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.
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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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Control
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 |
Characteristics |
3-4"Maksutov |
1+2+5+8+9 |
3-4" refractor
goto |
2+5+6+8+9 |
3-4"
Achromat |
1+2+5+6+9 |
3-4"APO |
1+2+3+5+6 |
6-10"
Cassegrain |
2+3+4+5+7 or 8 |
6"
F/5 Newtonian |
2+3+4+5+6+7+9 |
8-10"
Newtonian |
2+3+4+5+7 |
8
-10" F/4 Schmidt Newtonian |
2+3+4+5+7 |
6-10"
Dobsonian |
2+4+5+6+9 |
DSLR
+ lens+ small driven GEM |
1+3+4+6 |
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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:
Eyepieces
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
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