A beginners guide to astronomy

This section is filled with all sorts of information that will be useful to beginners.
from what type of equipment to buy, right through to how to use it.

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Starting out.
First, here is a beginner's guide to choosing binoculars or telescope and types of observing.

An interest in astronomy starts in many ways. Our eyes (if we are fortunate to have a good pair) will see in a suburban sky roughly 1500 stars. This increases to about 5000 in a rural area away from lights. See the Philips Dark Skies map for more information on finding a darker location. We can increase our star count with a pair of binoculars; you may have a pair. They are indispensable, easy to carry around and will show you a wide area of sky (about 7degrees) in more detail; revealing many interesting objects like nebulae, star clusters, comets and the moons of Jupiter.

Dark adaptation.
This is the process of allowing your eyes to become accustomed to the dark. Your pupils open up to allow more light in. It is important to be dark adapted when viewing the night sky whether it be with the naked eye, binoculars or a telescope. The exception is the Moon. This is already bright and seeing it in a telescope will often spoil your dark adaptation. Not to worry though, your dark adaptation will return after10 minutes, or you can buy filters to cut down the light if its a full moon for instance. Full dark adaptation may take 20 or 30 minutes.

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Averted vision
The edges of the eye's retina are more sensitive to light and movement, but not to colour. When one looks away, or to the side, a star can appear brighter. This method allows faint stars to become visible when they were not seen by direct vision. Astronomers often look around the field of view to pick up faint stars or objects at the threshold of visibility. There is a good summary in this power point pdf

Choosing Binoculars
You will see binoculars reviewed in magazines and on the internet. Our Links page has a wide range of suppliers, many of whom sell binoculars. Ask the supplier what he/she recommends or try a pair at one of our observing evenings. Sky and Telescope has a article on the subject.

Apart from optical and build quality, some other features should be understood. Binoculars are specified as 7 x 50 for instance (My pair is 8x40). The first number is the magnification, the second is the diameter of the front lens. (Called the "objective" or "object glass", or just "OG" for short ). The larger the better and usually more expensive. Larger also means higher magnification, smaller field area and greater physical size and weight. Small size was one of the best attributes of binoculars. However, don't buy binoculars with an OG less than 40mm. Typical sizes for night use are 8x40, 7x50, 10x50. Sizes greater than 10x50 will need to be supported by a tripod or other device for optimum use.

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Mounting Binoculars
The view through any pair of binoculars will be improved if supported to minimise shake. A tripod mounting bracket is a common solution. Improvement can be made by using a parallelogram mount such as on this link by Graham Wood, or maybe purchased. Some other examples of binocular mounts here.

Exit pupil
The exit pupil (measured in mm ) is sometimes given in the specification for binoculars. This is simply OG size divided by magnification and usually does not exceed 7mm and is typically 4 or 5mm. The exit pupil should be matched to your own dark adapted pupil size. The maximum size of the dark adapted pupil decreases with the age of the observe. So 10x50 or 15x80 are fine for most people. Some wonderfully views of the Milky Way can be had with say 8 x 50s from the darkness of mid-Wales, moors and National parks.

Scale in the Sky
Your hand held at arm length is a good way to estimate angles on the sky. The diagram below illustrates how to judge the angle between stars in the Plough (Great Bear, Ursa Major) and hence any other area of the sky. The width of a finger tip is about 1 degree and should cover the Moon completely - try it.

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The sky is measured in degrees. A full circle is 360 degrees. From horizon to zenith ( strait up) is 90 degrees. Your binoculars show only a 7 degree portion roughly, but it is useful to know more exactly because it will depend on the magnification you have.

Some scale markers can be found in the sky:

For Moon 0.5 degrees (deg), Andromeda galaxy 1deg, Orion's belt 2.5 deg, Caster to Polux 4.5 deg, the pointer stars in the Great Bear 5.5 deg, the sides of the Great Square of Pegasus 14 to16 deg.


Identifying what you see.
Sweeping the sky with binoculars on a warm August night is probably one of the delights of owning a pair of binoculars or a wide field low power telescope. How would you identify a misty blob or cluster?

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Well you really need to know the basic star patterns in relation to a map, and the only way to do this is take an atlas with you and a red torch to read it by. (Red light does not effect dark adaptation). First keep your head pointing in the same direction of the object but lower the binoculars so that you can see the whole area of sky. Raise the binoculars again to confirm the location of the object. Go to the star atlas and identify the area and the object or sketch the star pattern in a log book.

Stars maps and log books
A log book records important information which you might otherwise forget: Date, Time, Location, Instrument, field of view, direction in the sky, description, weather conditions.

A planisphere (eg Philips) enables you dial in the exact view of the sky and shows all the constellations above your horizon and their direction. This link contains a make your own planisphere. If the link doesn't work, there are copied here: bottom - top. For binocular users, a book devoted to the subject with star maps would be recommended. There are other options e.g. Norton's Star Atlas, Collins wild guide "Night Sky" and many others.#

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Estimating star brightness
How bright was it? I asked, "As bright as the Moon" was the answer. The was the description of a Leonid meteor which exploded on the night of November 16/17 1998. A meteor brighter or as bright as Venus is called a "Fireball" and much brighter events could be described as a "bolide". The SPA describes these terms here. Apart from the Sun and Moon, the planets Venus, Jupiter and Mars rank as the brightest objects - oh and of course the occasional "Great Comet" such as West (1976) Hale-Bopp (1997) and 2006P1 McNaught (2007) which was recently judged "as bright as Jupiter".

The magnitude scale
At the other end of the scale are stars that are just visible to the naked eye. Hipparchus in 120 BC put the brightness of stars into six groups, 1 to 6. (One being the brightest). However the notion of "1st magnitude stars" also includes stars like Sirius, which on the modern scale (Pogson) is magnitude -1.5. But this star is exceptional and all the brightest stars fall into the range -0.7 to +1.6. So including Venus the naked eye range of brightness is -4 to +6. In practical terms it useful to remember that a difference of 5 magnitudes is a difference of 100 in brightness, and that one magnitude is a 2.5 x difference. When estimating the brightness of objects, it usual to compare with a nearby star, and then go and find out the magnitude of the star from a chart, book, or sky software. Here is a short list of information.

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Moving on to telescopes
The move from binocular to telescope is a leap in performance. A history of the telescope can be found here. With a telescope comes many advantages: 1) Higher magnification to see planets as disks and detail on the disk, 2) greater light gathering ability - you can see fainter objects more easily like the Orion Nebula, the dumbbell and crab nebulae, stars in the Hercules cluster, and external galaxies like the Whirlpool. Telescopes also come in a variety of "configurations" to match your interests. So what about choosing a telescope ?

Choosing a telescope
Like binoculars, there are factors to consider before a purchase. If you join an astronomy group there will be opportunities to look through and use different types of telescope at no cost ! Here is a link to some of the terms used. Amateurs still make their own telescopes, and there are groups who specialise in this activity. Telescopes fall into catagories:

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Types of telescope
1) Glass element at the front (Objective): Refractor (Achromat and Apochromat(APO))
2) Glass and mirror (Catadioptric) : Cassegrain, Maksutov, Schmidt Newtonian,
Ritchey Chretien
3) Mirrors only: Newtonian refelector.

4) Mirrors with relay lenses: Some short tube Newtonians
5) Digital SLR on driven EQ mount, and eyepiece for the lens.

Mounts for telescopes
Telescopes are mounted and have controls to follow the stars. The possibilities are:

1) Alt-azimuth (e.g. a fork mount )
2) Equatorial ( e.g. German or "GEM" )

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The mounts themselves can have slow motion control to follow the stars. Examples are

1) Manual tracking only.
2) Sidereal drive motor (one or both axis)
3) Motors with speed control via a hand controller or computer (eg GoTo)

Choosing a telescope is dictated by your needs and circumstance and in what order you place the following :

1) Very portable, 2) Use - visual, 3) Use - photographic, 4) Deep sky objects (galaxies), 5) Moon, planets, clusters, bright nebulae, 6) Wide fields of view (2-3degrees), 7) EQmount-GOTO, 8) Alt-Az GOTO, 9) Low budget (less than £500 ish),

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Telescope guide.
This is not an exhaustive list.
Please note that mounts don't have to be of goto type if you buy the optics (OTA) and mount separately. Some instruments come with goto build in.

Instrument type
3-4" refractor goto
3-4" Achromat
6-10" Cassegrain
2+3+4+5+7 or 8
6" F/5 Newtonian
8-10" Newtonian
8 -10" F/4 Schmidt Newtonian
6-10" Dobsonian


DSLR + lens+ small driven GEM

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Tools of the trade
A well prepared amateur astronomer has accessories that wont be supplied with a telescope. Here are some of them:

The eyepiece magnifies the detail in the image formed by the lens or mirror. It's likely you may want to buy additional eyepieces at some point. The magnification is controlled by the focal length of the eyepiece: M = F/ f, where F= focal length of the telescope, f = focal length of the eyepiece, M is the magnification.

Telescopes are supplied with at least one eyepiece. There is an extensive range of eyepiece types and suppliers. They are offered in two draw-tube sizes: 1.25" and 2". New telescopes offer both fittings. Older (second-hand) telescopes may be 1.25" (31.7mm) only. The focuser and draw tube on older Newtonian reflectors can often be upgraded, but in most circumstances the 1.25" fitting is adequate for the beginner. Modern eyepiece designs types are:

TYPES: Plossl, Wide Angle, Ultra Wide Angle, Konig, Radian, Panoptic, Nagler.
MAKES: Teleview, Meade, Celestron, and a number of others.

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Barlow lens (2x, 3x)
A Barlow is used to increase the focal length of the telescope and increase the magnification. Useful if you have a few eyepieces and want to extend their magnification range. Barlows are also used to project larger images onto web cams. The results obtained on the planets and the Moon can be stunning. An alternative to the Barlow is a Powermate (x5)

Finder for the main telescope
An astronomical telescope has a finder which has been adjusted so that the cross hair indicates where the main telescope is pointing. Some finders are better than others. This picture shows a finder which most people would be pleased to have as their main telescope ! The finder on an amateur's telescope can be a simple device. This include 1) Red Dot Finder (using), 2) Telrad (using)

Red Torch
A torch with adjustable brightness LEDs (red and blue) is useful for reading maps.

Warm clothing for Winter.
Thermal under-ware, socks and gloves. Ski boots, overcoat, hoody.

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Filters can reduce light pollution and enhance contrast on faint objects. They can be used on the Moon to gut down glare, and coloured filters will bring out detail on planet, or reduce colour fringing.


Maidenhead AS document archive:

Bode's Law and the discovery of the asteroid

Earth's size, shape and mass

Nebula filters

Eyepiece projection calculation

150th Anniversary of Neptune's discovery 1846

Constellation Notes


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Last updated January 20th, 2007
by TimH


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