Stories from TechLast update: 2018/11/18
This page covers some of the things to consider when looking to buy a telescope.
The first thing to decide is your budget. In most cases, you get what you pay for with telescopes and the accessories. There are some exceptions to this rule in various ways, but quality and price are related.
It is highly recommended to do two other things before purchasing a telescope; ensure that it is a genuine hobby and not a passing fad, and know how to navigate the sky. The best solution to this is to find and study star charts, or use an application such as Stellarium.
Understanding some of the terms around telescopes is highly recommended in choosing the best item.
The focal length is the distance from the mirror to the eyepiece. A telescope with a longer focal length is capable of more magnification if required, meaning higher detailed images of objects can be produced, or dimmer objects can be viewed. Shorter focal lengths produce a wider field of view and a brighter image.
A focal length of f/9 or greater would be considered a long focal length. f/5 or less is considered shorter.
Therefore, if you are aiming to look at large objects such as clusters and nebulae, a short focal length is better. Close objects such as the moon and planets are better viewed through a long focal length.
The aperture is the size of the main mirror/lens of the telescope. The size specification is usually in inches or millimetres depending on the country of purchase.
This value can be worked out simply by using the formula: Focal length / Aperture = Focal ratio
The focal ratio is also known in some circles as the “F/Stop” or “Photographic Speed”.
When a telescope advertises a large number of magnification (e.g. 400x zoom!), it usually means its cheap. And not very good. Magnification is a poor indicator of how good a telescope is, and should be discarded for the moment. It does become more useful in regards to eyepieces and such, however a purchase should not be based on magnification alone.
Reflecting and Dobsonian telescopes will almost certainly receive knocks and bangs over their lifetime. The issue can also happen before the telescope arrives home as transportation can often cause the knocks, even though manufacturers and retailers do their best to try to minimise knocks. This causes the large mirror at the bottom of the tube to be moved out of alignment. Collimation is the process of setting the mirror correct again so that it correctly focuses light back up the tube towards the eyepiece.
Collimation requires a tool to realign the mirrors and ensure that the primary mirror is focusing correctly.
A Barlow lens is used in conjunction with other optics to increase the focal length. This has the effect of decreasing the field of view however.
The Barlow is placed immediately before the eyepiece and allows the image to increase the magnification.
Having a Barlow in your array of eyepices is very useful, as it effectively doubles the number of eyepieces. This is due to increasing the magnification of each eyepiece; so for example, a 48x lens with a Barlow becomes 96x.
Finally, a Barlow plus lens with outperform a single lens of the same magnification.
Chromatic Aberration is a type of distortion often occurring in shorter focal length telescopes. This occurs when the telescope lens can not focus all the different colours to the same point. Different colours of light have different wavelengths, and the Chromatic Abberation shows itself by blurring colours in the image.
Spherical Aberration is caused when the lens do not focus the light correctly onto a single point. Instead light passing through the centre of the lens is focused further away than light passing through the edge. This creates a halo effect where the object is out of focus.
There are two types of Spherical Aberration; positive and negative. Positive occurs when the rays are bent too much, and negative is when they are not bent enough.
Spherical Aberration is more pronounced at shorter focal ratios.
The effect can be minimised by using concave or convex lens, or using aspheric lenses.
Eye relief is the term used to indicate the maximum allowable distance your eye is required to be from the eyepiece.
The higher the magnification and larger the field of view required, the shorter the eye relief.
Another piece of advice to anyone thinking of buying a telescope is to manage your expectations. The fantastic colour and highly detailed pictures often released by NASA are taken either with the Hubble telescope, or land-based telescopes. These cost anywhere between several hundred million and several billion dollars. You won’t see that through a consumer telescope. You can see some fantastic views however of deep sky objects such as galaxies and nebula, however the choice of telescope will limit the quality of image.
There are plenty of things to see however even with some of the cheaper scopes. The bands of colour in Jupiter’s atmosphere, the four largest moons around Jupiter, the rings clearly visible around Saturn, highly detailed craters on the moon, stars not visible with the naked eye, the stars of the Pleiades, etc.
Types of telescope
There are several types of telescope, of which have their benefits and drawbacks.
A refracting telescope is the typical scope seen used by cartoon pirates. Light enters the telescope through the top of the tube, and through a lens. The lens is curved and focuses the light down the tube to a point where another lens sits within the eyepiece.
Refractor scopes never need to be cleaned internally or collimated because the assemblies are sealed. They are often light, and suitable for use on both equatorial or altazimuth mounts.
Often they are used for viewing planets and moons within the Solar System as they offer excellent viewing for detailed features.
One drawback of refractors is the position of the eyepiece. This sits at the bottom of the tube, meaning that you may need to sit in some awkward positions to be able to view the night-sky objects.
A reflector telescope, sometimes known as a Newtonian-Reflector, is one of the most common telescopes used. Light enters the tube and bounces off a large mirror at the bottom which focuses the light. The focused light then travels back up the tube to where it entered to another smaller mirror. The smaller mirror is angled at 90 degrees and focuses the light again onto the eyepiece.
Reflector scopes need to be cleaned every now and again due to their open nature. The mirror at the bottom of the tube will need to be carefully removed and cleaned. Collimation will also be required every so often to account for imperfections in the positioning of the mirrors.
In terms of the optical tube assembly, the way the telescope works is the same as the reflector. The only difference is in the mount, which sits on the floor, and usually holds the telescope near to the ground rather than in the middle of the tube.
Schmidt-Cassegrain telescopes work in a similar way to a reflector. The light enters the tube assembly, passing through the “corrector plate” at the top of the tube. The light hits a large mirror placed at the bottom of the scope. This mirror is disc-shaped with a hole in the centre. The light is then focused to a smaller mirror placed at the top of the telescope. This smaller mirror then focuses the light again back towards the hole in the large mirror where the eyepiece is placed.
The design of this telescope makes it very compact compared to the tube assemblies of reflector and Dobsonian scopes.
A Maskutov is very similar to a Schmidt-Cassegrain telescope, although there are quite a few variants of Maksutov.
The major type uses a convex secondary mirror to focus the light into the eyepiece.
A major benefit of Maksutov scopes are that they have sealed tubes, which minimises cleaning and ensures that the scope does not require collimating.
A Schmidt-Newtonian telescope is very similar to a reflector, however it has a corrector at the top of the tube assembly to create a sealed tube. The corrector plate corrects Spherical Aberration, and reduces coma on the image.
Schmidt-Newtonian scopes are more costly than a reflector due to the extra corrector plate, however they are cheaper than Schmidt-Cassegrain as they don’t have curved mirrors.
Most Schmidt-Newtonian scopes have short focal lengths which makes them suitable for astro-photography.
Schmidt-Cassegrain vs Maksutov-Cassegrain
Maksutov-Cassegrain telescopes are often heavier than their Schmidt counterparts, therefore they need sturdier mounts. They also get particularly expensive at around 200mm mirror sizes as the optics become more expensive to produce.
The Schmidt-Cassegrain has the benefit of cooling quicker so you can start using it sooner, and a Schmidt will give a wider field of view.
On the other hand, Maksutov scopes give a higher contrast and slightly better images, however the emphasis is definitely on the slightly. This may be more noticeable on planets, or the Moon, however to the untrained eye it would definitely be hard to tell.
If you are planning on spending around £150, there are two suggestions. The first would be to consider purchasing a pair of binoculars. For £150, you will have something that will beat any telescope at the same price point. The second may be to consider waiting until you have a little more money available, as a telescope in the £200-250 range will be more enjoyable.
What’s in a mount?
Altazimuth mounts are more basic and lighter in weight. The mount can move the telescope in any direction, but does not allow tracking of an object in the same way an Equatorial mount does.
An altazimuth are great for quickly moving between objects in various parts of the sky.
An equatorial mount is more complicated than an altazimuth mount.
Firstly, a little astronomy. Polaris in the northern hemisphere is known as the pole star, because it appears that all the other stars in the sky rotate around it. This is by coincidence the axis on which Earth rotates almost lines up perfectly with Polaris.
An equatorial mount requires some setup before you can view any stars. First, you must locate Polaris, and then know the direction of both east and west. The telescope will then rotate from the east, across the sky, down to the west.
This rotation makes it much easier to follow a single star across the sky, and is ideal for astrophotography (in combination with an autotrack system).
Equatorial mounts tend to be heavy and bulkier than Altazimuth mounts due to the extra gears, knobs, and counter-weights which are required. Also, they must be heavier to be able to support both a telescope tube, and potentially heavy camera.
A Dobsonian telescope is a combination of a reflecting telescope, and an Altazimuth mount. However, whereas an Altazimuth mount holds the telescope on legs and connects near the middle of the tube assembly, a Dobsonian mount holds it near the bottom on a flat base.
Dobsonian telescopes can be used to quickly hop between objects in the sky. They are also typically cheaper than the same sized scope utilising an Equatorial mount.
Finally, a Dobsonian mount is able to hold larger telescopes, and the weight distribution is easier to manage.
Huygens eyepieces perform fairly well on telescopes with focal ratios of 15 or more, but they provide a narrow field of view. They also have fairly pool image quality.
You won’t typically find any of this type used any more, except on the cheapest of telescopes.
A Plossl eyepiece is relatively popular as they are quite versatile, work with many types of telescope, and are suitable for a range of viewing. The cost of the eyepiece can be quite high however, and cheaper models can exhibit some internal reflection.
Orthoscopic eyepieces are one of the best as they have very little chromatic and spherical aberration, and provide a wide field of view.
GoTo systems are a combination of a computer and mount. The computer allows the user to input the object they wish to locate. The telescope is then automatically driven to the location of the object using gears and motors on the mount.
GoTo system provide a nice system for quickly and easily moving between locations in the sky. They can also be used on both alt-azimuth and equatorial mounts.
They do however pose several issues which affect newcomers more:
- Cheaper GoTo systems aren’t particularly great. They sometimes have limited databases of objects, or they aren’t particularly user friendly.
- The cost of GoTo system adds to the cost of the telescope, and if you are looking at a cheaper scope, it would be highly recommended to go with something that has better optics.
- GoTo requires a more expensive mount, thereby taking more money that you’re spending away from the optics of the scope.
- Using a GoTo can often take away from the experience of learning the sky, and navigating objects simply be sight.
- An additional GoTo adds weight, making it harder to take the telescope on ventures out.
- Before usage, you will need to setup the scope to be able to find objects correctly in a similar way to setting up a scope on an equatorial mount.
I’d really recommend against buying a GoTo unless you are spending more than £800. At this price point, you can get a good combination of optics, mount and GoTo without feeling like something is being skimped on.
A combination of star charts and software such as Stellarium can really be as good as a GoTo system.
To be able to grab fantastic images with long exposures, a telescope with a motor drive is going to be a requirement. This allows the telescope to follow the object across the sky throughout the exposure.
A sturdy mount is also needed. Many cheaper equatorial mounts are built only to support the weight of the telescope assembly, and when a heavy camera is added, it can be simply too much weight for the mount.
Optical Tube Assemblies
If you wish to purchase separate parts of the telescope, Optical Tube Assemblies can be purchased on their own. When picking an OTA, it is important that you also choose the a suitable stand for use with it. For example, too heavy an OTA on a mount not suitable may cause it to fall over.
Below is a list of manufacturers which make excellent telescopes, and are highly regarded.
Questions? Any questions?
Where can I go to learn more?
Stargazers Lounge is a popular forum. The Heretic’s Guide to Choosing and Buying Your First Telescope, written by Michael J Edelman was also quite interesting and covers many questions asked by beginners.
Which star atlas’ would you recommend?
A key recommendation is The Cambridge Star Atlas which was authored by Wil Tirion.