You don't actually need an telescope to get started in deep sky astronomy, a pair of 7x50 or 10x50 binoculars are a very good starting point and best of all most people will probably already own a pair! To decide whether a pair of binoculars are suitable for astronomy the exit pupil needs to be at least 5mm in diameter, this is the diameter of the beam of light as it exits the binoculars eye lenses. Binoculars have numbers marked on them often near the manufacturers name in the form of 8x25, 7x50 or 10x50 for example, the first number is is the magnification and the second is the diameter of the objective (front lens) in millimetres.
To find the exit pupil divide the aperture by the magnification and the result will be the exit pupil in millimetres. Binoculars suitable for astronomy should have an exit pupil of at least 5mm, any smaller and the image will be too dim to be useful in astronomy. For example a pair of 8x25 binoculars have an exit pupil of 3.1mm which will produce a very dim field of view (these are more suitable for daytime activities); 7x50 and 10x50 binoculars both have an exit pupil of 5mm, perfect for astronomy.
If using binoculars then I strongly recommend attaching them to a sturdy support as hand holding a pair of 7x50 light as they are will result in aching arms and the image will jump around alarmingly after a very short time.
Telescopes come in a few forms, refractor, reflector and catadioptrics, a refractor uses lenses, a reflector uses mirrors to form a image and catadioptrics use both and your circumstances may be the deciding factor to which telescope you purchase. All three types of scope have their own advantages and weakness and what you decide to buy will depend on several factors: How much are you willing to spend, Are you going to house the telescope permanently in an observatory or will you want to be able to transport it to a dark site and what types of objects do you want to study.
For the observer who wishes to observe all types of deep sky object but is on a budget or doesn't have room to store a large telescope then a simple 4 inch reflector on a simple equatorial mounting or a 6 inch Newtonian on a Dobsonian mount may suffice. These telescopes are very portable, small enough to hide away in a corner but capable of showing many wonderful sights in the night sky. Alternatively one may purchase one of the excellent 3.5 inch refractors which are available in short and long focal lengths and will fit easily on the back seat of a car.
The more serious observer may opt for a 8 inch or 10 inch reflector, either of which will show detail in many of the brighter objects and will still be compact enough if on a Dobsonian mounting to be very portable though Newtonian reflectors above 10 inches aperture can be awkward to transport.
If storage is a problem then a small 5 inch catadioptric on a small equatorial mounting may be ideal, a telescope of this size is quite short yet can give superior views to a reflector of similar aperture however larger catadioptrics are quite expensive.
When purchasing a telescope the deciding factor will be it's aperture as deep sky objects are often very faint and the larger the aperture the fainter objects you will be able to see and with finer resolution so it may seem that given a large enough budget choosing the largest aperture is the way to go but beware as large scopes can be a mixed blessing because the larger the aperture the larger and heavier the telescope becomes! Where are you going to store that 220lb 16 inch Newtonian reflector? could you move by yourself, a telescope that is difficult to handle is a telescope that will not be used so perhaps that smaller more manageable 8inch reflector is a better choice after all.
Where you live will also determine the largest aperture that you will be able to use. Large optics collect more light than small ones but they also collect more light pollution so large apertures are only practical if you observe away from urban lighting, a large aperture if used under urban lighting will give poor washed out view. You may be able to transport the instrument to dark skies but then you have the issue of the physical size and weight. Your first telescope has to be a compromise: small enough for easy portability unless housed in an observatory but with a large enough aperture for serious deep sky study but not so large to be seriously affected by local light pollution.
Some deep sky objects are seriously affected by light pollution namely nebulae and galaxies because the light from these objects is usually spread out over a large area making them very diffuse and therefore challenging to see under light skies but you can purchase special filters to help combat the effects of light pollution called nebula filters.
These filters essentially block out any unwanted light whether it be natural airglow or street lighting by only transmitting a few specific wavelengths in a small portion of the green part of the spectrum while blocking out the unwanted blue and yellow on either side. What a nebula filter does is not make the object any brighter but makes the background darker enhancing the contrast between sky and object.
Nebula filters come in three categories, broadband, narrow band and line filters. Broadband filters have the widest band pass making them suitable for most deepsky objects but the wider band pass means the less contrast gain on an object. A narrow band filter is used to observe emission and planetary nebulae: the band pass is narrow enough to reject most light pollution and is now regarded as a filter for general use and line filters are specialist filters designed for observing certain choice objects. One word of caution when using narrow band filters is that the light rejection is so great that it includes starlight so any field stars within the eyepiece will be considerable dimmed, even more if using a line filter and with a smaller telescope the field of view can become so dark that it can be difficult to see anything.
Deep sky objects fall into four categories: clusters, nebulae, galaxies and double stars and contain various sub-types.
Open clusters populate the spiral arms of our galaxy and are collections of gravitationally bound stars, They range from a few dozen members barely distinguishable from the background star field to clusters of several hundred stars a few tens to perhaps a hundred light years across.
Open clusters vary considerably in age, some such as the Pleiades in Taurus are relatively young at around 50 million years to NGC6791 in Lyra which is 600 million years old. A few open clusters such as the Pleiades are so close that they are resolvable;e with the unaided eye and binoculars begin to resolve individual stars in some of the larger clusters seen against a fuzzy background of unresolved stars. Telescopes will resolve many more stars thus resolve most clusters.
Globular clusters are huge spherical concentrations of stars orbiting high above the galactic plane and some are thought to be the oldest objects in the universe. Globulars vary in distance and size, the northern hemispheres best globular cluster M13 in Hercules is 25,000 light years distant and is about 120 light years across containing about 30,000 stars. In comparison NGC2419 in Lynx is called the intergalactic wanderer and is 200,000 light years away.
Observers in the southern hemisphere have the finest example of a globular cluster in the entire sky, omega Centauri lying about 17,000 light years away it is one of the closest objects of it's class to the Earth, also one of the largest with a diameter of at least 150 light years and containing over a million stars.
Some globular clusters are visible with the unaided eye if you know where to look resembling faint slightly fuzzy stars and binoculars will show many of the brighter objects but only as nebulous discs; they will not reveal any individual stars. What you see in a telescope is depends what aperture you are using, a 4 inch telescope will just begin to resolve the extreme outer areas of the bigger, brighter globulars such as M13 but more compact objects will not be resolved. Larger telescopes will resolve many globulars to the core giving an almost unreal three dimensional appearance.
Nebulae are regions of gas, mostly hydrogen and dust that populate most of our galaxy and range from a few light years to hundreds of light years in diameter.
Emission nebulae (diffuse) are huge clouds of highly tenons gas, chiefly hydrogen that shines by its own light. Ultraviolet radiation from nearby hot stars causes the gas in the nebula to become ionised and thus shine. There are over 400 diffuse nebulae in the sky and a few are visible to the unaided eye and many are visible in binoculars and under good conditions a telescope will reveal in the brighter examples a wealth of intricate detail. The best knows diffuse nebula is M42 in the constellation Orion, 1600 light distant and visible with the unaided eye as a fuzzy star in the sword of Orion.
Dark nebulae appear as areas of the sky which contain no or few stars, the dust they contain absorbs light dimming or blotting out anything that is behind the nebula so these objects are more noticeable if they are situated in front of a bright source such as a milky way star field or a bright nebula. Not all dark nebulae are equally dark; they are assigned a value on a scale of 1-6 to indicate their opacity, 6 being the most opaque. This type of nebula can have any shape ranging from long elongated areas such as the great rift in the Cygnus region of the Milky Way or the more compact coal sack in the southern constellation Crux to the small but distinctive Horse head nebula in Orion.
Reflection nebulae are clouds of dust that reflect light from a nearby star, this happens if the nebula contains dust consisting of particles the size of cigarette smoke or if the star is not sufficiently hot enough to ionise the gas and make it emit light. Reflection nebulae are more difficult to observe than other types of nebulae.
Planetary nebulae were first described by William Herschel in 1782 , he named them for their unique appearance through a telescope that reminded him of the planet Uranus but the term m planetary nebula is misleading as these objects have nothing to do with planets as they consist of a hot star with a temperature of 100,000 degrees Kelvin surrounded by a shell of ionised gas resulting from the ejection of the stars outer atmosphere in the latter stages of its life. Planetary nebulae seen through a telescope vary in shape and size, they can appear as nebulous discs; some have an hourglass shape like the Dumbbell nebula in Vulpecula while others have a smoke ring shape like the ring nebula in Lyra and some appear simply as a star like point of light.
Supernova remnants (SNR) are the remains of massive stars that end their life by blowing themselves apart in a supernova. The remains of the star are scattered throughout the surrounding region forming delicate strands of nebulosity that gradually disperse into space. The most famous SNR is the Crab nebula in Taurus witnessed by Chinese astronomers in the year 1054AD. Most objects of this type are too faint for binoculars but telescopes of sufficient aperture often reveal beautiful wispy ghost-like structures especially if a nebula filter is used.
Galaxies are the most numerous deep sky objects and well over a thousand of these fascinating objects listed in the NGC an IC catalogues are brighter than 13th magnitude, within the reach of a modest backyard telescope but the vast majority appear as misty patches of light and their often low surface brightness makes their internal structure difficult to pick out unless using a large aperture.
A galaxy's surface brightness is derived by dividing its magnitude by its area and is a more reliable indication of its visibility with a given aperture. The given magnitude of a galaxy in a typical deep sky catalogue is usually it's integrated magnitude which is the magnitude of the object if it were compressed into a star like point of light but nearly all galaxies in a telescope appear as extended objects which means their light is spread out over a larger area of sky. This means they will have a lower surface brightness and will appear to the observer much fainter than their integrated magnitudes suggest.
Elliptical galaxies as their name suggests are symmetrical in shape and differ from other galaxies in having practically no interstellar gas and dust and are dominated by old giant stars.
The best elliptical galaxy for small apertures is M87 at the heart off the Virgo super cluster of galaxies in Virgo, this is one of the largest galaxies known with an estimated mass of several trillion suns and is several times as massive as an average galaxy.
Spiral galaxies are systems whose stars are arranged in a spiral pattern and come in two varieties: regular spirals and barred spirals. Regular spirals have arms that extend from a central nucleus, they vary in that some have tightly would arms while others have a looser spiral structure. In barred spirals the arms extend from a central bar straddling the nucleus.
Irregular galaxies have no symmetry and appear as structureless, often uniform, areas of light in a telescope.
Many of the stars in the night sky are either double or multiple and even a small telescope is capable of resolving hundreds of these often beautifully coloured combinations. There are two types of double stars; optical and physical, optical doubles have no association with each other; they lie at different distances yet because they lie on much the same line of sight as sen from the Earth they appear to be close together. Physical doubles are binary stars - two stars gravitationally bound together orbiting each other in a period of months in some systems while others may take tens, hundreds or even thousands of years to complete a single orbit. A binary system with more than two members is called a multiple star.