Types of telescope

Advantages of Newtonian scopes

They have a simple design that gives you a lot of aperture for your dollar.

To keep the tube lengths manageable, most Newtonians have a fairly short focal length. This means they have very wide true fields of view, so they are great for deep sky objects like galaxies, star clusters and nebulae.

Small Newtonians are fairly portable. For example, the tube of a typical 8 inch Newtonian may be 75 centimetres long, easily fitting in a car for transport to a dark site. Larger scopes can get quite bulky, though – above 12 inches and transport becomes more of a hassle.

The cool-down time is short because they usually have an open tube. Getting the telescope close to ambient temperature is important for high magnification views, such as planets.

You can use them for astrophotography, but not as easily as some other scopes.

Disadvantages of Newtonian scopes

The short focal length (or fast focal ratio) is a double edged sword. While it gives great wide angle views and shorter photo times, you need good quality eyepieces in order to get sharp images from edge to edge.

Also, shorter focal lengths make it harder to get high magnification. Inexpensive eyepieces that will deliver high magnification in a Newtonian will tend to have small eye lenses and very little eye relief, so can be uncomfortable to use for beginners.

They need occasional maintenance, but this is not difficult. The mirrors can get bumped out of alignment, especially if the scope is transported often. After a little practice, adjusting the scope takes just a minute or two.

All telescopes have some form of optical aberration; a Newtonian’s peculiarity is called ‘coma’ where stars appear slightly wedge shaped at the edge of the field of view. This is more of an issue for short focal length scopes, and is relatively unimportant if the focal ratio is f/6 or bigger.

The image is upside down, so Newtonians are for stargazing rather than observing the land.

Newtonians are best for:

• The Moon, planets and fainter deep sky objects

• People who are less bothered by portability

Advantages of Dobsonians

Because they are Newtonians, they have most of the same advantages and disadvantages that were listed above.

Above all, Dobsonians are outstanding value for money. They give the most aperture per dollar of any scope on the market. Because the mounts are so robust, the scopes can be made very large – more than 30 inches in aperture, for example. It is no surprise that they are nicknamed ‘light buckets’.

The mounts are immensely stable, so they have fewer annoying jiggles.

Dobsonians are breeze to set up – place the base where you want it, and slot the tube on top.

Disadvantages of Dobsonians

Dobsonians are no-frills, manual scopes. This means they do not track the stars as they drift across the sky unless you add motorised tracking equipment.

They are not suitable for astrophotography, not only because they have a manual Alt-Az mount but also because most focusers on a Dob do not have enough in-travel to allow digital SLR cameras to focus.

Dobsonians are best for:

• Moon, planets and fainter deep sky objects

• Those on a budget who want a lot of scope with no frills

Advantages of Ritchey-Chretiens

High quality images with relatively few distortions.

Great for serious astrophotography.

Generally shorter exposure times than comparable compound telescopes like Schmidt-Cassegrains because they usually have a shorter focal ratio (f/6 to f/9, rather than f/10).

Compact, light, and easy to transport.

Disdvantages of Ritchey-Chretiens

Cost. It is difficult to manufacture the hyperbolic mirrors that create such images, so these scopes are more expensive than other reflecting telescopes of the same aperture.

Ritchey-Chretiens are best for:

• Astrophotography

Advantages of Cassegrains

Very good, all-purpose telescope. Excellent for deep sky viewing or for astrophotography. Very good for planetary and lunar observing.

They are very compact. Their design ‘folds up’ the focal length into a much shorter tube than a Newtonian or a Dobsonian. This makes them extremely portable. For example, a typical 8 inch SCT has a tube about 45cm long, which is a third of the length of an 8 inch Dobsonian.

Excellent optical quality with sharp images over a wide field.

Most SCTs come with motor-driven tracking and computerised goto functionality.

SCTs are nearly maintenance free. They require collimation (optical alignment) from time to time, but much less than Newtonians or Dobsonians. They will tend to hold their alignment from year to year, and the longer focal lengths mean that collimation errors are less important.

They are well suited to astrophotography, both with an everyday DSLR cameras or specialist CCD astrophotography camera.

Disadvantages of Cassegrains

The correcting lens adds cost when compared with a Newtonian, as does motorised tracking and goto functionality. Cassegrains scopes are considerably more expensive than Newtonians of the same aperture, but less expensive than a good refractor of the same size.

The longer focal length gives a narrower field of view, especially when used with 1.25 inch visual back and diagonal. You can fit 2 inch optics at the back but this adds to the cost.

While the telescopes are very compact, the mounts can be heavy. Make sure you pay attention to the all-up weight, including the mount.

Because they have closed tubes, they take much longer to cool down to ambient temperature.

Cassegains are best for:

• General purpose viewing, for planetary and deep sky viewing and astrophotography

• People with a larger budget who want lots of aperture with portability

Advantages of refractors

Crystal clear, high contrast images (as long as the scope has good quality lenses).

Easy to use and robust.

Disadvantages of refractors

If you want to observe the fainter deep sky objects, a 4 inch scope will do the job, but is marginal. But a 6 inch refractor is expensive, heavy, has a long tube and requires a large expensive mount.

Refractors are best for:

• Observers who want the best possible image quality