So You Are Interested In Astronomy!

Or perhaps you have a child or grandchild who is.

Should you buy a telescope?

Not to begin with! And don't be tempted by a 'bargain' telescope of the kind commonly seen in some high street shops. These telescopes are typically of poor optical quality and are provided with very flimsy mounts. They will almost certainly disappoint.

Practical astronomy is necessarily a fairly technical and complex subject. Until you have at least some knowledge of the night sky, a telescope would be of little use, even if you understood how to use it.

In fact, you might not need a telescope.

So how should you start?

The Internet is a great source of information relating to astronomy and your local library will almost certainly have books about astronomy. Some of these might be rather hard going, but there are good books for beginners. Magazines such as 'BBC Sky At Night' and 'Astronomy Now' often contain information aimed at beginners, including reviews of books, DVDs and astronomical equipment. Have a look at the reference section on this web site.

Joining a local astronomical society could be a good start. Astronomers are usually only too pleased to help newcomers. You will get to talk about astronomy, and have opportunities to see and use telescopes, etc.

If you live near to Kendal, why not join the 'Eddington Astronomical Society'?

Even if you prefer to start off on your own, do get a good pair of binoculars. For more information about choosing and using binoculars,  click here. Then, get yourself a torch fitted with a red filter and a star map for the current month. Star maps are included in the magazines mentioned above. Go outside after dark, look up and begin to relate what you see to the star map.

In the unlikely event that you are not amazed by what you see and you loose interest, you will still have a good pair of binoculars that can be used for bird watching or a host of other terrestrial activities. An astronomical telescope would have been a waste of money, especially if it was one of those high street 'bargains'.

I now have two telescopes, but still use my binoculars for viewing 'wide sky' subjects.

What Next?

After you have given yourself say 6 to 12 months of stargazing through your binoculars and learning about astronomy, you will begin to have some ideas of what really interests you. This might be the time to consider, and I mean consider, buying a telescope. Astronomical telescopes come in many types, sizes and makes. Very confusing!

Don't forget that the telescope must be supported by a suitable mount which must allow you to follow the movement of stars, etc as they move across the sky. Again, confusing!

The best telescope and mount for you depends on what you want to do with it, and your budget. You will need to decide what might be best for yourself, but the following notes might help. These notes are indended provide a basic introduction. You should still do your own detailed research.

Telescopes:

The two main types, but by no means the only types of telescope are 'refractors' and 'reflectors'.

Refractors use lenses to focus incoming light to a point as is done in a typical camera. Refractors all suffer from chromatic aberation. ie light of different colours is not brought to exactly the same point of focus. ACHromatic refractors typically use two element lenses to reduce chromatic aberation to sensibly low levels. True APOchromatic refractors use three (or even four!) element lenses to reduce chromatic aberation to practically negligable levels. By using special kinds of glass, near apochromatic performance can be achieved while using only two elements to make up a lens. These specialialised two element lenses are often advertised as being apochromatic.

Needless to say, apochromatic telescopes are expensive. The majority of refractors in use are achromatic and can give remarkably good results.

Reflector telescopes use concave mirrors instead of lenses. A concave mirror reflects light back towards a point of focus in the direction from which it came, in the same way that an ordinary shaving mirror will do. In the simplest form, a small flat secondary mirror is fixed just inside the point of focus so as to reflect the focused light out sideways and so then into an eyepiece. This is the basis of a Newtonian telescope. Mirrors are silvered (actually aluminised) on the front surfaces so that no refraction of light can occur. Reflector telescopes do not suffer from any chromatic aberation whatsoever.

Conventional refractors and reflectors are relatively bulky. This can be overcome by 'folding' the light path back on itself as is done in Schmidt Cassigrain telescopes (SCTs) and Maksutov Cassigrain telescopes (MCTs). Both of these types use a main concave reflector mirror, but unlike Newtonian telescopes where focused light is diverted sideways, in SCTs and MCTs, a secondary mirror is used to reflect focused light back down through a central hole in the main mirror. The secondary mirror is supported on a glass 'corrector' lens fixed at the top of the telescope tube. In practice the corrector lens causes negligable chromatic aberation. Although SCTs and MCTs are quite compact, they can be surpprisingly heavy, especially MCTs. SCTs and small MCTs are very popular, but are also relatively expensive. They also have long focal lengths (>2000mm) when compared with refractors and Newtonian telescopes (~1000mm).

Enough about types of telescope. There are other types, but these are mostly derived from the basic types described above. I'll leave you to find out about them. Just remember that the perfect telescope has yet to be made. Each design has it's own merits, but all have limitations.

There are a few other points to keep in mind:

An astronomical telescope could be thought of as a 'light bucket' in that it collects light. The bigger the bucket, the more light it will collect so that feinter objects become more visible. It's perhaps not so obvious that bigger telescopes are also capable of resolving greater detail than can a small telescope. Theoretical resolving power is directly proportional to the diameter of the primary mirror or lens and does not depend on the type of telescope used.

Telescopes are often refered to in terms of their 'aperture'. This is calculated by dividing the focal length of the telescope by the diameter of the primary mirror or objective lens, just as is done for camera lenses. The result is expressed as an 'F' number. Typical SCTs, MCTs and refractors have 'F' numbers of 9 or more. Many Newtonian telescopes have 'F' numbers of less than 5, although long focal length Newtonians are available with 'F' numbers of 10 or more. The 'F' number of a telescope can be a measure of it's light gathering power, but it is also significant when selecting what type of eyepiece to use. (see below)

Note: You will also see 'f' used to refer to the focal length of a telescope (eg as f=1200mm). More confusion?

Don't pay any attention to manufacturer's claims along the lines of '500x magnification'. On rare nights when the atmosphere is very stable, 500x magnification might be useful on a Newtonian telescope with 250mm (10") diameter mirror. Under the same conditions, a 60mm diameter refractor would have a maximum useful magnification of only 120x, nomatter what it says on the box!

Unless you can be very sure of what kind of telescope you want, then consider buying a 100mm to 150mm (mirror diameter) Newtonian telescope. A Newtonian will definately give you the 'best bang for your buck', especially in this very popular size range.

Telescope Mounts:

A telescope is of little use without a suitable support/mount. The two main classes of mount are Alt/Az (Altitude/Azimouth), and Equatorial mounts.

At first sight, Alt/Az mounts appear to be very simple to use. There's a horizontal plane of rotation and a vertical plane of rotation. Simple! Well, actually not so simple. If you want to follow the movement of a star across the sky, you must keep readjusting in both planes of rotation.

Equatorial mounts also provide two working planes of rotation, known as RA (Right Ascension) and DEC (Declination), but also include manually set and lockable Altitude and Azimuth settings. The latter two are used to align the RA axis of rotation to the celestial pole. In the Northern hemisphere, this means aligning to Polaris, the pole star (actually to a point very close to Polaris). Once this is done, the intended subject is brought within the field of view by adjusting both the RA and DEC. Then, to follow a star across the sky, only the RA need be adjusted. In practice, this is very much easier than using a simple Alt/Az mount!

Both types of mount can be motor driven and automated so as to be able track a single star accurately. BUT, if you intend to do any long exposure astrophotograpy, an equatorial mount is essencial. Alt/Az mounts suffer from 'field rotation' which means that although a central star can be tracked accurately, other stars within the field of view will appear as streaks or arcs around the central star.

Alt/Az fork mounted SCT/MCT systems from the likes of Meade and Celestron can be set up to operate as equatorial mounts by fitting an 'equatorial wedge'. This solution works, but is not as stable a platform, or as flexible a system as that provided by a true equatorial mount.

The mounts described above are typically controlled via gear/worm drives, so adjustment is done by turning a knob (or in the case of motorised drives, by pressing a button). Dobsonian mounts are simple Alt/Az mounts, but have no gears/worm drives for adjustment. They are best described as 'push to' mounts. Dobsonian mounts are mostly used with medium to large Newtonian telescopes, but do remember that the only control is to 'push' the telescope to the position required. You must then keep 'pushing' (in both Alt and Az axes) to follow your subject. Because of their simplicity, Dobsonian mounts are relatively cheap.

Eyepieces:

Telescopes are typically supplied with two eyepieces, chosen to give low magnification (eg 50x) and medium/high magnification (eg 120x). Nowadays, Ploessl or Super Ploessl types are popular (and hence are relatively cheap) with all optical surfaces multicoated to minimise reflections and light scatter. Magnification is calculated by dividing the focal length of the telescope by the focal length of the eyepiece.

As with telescopes and mounts, there are many types of eyepieces, some of which are not suited to wide aperture (ie low 'F' number) telescopes (eg Kellner and some ultra wide angle types). For now, just stick with standard Ploessl/Super Ploessl types and you won't go far wrong.

Manufacturers of astronomical equipment:

Don't let labels of well known suppliers of equipment fool you. Much of the astronomical equipment now available is manufactured in China by companies such as Synta. For example, the highly regarded Sky Watcher EQ6 mount is made in China by Synta, but is labeled and marketed as coming from at least three well known western companies (eg as the Orion Atlas). This gives the Chinese manufacturers (and us) considerable benefits from economies of scale. Chinese made lenses and mirrors are also good. I have a 5" refractor telescope and a 10" Newtonian telescope, both Chinese made. They perform very well indeed. I've seen eyepieces labeled as coming from Meade, but stamped with 'made in China'. Of course, you are likely to get the quality that you pay for, but Chinese sourced equipment can be very competitive in both price and quality.

Summary:

You will read about and hear a lot of confusing claims and counter claims about telescopes, mounts, etc. If you go beyond using a pair of binoculars, the subject does begin to get rather 'techy'. Don't worry!. Just take your own time to absorb it and to decide what is appropriate for you. If you are interested in 'wide sky' views, then your major financial outlay could be for a pair of binoculars. Even if you decide to invest in a telescope system, I doubt that you will need to take out a second mortgage. Good luck.

P.S.
I haven't covered the subject of astrophotography to any extent. This is a very specialised and extensive topic. You really will need to do a lot of research here. Have a look at the sections about astrophotography and spectroscopy on this web site.

Ken Hough

Updated: 08/01/08