As we move on in our new hobby you will start to think about getting a telescope. So what are the best affordable telescopes for beginners? First though, we need to understand what types of telescopes there are, how they work and what they are best at doing.
The Refractor Telescope
The refractor telescope was the first type of telescope that was turned to the skies by Galileo. With this telescope he made the first observations of the mountains on the Moon, the moons of Jupiter, the phases of Venus and the rings of Saturn.
Principle operation of a Refractor telescope.
Refracting telescopes typically have a lens at the front, then a long tube, then an eyepiece instrumentation at the rear, where the telescope view comes to focus. Originally refracting telescopes had one element but today most lenses are made with two or three elements.
Like all telescopes today they come with a variety of eyepieces and accessories. Eye pieces can range from 6mm to 25mm or more. There are two standard sizes, 1.25 inches and 2 inches. There are larger sizes but the amateur is unlikely to come across these. The size depends on the type of refractor you get but the standard size is 1.25 inches.
Modern Refractor upon a German Mount.
Refractors are a great introductory telescope, especially for children. The smaller ones come with either 2 to 3 inch objective lens and a good tripod, so easy for a budding young astronomer to move around and put together. They usually come with a altazimuth mount, which means they operate left right, up down. This is sufficient if you are just learning the night sky but you will find If you are going to use a refractor as your main telescope a maximin size of object lens should be no more that 6 inches. Over 3inch would need a sturdy mount with a drive.
These telescopes are generally best for viewing the moon and planets. The can also show M42, The Sword of Orion’ and M31, the ‘Andromeda Galaxy’. Globular clusters, like M13 in Hercules.
M42 ‘The Sword of Orion
M31 ‘The Andromeda Galaxy’
M13 Great Cluster in Hercules.
These telescopes won’t be able to show these objects in detail but will give lovely views all the same. The refractor is also the best instrument of observing variable stars. These stars vary in brightness either because they fluctuate in size over a period of time. Other stars have a companion, or companions. These companion stars pass in front of the main, from our perspective, star causing its light to dip in brightness. Astronomers who study variable stars report their observations to either the AAVSO website in America or the BAA in the United Kingdom.
Variable star observing is a whole subject on its own and I will deal with it in a later post.
The Reflector Telescope
Principle operation of a reflector telescope.
The reflector telescope is type of instrument that is derived from the fact that the primary mirror reflects the light back to a focus instead of refracting it. The primary mirror usually has a concave spherical or parabolic shape, and, as it reflects the light, it inverts the image at the focal plane. The diagram shows how this works.
The primary mirror is located at the lower end of the telescope tube in a reflector and has its front surface coated with an extremely thin film of metal, such as silver .The back of the mirror is usually made of glass, although other materials have been used from time to time. Pyrex was the principal glass of choice for many of the older large telescopes, but new technology has led to the development and widespread use of a number of glasses with very low coefficients of expansion. A low coefficient means that the shape of the mirror will not change significantly as the temperature of the telescope changes during the night. Since the back of the mirror serves only to provide the desired form and and shape of the mirror. The shape of the mirror determines the focal length of the telescope this affects the length of the tube within which the optics are housed. Reflecting telescopes are generally mounted on what is called a German or Equatorial mount. These mounts have their centre of axis pointer at due north.
As with the refracting telescope there are many eyepieces and accessories available. Reflecting telescopes are widely used for astrophotography as they lend themselves very good having a telescope drive fitted.
The Dobsonian Telescope
A Dobsonian telescope is a variation of the reflecting telescope. Designed in 1965 by John Dobson he created a type of telescope that could easily be built by the amateur using ordinary tools and a small outlay. The Dobsonian has a tube and a mirror, which reflects back to a focal point. This telescope is mounted on what is called a Dobsonian mount.
A simple Dobsonian Telescope
The classical Dobsonian mount consists of a flat horizontal “ground board” platform on top of are attached three of seven supports. Upon these three supports rests a box construction called a “rocker box”. A loose center-bolt keeps the rocker box centered and allows it to pivot above the ground board. On opposing sides of the rocker box, semicircular depressions are cut out from the top edge of each wall (the rocker box is open on the top and on the back. Each depression has a widely spaced pair of supports installed in the cut. The telescope optical tube assembly has two large round trunnions (or arc-shaped rails for larger telescopes) secured on the left and right sides. Their common axis intersects the entre of gravity of the telescope OTA. The trunnions (commonly known as altitude bearings) rest atop the aforementioned four supports in the top cutouts of the rocker box. To raise the telescope (altitude), just lift the tube and the trunnions will slide over the four supports. To move the telescope left or right (azimuth), push or pull the top rim of the OTA (some have a dedicated handle) so that the pivoting rocker box slides over the ground board’s three supports.
Astronomers who embark on building their own telescopes invariably start with building a Dobsonian. Its simplicity in design and easily sourced components make it a worthwhile project for those who are good with their hands. There are many plans for building a Dobsonian that can be found on the internet. Some showing how people have made their telescope mobile by adding wheels. Others have changed it so that they can be polar aligned. Others have have embarked on actually grinding their own mirrors.
One advantage of Dobsonian telescope is that they will support large mirrors and many Dobsonian have been made with very large mirrors, some as big as 20 inches, or 5000 millimeters. This is why these telescopes have acquired the nick name of “Light Buckets”.
New or Second Hand?
This can be a mine field for those new to the hobby and buying their first telescope. Many stores sell simple altazimuth telescope that is on a simple tripod. There is nothing wrong with these telescope but the optics may not be up to standard for use as a astronomical instrument. If your budget can only afford this type of telescope then all well and good. You will learn how to use a telescope so when you have the resources available to buy a better telescope you will be in a position to use it better.
There are some astronomy shops that sell second hand telescopes and you can be fairly assured that you are getting an instrument that is in very good condition. If you plan to buy privately then I would take along someone who is knowledgeable about telescopes, a friend from your astronomical society would be ideal. Many members would be glad to help you as they would remember when they brought their first telescope.
When your budget allows then seriously consider buying new from a astronomy shop. There are many that can be found on the interenet and prices are very reasonable with prices for an entry level telescope around £150.
What Should I Look for When I Buy a Telescope?
The first thing I would look at when buying a telescope is I would ask about its history. What did the seller use the telescope for? Where did he keep it and why is he selling. Then look at the tub. Is it dented in some way. Small dents are ok but major ones are definatly one to steer clear of. Some telescopes don’t have a tube but the optics are held in place by rods of metal or wood. Check for any warping or splits in these rods.
Next I would look at the optics. If it is a refractor telescope then check the object lens, If it is a reflector or a dobsonian I would check the mirror. Is it chipped or cracked? Does the mirror need resilvering, your friend who is with you will be able to tell you this. Resivering a mirror does not cost a lot of money and there are many companies to be found on the internet who do this type of work for a resonable cost.
Try using a eyepiece in the telescope and look at a distant object, does it focus ok. If everything is ok then look at the extras, like other eyepieces. Are they chipped, is the lens in the eyepieces ok. Check each eyepiece on a distant object. Then look at the mount. Is it sturdy enough. If you plan to use it for astrophotography then make really sure it can carry the weight of the telescope and your camera setup easily without buckling. Then discuss the telescope with your friend. Has he noticed anything wrong or has he a question to ask. If all seems ok and you are happy with it then go ahead and purchase it. But if you come across any cracks, splits or any problem that you cannot easily fix then steer clear. There are many telescopes out there that are in very good condition and would suit your needs well.
Clear Skies
Dave