Selecting your first telescope can be a daunting task. There are many different specs and options available, and getting the selection right is important.
Some important things to consider;
- Do I need it to be easily portable? A bigger telescope will of course allow you too see more. But if you live in the middle of a city with lots of light pollution, you might want to find a dark sky area and will need to load in your car. Consider this before buying a big, heavy telescope.
- Am I only interested in casual observing and looking at the planets or astrophotography and the study of deep space objects? While all types of telescopes will allow you to look at whatever you want, as you refine your interest some types of telescope will be better suited than others.
Read on to find out more about what will be the right choice for you.....
Telescopes come in many different sub-types, but they all have some common pro's and con's.
They offer the ability to configure your setup according to what you want to see. You can change the eyepiece, which dictates the magnification factor, change the mount which dictates how the telescope tracks your target as well as add many different filters, astro-cameras and a multitude of other accessories to tailor your experience. They also offer a much greater ability to collect light than most binoculars and therefore offer a much better range of viewing options.
However, they are bigger and heavier than most binoculars, and so need more planning and time before you go star gazing. They require some preparation before they can be used to full effect, and some basic experience on how to set them up and what type of equipment to use, and when to use it will also improve their performance and your viewing experience.
Let's take a look at the different types of telescopes and mounts that are available and their pro's and con's to help you make an informed purchase.
Many newcomers to astronomy make a common mistake, and assume that the magnifying power of telescopes is the most important factor with the idea that the higher this value is the better the view will be, right? Wrong!
No matter how much you magnify it, the light coming from an object stays the same, and the more you magnify it, the more you spread that light over a larger area and so the dimmer the view will be.
You need a balanced combination of magnification and light collecting ability to have a great viewing experience. You can configure a telescope with various magnifications just by changing eyepieces, but without a sufficiently large aperture, the high magnification is worthless — it will just result in showing a blurry, dim mess. Not much fun!
Ultimately, all optical devices like binoculars and telescopes are designed to magnify and brighten the object being viewed. Different telescopes work in different ways, so how they accomplish their goal is also different, but the single most important factor to consider when selecting your optical equipment is its light collecting ability.
Simply put, the diameter of the primary light-gathering lens or mirror, also known as it’s aperture, dictates how much light is collected. A larger lens or mirror can gather more light from the object being viewed, which means that the telescope is able to provide a brighter and sharper image.
Capturing more light from the viewed object also means that you can apply more magnification while maintaining clarity. Put simply, a larger aperture means more magnification is possible without resulting in a fuzzy image.
So, a larger aperture with a smaller magnification will give better results than a small aperture and large magnification.
This is also very important and this where the eyepiece comes in to play. Unlike Binoculars, whose eyepieces are permanently attached to the device, a Telescope can be configured with different eyepieces allowing you to increase or decrease the magnification. Ideally, you want a range of eyepieces from high power (5-10mm) to low power (25mm and above) options to give you the best range of viewing options.
As a general rule, a telescope has a magnification potential of 2x per millimetre of aperture. This is sometimes known as the MUM factor (Maximum Useable Magnification). This value is used to calculate the maximum magnification that can be used with a certain aperture and still deliver acceptable results. For example, a 125mm (5”) aperture telescope has a potential MUM factor of 250x magnification depending on other variables like light pollution and other seeing conditions.
However, we also need to consider the eyepiece magnifier and telescope focal length.
To work out the magnifying power of a particular combination of equipment, you must divide the focal length of the telescope by the focal length of the eyepiece you have fitted to it.
For example, a 1000mm focal length telescope fitted with a 20mm eyepiece will magnify by 50x, but if you change the eyepiece to a 5MM it increases to 200x. If the focal length of the telescope were 600mm instead of 1000mm the magnification using the same eyepieces would alter to the 20mm magnifying by 30x and the 5mm by 120x. So the longer the focal length of the telescope the more magnification it can deliver.
If this figure is less than the MUM figure, all is well, but if it is more than 10% higher you may want to rethink the combination you are using.
A refracting telescope has an object lens at the front of the tube and light exits at the end through the eyepiece.
The refractor has the advantage that since both ends of the tube are sealed by lenses, dust and moisture are not able to enter the tube so ensuring the quality of the visual output. Additionally, the fixed optics do not normally require collimation and they do not have a central tube obstruction that reduces light collection. A refractor is a great telescope for beginners and gives higher quality images of the Moon and planets.
However, because a lot of stargazing means pointing the telescope at high angles you will need to stoop and crane your neck to look through the eyepiece of a refractor, so buying a 90° adapter is recommended to make your viewing experience more comfortable. Additionally, refractors can suffer from something called Chromatic Aberration, which is a colour distortion around the object being viewed. We feature both Achromat and Apochromat refractors that are designed to suppress this by using additional lenses to correct the aberration.
A Refractor on an Alt-Az mount with finder scope and 90° eyepiece adapter.
As the name suggests, a reflector telescope, also known as a Newtonian Reflector, collects light from a mirror at the end of the tube and reflects it back up the tube to a focussing prism at the other end and then through the eyepiece.
The benefits of a Newtonian are that because it is cheaper to make mirrors than lenses, you get more aperture for your money, and they don’t suffer from Chromatic Aberration. However, you may need to adjust the optics occasionally with a process called Collimation. This simply means ensuring that the optical elements within your telescope are properly aligned to give the best possible image.
A Newtonian Reflector telescope on an Equatorial mount.
Essentially this is a combination of a refractor and a reflector. The light is folded twice by using a mixture of lenses and mirrors.
The main advantage of the design of these telescopes is that because the main tube is much shorter than refractors and reflectors, they provide a much more convenient and portable option, but with a long focal length.
A Cassegrain type Optical Tube Assembly (OTA)
We have compiled a special collection of Telescopes with the novice astronomer in mind. These telescopes will allow you to setup and get going with everything you need to begin exploring the night sky right away. This collection includes examples of both Refractor and Reflector Telescopes so you can choose the perfect option for you.
For a little more investment, it is well worth considering a telescope that has a GoTo motor drive. This allows you to easily find a specific objects from the onboard library once the Telescope is aligned with your location, a process which normally can be completed in less than 5 minutes.
The last part of the equation is the mount which supports the telescope and keeps it steady allowing you to take in the view!
This is the most common mount and is best for beginners. It allows you to move the telescope laterally and vertically in a simple up/down and side to side motion. They are easy to setup and use.
An example of an Alt-Az mount
These mounts are for the more serious observer and particularly for Astro photographers. These mounts allow you to track the same object as it moves across the sky. Celestial objects follow the equatorial arc and this movement is replicated by the equatorial mount. The whole setup is tilted to align with the latitude from which you are viewing the sky. Most Equatorial mounts come with a computer controlled motor drive to track the desired target for you. This type of mount does need a bit more time and expertise to set up properly, particularly if you plan to take long exposure photographs.
An Equatorial drive with GoTo motor drive and computer controller
A Dobsonian telescope is a Newtonian mirror telescope, which is suspended in a so-called rockerbox, which replaces the mount and tripod.
A Dobsonian telescope is therefore very simple, so that here the money is almost exclusively invested in the optics. Essentially this is just a large spinning plate to which the telescope and support cradle are attached. You then simply rotate the telescope in any direction you want and point away! Most of these mount do not come with a motor drive option. Dobsonian telescopes are particularly good for beginners who want to start with a large telescope for relatively little money. In many cases, the OTA (Optical Tube Assembly) can also later be easily mounted on an astronomical mount with a tripod.
An example of a Reflector on a Dobsonian mount.
When looking at a telescope, you will often find that there a number of abbreviations and numbers in the description. They tell you some important information, and breaks down as follows:
Example from this site-
Omegon Refractor Telescope AC 90/1000 EQ-2
Omegon - Telescope Manufacturer
Refractor - Type of build of telescope
AC - Model series
90 - Aperture of the telescope in millimetres
1000 - Focal length of the telescope in millimetres.
EQ - denotes Equatorial mount (AZ would denote Alt-Azimuth)
2 - Mount support rating. Higher rating required for heavier telescopes
You will sometimes see the term OTA on a telescope page. This means 'Optical tube assembly', and indicates that you only get the tube included in your purchase. You will need to purchase the mount and tripod separately if you do not already have them.