece at the opposite end of the tube.

Advantages
ﾷ Easy to use  due to the simplicity of design.
ﾷ Good for distant terrestrial viewing
ﾷ Excellent for lunar, planetary and binary stargazing especially with larger apertures
ﾷ Sealed tube protects optics and reduces image degrading air currents
ﾷ Rugged, need little or no maintenance

Disadvantages
ﾷ Generally have small apertures, typically 3 to 5 inches
ﾷ Less suited for viewing small and faint deep sky objects such as distant galaxies and nebulae
ﾷ Heavier, longer and bulkier than equivalent aperture reflectors and catadioptrics
ﾷ Limited practical usefulness
ﾷ Good-quality refractors cost more per inch of aperture than any other kind of telescope.

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!