One of the questions I get asked most often is:

“I’m interested in getting started in astronomy, can you recommend a telescope?”

The problem with that question is the answer depends on where your interests lie, do you want to use it to image and/or observe the Sun, Moon and planets, the deep sky (stars, nebula, galaxies), or a little bit of everything?

So it really pays to think about how you want to use your scope and the various options available to you before parting with your hard earned cash.

Types of Telescope

Before we go any further it is probably worth briefly describing a little about the history and limitations of the various types of telescope available to the amateur astronomer.

Refractors

A refractor is probably what most people think of when asked to visualise a telescope, and was the first to be systematically employed for astronomy by Galileo Galilei in 1610. If you travel to Syon House just outside London you will see that Englishman Thomas Harriot could also claim some firsts, including being the first to use a refractor to record a telescopic observation of the Moon and sunspots.

Plaque at Syon Park, Brentford celebrating the achievements of Thomas Harroit

In simple terms the refractor focusses light via a bi-convex lens (the objective). You can see from the diagram below that a simple refractor does have a problem in that the glass used to make the objective lens has a refractive index which decreases as the wavelength of light increases. As a result of this if you move the focal plane to the point of blue focus you will see a sharp blue point of light surrounded by a red halo, moving the focal plane to the red point of focus will result in a sharp red point of light with a blue halo, this is called chromatic aberration. A compromise would be moving the focal plane to the ‘circle of least confusion’, but at this point nothing is properly in focus!

Chromatic aberration in a refractor & how to rectify it. Source Wikipedia

Never fear, modern refractors shouldn’t suffer from chromatic aberration due to the efforts of English optician John Dollond (still on the UK high street as the opticians Dollond & Aitchison). He patented the achromat in 1758 which corrects chromatic aberration by using two lenses made of different materials (crown and flint glass). The crown glass converges the light & introduces chromatic aberration, the flint glass element has a refractive index which strongly affects blue light thus correcting the chromatic aberration.

The addition of a third lens (triplet) will bring the green wavelength of light to the same focus as well, these are called apochromats (APOs) resulting in red, green and blue wavelengths all being brought to the same focus. Pretty much every astronomical refractor on the market today will be an APO but I think it is always useful to know what this actually means. Addition of further lenses will offer benefits such as field flattening (round stars at the edge of the field of view) or focal length reduction.

Pros and cons

If you are considering buying a refractor here are some pros and cons. First the good points:

Refractors are generally stable against temperature changes over the course of a night as the expansion and contraction of the front and back surfaces of the lens tend to cancel each other out.
They require minimal maintenance as they are sealed unit
Collimation is not required
Capable of producing reliable, high quality images
Compact and easily transportable

On the flip side:

High quality lenses are much more difficult and expensive to manufacture than mirrors so if you want an aperture comparable with even the smallest of reflecting telescopes it will come with quite a high price tag.
Traditional refractors tend to have quite a long focal length meaning that the image is spread over a large area of the focal plane effectively dimming the image, however refractors with short focal lengths (‘fast’) are available and there are some reasonably priced, good quality fast refractors on the market.

Reflecting Telescopes

Reflecting telescopes, as the name suggests, use mirrors to collect and focus light and so are free from chromatic aberration. This doesn’t mean that reflectors are problem free, with a spherical mirror (the mirror being a section of a sphere) light rays from the edge of the mirror achieve focus nearer the mirror than those from the centre. This causes blurring known as spherical aberration. You may have heard that term before as this is what plagued the Hubble Space Telescope before the shuttle mission to repair it. Spherical aberration is eliminated by using a parabolic mirror where the reflected light achieves focus at the same point.

In the section on refractors you may recall I mentioned field flattening, this corrects yet another aberration called off-axis aberration where objects at the edge of the field of view suffer from various types of distortion, coma being the one that causes the most angst. At the centre of the field your stars may be sharp points but at the edge they may appear distorted and comet like (hence the name). Field curvature is also a common issue where the best focus lies on a curved plane resulting in the stars at the edge being blurred, field flattening lenses are employed to correct this issue. I’ll touch upon how these problems have been addressed in the various designs available.

Newtonian – Isaac Newton is credited with making the first reflecting telescope in 1668 coming up with the eponymous design shown in the diagram below. The telescope consists of two mirrors, the primary which is a concave parabola and the secondary which is flat. The primary collects the light and the secondary, inclined at an angle of 45 degrees, directs it so that the point of focus is to the side of the primary mirror. This design still works well for small amateur scopes but for larger scopes this configuration becomes impractical.

Newtonian Reflector. Credit: oneminuteastronomer.com

Cassegrain - Hot on the heals of Newton in 1672 Laurent Cassegrain developed the design which carries his name. The primary mirror is the same as in the Newtonian design but the secondary, rather than being flat is convex. This reflects the light back to the centre of the primary where it then passes through a hole and comes to focus just behind it. This has the benefit of enabling instrumentation to be fitted without the balance problems of a Newtonian design and delivers long focal lengths without the corresponding increase in the length of the optical tube.

Cassegrain Reflector. Credit Vik Dhilon (http://slittlefair.staff.shef.ac.uk/teaching/phy217/lectures/telescopes/L07/index.html

Ritchey-Chretien - Remember off-axis aberrations? The Newtonian and Cassegrain designs both suffer from coma so to rectify this George Ritchey and Henri Chretien jointly developed the Ritchey-Chretien telescope in the early 1900s. This is basically a modified Cassegrain with a concave hyperbolic primary and concave hyperbolic secondary, removing spherical aberration and coma and reducing the effects of field curvature. The payoff is that the secondary mirror is larger and covers some of the primary but imaging performance over a wide field of view is much improved on the Newtonian or Cassegrain designs. For the amateur they do tend to be expensive due to the increased complexity of producing hyperbolic mirrors.

Catadioptric Designs

Celestron Edge HD Schmidt-Cassegrain Telescope in my home observatory

You don’t have to limit yourself to only using mirrors or lenses, you can of course combine the two in what is known as a catadioptric telescope. Three of the most common are the Schmidt, Maksutov-Cassegrain and Schmidt-Cassegrain.

Schmidt –Also known as the Schmidt Camera, was designed to image large fields of view. A spherical primary mirror receives its light through a thin aspherical lens (corrector plate) which compensates for spherical aberrations. The spherical mirror means that this design is free from astigmatism.
Maksutov-Cassegrain – This design combines a spherical mirror with a weakly negative meniscus lens or corrector plate which corrects the problems of off-axis and chromatic aberration. Patented in 1941 by Russian optician Dmitri Dmitrievich Maksutov it is based on the Schmidt camera and uses the spherical errors of a negative lens to correct the errors of the spherical primary mirror. In the Cassegrain version there is an integrated secondary using all spherical elements, simplifying the build and making this design cheap to manufacture. A 127mm ‘Mak’ makes a great first telescope.

Schmidt-Cassegrain - As the name suggests this is a hybrid of the Schmidt and Cassegrain designs. The big advantage of this design for the amateur astronomer is that it is cheap to mass produce as it uses spherical mirrors, spherical aberration being dealt with by a corrector plate. This design does suffer from off-axis aberrations as the corrector lens is not placed at the centre of curvature, this is corrected for in more expensive models of Schmidt-Cassegrain such as Celestron’s Edge HD range. This design delivers a long focal length in a short tube resulting in a powerful, compact and portable scope. The focal ratio is typically f/10 and they come in a range of apertures up to around 14” (356mm). There is a practical upper limit of around 16” (406mm) as manufacture of a corrector for larger apertures is costly and can suffer from flexure making it impractical to go much larger with this design.

Schmidt-Cassegrain. Credit:oneminuteastronomer.com

Pros and cons

If you are considering buying a reflector or catadioptric scope here are some pros and cons. First the good points:

Reflectors and catadioptric telescopes deliver much greater apertures and hence light gathering power than refractors in a similar price range.
The compact design of Schmidt-Cassegrain and Maksutov-Cassegrain scopes deliver long focal lengths in a small package (for smaller aperture scopes). Anything up to an 8” scope (203mm) will be easy to handle and transport and the smaller 5” (127mm) ‘Maks’ can fit in a small (ish) bag while delivering typical focal lengths of 1500mm.

On the flip side:

They can suffer from changes in temperature through the course of an observing session so you need to be mindful that focus may shift.
Newtonian and truss designs are not sealed units so suffer from collimation issues. You will have to learn how to collimate your scope to achieve the best results. Collimation will also have to be performed on Schmidt-Cassegrains but if you are not transporting it regularly this will not be as much of an issue as with a Newtonian for example. The good news is collimation should not be an issue with Maksutov-Cassegrain scopes.
They can be large and cumbersome, especially the Newtonian design where the focal length of the scope is reflected in its length rather than being folded as in the Cassegrain design.

Imaging versus Observing

Of course you aren’t just going to buy a telescope, you are also going to be buying something for it to sit on so think about whether you will primarily use your scope for visual observing or want to try your hand at astrophotography. This is important as it will have a bearing on the type of mount you will require.

For the beginner in visual astronomy and astrophotography I would recommend a driven ‘Go-To’ mount. Once properly aligned all you will need to do is select the target you want to observe on the hand controller, or in some cases via an app. This is particularly useful in urban locations as often you simply can’t see the stars required to star-hop to an object, and anyway this is a tricky skill to learn when just setting out.

There are some bargains to be had if you go down the un-driven route with ‘Dobsonian’ scopes offering large aperture at relatively low prices but I would leave these until you are more familiar with the sky and have established where your interests lie. It can be a frustrating activity trying to find an object and keep it in view at high magnification!

If you are a visual observer or are just planning on shooting the Sun, Moon or planets then you should be able to get away with a simple alt-azimuth driven mount. These track the object you have selected in two axes in a kind of stepping motion so your target will always stay in view, but this won’t be suitable for long exposure photography. For long exposures you will require a sturdy, polar aligned equatorial mount. This tracks in one axis meaning that the scope will smoothly track your chosen object from east to west. Equatorial mounts are more complex to set up and align and are normally much heavier than a simple ‘Alt-Az’ so if you think this may be a problem for you start off with the simpler Alt-Azimuth mount. You can always purchase an equatorial mount later as your interest develops while retaining your scope.

The Sun, Moon and Planets

The primary consideration when photographing or observing the Sun, Moon or planets is image scale, you want your scope to deliver high magnification so a long focal length is required. Traditionally the recommendation would have been a long focal length refractor but these are expensive, or if cheap low quality so I would recommend a Schmidt-Cassegrain or Matsukov-Cassegrain design. Go for the largest aperture you can afford in your price range as at long focal lengths light gathering power and resolution is important. If you go for an 8” scope you’ll find this should serve you well for years.

The Deep Sky

If your interests are in observing star clusters, galaxies such as The Andromeda Galaxy M31 or the Orion Nebula M42 then high magnification is not what you are after. Here a fast refractor will produce beautiful wide field views and a small objective lens should not be a problem.

A lot of people don’t appreciate the size of many deep sky objects, M31 covers an area of over 3 degrees, that’s 6 full Moon widths so you need a wide field of view to fit it in! A 65mm f/6 refractor will provide you with lovely wide field views and will also enable you to take beautiful images when coupled with a camera and the right mount. When looking at potential purchases bear in mind the issues mentioned in the refractor section of this post. You want to make sure it is an APO and if possible also delivers a flat field giving you round stars right to the edge of the field of view.

A Little Bit of Everything

It’s likely that if you are just starting out you won’t know exactly where your interests lie. If this is the case I would lean towards a scope that is going to deliver good views of the Sun (with a suitable filter), Moon and Planets while also enabling you to view brighter deep sky objects such as open and globular clusters. A 5” (127mm) or 6” (152mm) Schmidt-Cassegrain or Maksutov design will serve as a good all-rounder, delivering reasonable aperture and focal length.

A word on Eyepieces

First you need to know how to calculate the magnification your eyepiece will deliver. All you need to do is work out the focal length of your scope by multiplying the aperture in mm by the focal ratio, so a scope with a 203mm aperture and focal ratio of f/10 will have a focal length of 2030mm. The magnification provided by your eyepiece is calculated by dividing the focal length of the telescope by the focal length of the eyepiece, so a 25mm eyepiece will deliver a magnification of 2030/25 = 81 a magnification of 81 times (81x).

Typically a telescope will be supplied with a 25mm eyepiece, you may want to supplement this with a 9mm for higher magnification or invest in a Barlow lens or Powermate. These will effectively increase the focal length of you scope by a stated amount, e.g. 2.5x , so your 2030mm scope will deliver an effective focal length of 5075mm with a 2.5x Barlow or Powermate, with a 25mm eyepiece delivering a magnification of 203x. Barlows and Powermates are especially useful for high resolution planetary, lunar and solar imaging, so if you think this is going to be your thing then I would recommend investing in one.

Generally the eyepieces supplied with cheaper telescopes will not be of great quality but will suffice when starting out. If you do decide the hobby is for you it makes sense to upgrade to a more expensive model at some stage.

Still Can’t Decide?

Here is a summary and a few key points to consider:

Don’t run before you can walk. Start small while you dip your toes in the water, if you get the astronomy bug there’s plenty time to upgrade as your skills develop.
The best scope for you is the one you will use. Don’t choose something that is heavy or difficult to transport and set up. You’ll end up cursing it and coming up with excuses not to spend a night under the stars.
For planetary, lunar and solar observing and imaging you need a long focal length.
If you want to observe or photograph the deep sky a short focal length is best, ideally coupled with a large aperture.
If you want to take long exposures you’ll need an equatorial mount.
If you are still unsure about what you should buy you could go for a pair of binoculars. A pair of 10x50s will give great wide-field views, are relatively cheap, easily transportable and can be used for other things if you decide astronomy isn’t your thing after all.