The biggest headache in astronomy comes when one starts thinking about buying the first telescope. But actually, this need not be a headache. One simply has to consider a number of factors and then decide according to what suits him or her best, then the process of choosing starts loosing its menacing look. But first, you need to know the basics. Before even considering buying a telescope, let us see whether you actually need a telescope! Yes, that’s right. one does not necessarily need a telescope in order to be able to view interesting objects in the sky. As a matter of fact, it is often recommended that the first step should NOT be to buy a telescope, but rather invest in a good pair of binoculars. Let’s see why.
Do I need a telescope?
The first two questions to ask yourself are:
1) How interested am I in the hobby? Am I still a beginner who is looking around and trying to explore this new subject?
2) What is my budget?
It is of paramount importance that you answer these two questions. If you have just begun roaming about in the vast subject of astronomy, then you really should NOT buy a telescope immediately. Consider investing in a good pair of binoculars, then go out to a dark site, obtain a skychart and start learning the constellations and the objects you can find within their boundaries. In so doing, you shall start familiarizing yourself with the night sky. THIS should he your first step. It is more important than buying a telescope at once. After all, what is the use of a telescope if you don’t know where to look and what to see?
Secondly, if your budget lies, say, in the range between Lm30 and Lm50, then you cannot expect to be able to get a good telescope with that amount. So now what? Should you give up? Of course not! If we were to be discouraged by prices, then no one of us would be an astronomer. If your budget ranges in this territory, then do as suggested above: buy a pair of good binoculars. You will be able to see a lot and learn a lot.
Now, if you have answered these two questions, you have already gone through this step, and you think that now you are ready to get your first telescope, move on to the next section.
Ok, so you’re ready to get your first telescope. Great! Then, let us first see some basics about telescopes. For example, what does a telescope do and how does it do that? How many types of telescopes exist? (There are three main types and later on we shall be looking at how these three different kinds of telescopes work. You’ll need to know this in order to form a wise decision as to what you need.)
The main purpose of any telescope is to gather light from an object. The pupil of the human eye has too much a small area as to be able to collect enough light from very faint objects such as remote galaxies nebulae and allow one to view them. So, the larger the area of a telescope, the more light it gathers and the greater the resolution. Therefore finer details can be seen. So one should buy the biggest aperture telescope affordable whilst considering matters of portability (the larger the telescope, the greater the weight and the less portable it is).
Magnification is NOT IMPORTANT. Magnification depends on the type of ocular (eyepiece) used. So it can be changed as appropriate. The highest useful magnification is determined by the formula:
Highest useful magnification = Aperture of telescope (in inches) X 60
The practical highest magnification is usually less than this because of seeing conditions. Large magnification is usually used when studying lunar or planetary features. Deep sky objects are generally large enough to be visualized well at low to moderate magnification. The reason we don’t see them with our naked eye is due to the fact that they are very faint. Remember that it is the telescope aperture that has to be increased in order to increase brightness and contrast, and not the magnification.
There exist three main types of telescope. These are the refractor, the reflector and the modern Schmidt Cassegrain (Catadioptric). All have their own advantages and disadvantages and the choice of a particular type of telescope depends mainly on what type of observing one is to put the telescope to. Move on to the next page to learn more about the types of telescopes.
The refractor is the classic long, thin tube telescope design. It was invented in 1609 by the famous Italian scientist and inventor Galileo Galilei. The first telescope was amongst the smallest compared with today’s vast selection. It magnified only sixty times but that was enough to reveal that the moon wasn’t smooth and shiny, but heavily cratered, and also showed mountains and maria (seas). The refractor also confirmed Copernicus’ idea that the planets revolve around the sun, even though this resulted in Galileo’s imprisonment on the decision of the church.
The refractor works as follows:
An objective lens collects more light than our eye because it has a larger area, and brings it to focus at the eyepiece. The eyepiece magnifies the image, and so we see a brighter, larger image.
- The refractor is excellent for lunar, planetary, and binary star observing especially in larger apertures.
- The objective lens is permanently mounted and aligned needing no collimation.
- It requires little or no maintenance.
- The images are of high contrast with no secondary mirror or diagonal obstruction.
- Sealed optical tube reduces image degrading air currents and protects optics.
- Good for distant terrestrial viewing.
- More expensive per inch of aperture than Newtonians or Catadioptrics.
- Heavier, longer and bulkier than equivalent aperture Newtonians and Catadioptrics.
- The cost and bulk factors limit the practical useful maximum size objective to small apertures.
- A good quality 5 inch refractor costs about 510,000 and will probably weigh several hundred pounds.
- It is less suited for viewing small and faint deep sky objects such as nebulae and distant galaxies because of practical aperture limitations and long focal ratio (f/11 or slower).
- The long focal ratio makes photography of deep sky objects more difficult.
- There is some colour aberration in the achromatic design.
The Newtonian Reflector
The Newtonian reflector was invented by the English scientist Sir Isaac Newton. It is essentially an open tube containing a concave, primary mirror on the back part which directs light onto a small plane secondary mirror suspended in the middle of the tube. This secondary mirror reflects light at an angle of 45 degrees, directing it towards the eyepiece which magnifies the image.
The reflector uses no lenses, just mirrors to obtain an image. This greatly reduces the cost of these telescopes.
- The reflector has the lowest cost per inch of aperture compared to refractors and Catadioptrics since mirrors can be produced at less cost than lenses in medium to large apertures.
- Excellent for viewing faint deep sky objects such as nebulae, star clusters and remote galaxies, because of their fast focal ratios (f/4 to f/8).
- Reasonably good for lunar and planetary work especially when used with a Barlow lens.
- Do not suffer much from optical aberrations and deliver very bright images.
- Reasonably compact and portable up to focal lengths of 1000mm.
- Good for deep sky astrophotography (but not as convenient and more difficult to use than Catadioptrics).
- Open optical tube design allow image-degrading air currents and air contaminants, which over a period of time will degrade the mirror coatings and cause the telescope performance to suffer.
- More fragile than Refractors and Catadioptrics and thus require more maintenance (such as collimation).
- Generally not suited for terrestrial applications.
- Suffer from off-axis coma.
- Slight light loss due to secondary (diagonal) obstruction when compared with refractors.
- Large apertures (over 8 inches) are heavy, bulky and tend to be expensive.
The Schmidt Cassegrain (Catadioptric)
The Schmidt Cassegrain uses a combination of lenses and mirrors to produce razor sharp images. The enters through a thin aspheric Schmidt correcting lens, strikes the spherical primary mirror which directs light on the small, plane, secondary mirror. This reflects light out an opening in the rear of the instrument.
Nowadays, this is the most popular type of instrument with the most modern design.
- Best all-around, all-purpose telescope design.
- Combines the optical advantages of both lenses and mirrors while cancelling their disadvantages.
- Excellent optics with razor sharp images over a wide field.
- Excellent for deep sky observing or astrophotography with fast films or CCD’s.
- Very good for lunar, planetary and binary star observing or photography.
- The focal ratio is generally around f110. This is useful for all types of photography.
- Most versatile type of telescope.
- The closed tube design reduces image degrading air currents.
- Most are extremely compact and portable.
- Easy to use.
- Durable and virtually maintenance free (except for occasional collimation if the telescope is bumped or jerked severely).
- There are more accessories available than with other types of telescope.
- Large apertures at reasonable prices and less expensive than equivalent aperture refractors.
- Best near focus capability of any type of telescope.
- Excellent for terrestrial viewing or photography.
- More expensive than Newtonians of equal aperture.
- It is not what people expect a telescope to look like.
- There is slight light loss due to secondary mirror obstruction compared refractors.
Ok, so by now you should have obtained a good general idea of the three main types of telescopes. So what comes next?
Now, it’s opportune to take a look at different brands of telescopes that exist out there and see what they offer. You can find an extensive list of telescope manufacturers in the Links section of our website. It’s best to compare the prices of different models with your available budget and see what you can afford. It is recommended that you do not buy a refractor with an aperture less than 3 inches in diameter, or a reflector with an aperture less than 6 inches in diameter. The larger the aperture, the better. But keep in mind that if you live in a light polluted area, you’ll most probably want to travel to some dark site. Whilst a larger telescope reveals more, it is also heavier. So if you know that you’ll be travelling a lot try to compromise between aperture and size/weight.
Choosing your first telescope should not be a fast process. Take your time to think thoroughly. There is a big chance you’ll need help at some point. In that case, do not hesitate to send an email to us. You can find contact details in the Contact section of our website.
One final word: AVOID ALL TEMPTATION TO BUY A VERY SMALL (CHEAP) TELESCOPE that claims to have magnifications up to 600X! Such telescopes are not of a good quality. In fact, they are often called trash scopes. Keep in mind the guidelines you read above.