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Types of Telescopic Mount
To a large extent, a telescope is only as good as its tripod and mounting. A telescope is
used to magnify the sky, but unfortunately it also magnifies vibrations.
A telescope mount has two primary functions:
1. Support the telescope firmly so that objects can be viewed and photographed
without vibrations
2. Provide a system for smooth controlled movement to point and guide the instrument
There are two major types of mounts for astronomical telescopes: Altazimuth and
Equatorial.
Altazimuth Mount
Altazimuth (sometimes called alt-az) is the simplest type of mount with two motions,
altitude (vertical) and azimuth (horizontal): thus the name Altazimuth. Good Altazimuth
mounts have slow-motion knobs for making precise adjustments, aiding smooth tracking
across the sky. These type mounts are generally good for terrestrial observing and for
scanning the sky at lower power but not for deep sky photography. Some Altazimuth
mounts are now computer driven and allow a telescope to track the sky more accurately.
This is generally good for visual use but can lose tracking on longer exposure
astrophotography.
In addition to a standard Altazimuth, mounted on the top of a tripod, there is also the
Dobsonian Mount.
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Dobsonian Mount
The Dobsonian mount is a newer, modified version of the Altazimuth mount. This mount
was invented in the 1970's by John Dobsonian. Dobsonian mounts are mounted on the
ground by a heavy platform, and designed to support massively sized Newtonian
Reflectors, while keeping a steady image. It is common for Dobsonian telescopes to have
very large apertures - anywhere between 6 and 20+ inches!
Equatorial Mount
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Equatorial mounts are superior to non-computerized Altazimuth mounts for astronomical
observing over long periods of time and absolutely necessary for astrophotography. As the
earth rotates around its axis, the stationary stars appear to move across the sky. If you are
observing them using an Altazimuth mount, they will quickly float out of view in both axes.
A telescope on a properly aligned equatorial mount can be aimed at a celestial object and
easily guided either by either manual slow-motion controls or by an electric motor.
Aligning an Equatorial Mount
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Telescope Types
Refractor
The eyepiece ('e') is fitted into the end of the drawtube 'd'. Usually the end of the drawtube
has a small clamp-screw to hold the eyepiece safely in place.
The drawtube should protrude about 150mm beyond the end of the main tube ('m') of the
telescope. The entire drawtube needs to be about 230mm (9inches) long, because the
plug 'p' needs to be about 75mm long. The drawtube needs to be a neat, sliding fit inside
the plug 'p'. You can line the plug with baize to help the drawtube glide smoothly.
You can cut a suitable piece of baize from the middle of a friend's billiard table. You may
wish to provide the plug with a clamp screw also, so you can hold the draw-tube in a fixed
position once you have focussed the telescope by sliding the draw tube to the position that
gives the best image, seen through the eyepiece.
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Reflectors
The second type of telescope, the reflector, uses (as its name suggests) mirrors to gather
and focus light from the object under scrutiny. In its most commonly encountered form the Newtonian, which has been around for in excess of three centuries - there's a
specially-curved concave (dish-shaped) primary mirror at the bottom end of the telescope.
Near the top of the tube, a small inclined secondary mirror directs the light reflected from
the primary to the side of the tube where it's met by a conveniently-placed eyepiece. If you
want the largest aperture for your money, then the reflector is unquestionably the scope for
you. When well made and maintained they can provide sharp, contrasty images of all
manner of celestial objects at a small fraction of the cost of an equal aperture refractor.
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Catadioptrics
Then there's the third category of telescope, the catadioptric, or compound telescope as
they are occasionally referred. These came about in the 1930s out of a desire to marry the
best characteristics of refractors and reflectors. This is why they employ lenses and
mirrors to form an image. The greatest appeal of these instruments is that in their
commonly-encountered forms (the Schmidt-Cassegrain and Maksutov-Cassegrain) they
are very compact - their tube lengths are two to three times the aperture of the scope due
to the 'optical folding' of the light passing through them. The smaller tubes dictate more
manageable (and consequently lighter) mounts and tripods. The practical upshot is that
you can obtain a large aperture and a long-focus telescope that's very transportable.
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Advice on buying and using binoculars
The Pleiades Cluster - best seen in Binoculars.
Why use Binoculars?
Put in a nutshell, they are a wonderful aid to learning the night sky and may well give you
the best view possible of some of the night sky wonders such as the Andromeda Nebula
and Comets such as Hale Bopp. The fact that both your eyes are used can be a real asset
too. They allow you to sweep large areas of sky and observe objects, such as the Pleiades
Cluster in Taurus that are simply too large to be seen in the field of view of most
telescopes.
How do binoculars differ?
There is a bewildering array of binoculars to choose from. Let’s first discuss the
specification that defines the characteristics of a pair of binoculars and then point out the
ideal specification for some of the different uses to which they are put. You can skip this if
you like and drop down to the section headed "What’s best for my requirements?"
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For astronomical use it is useful to have a tripod mount
Magnification
This is the first number given in the basic specification of a pair of binoculars. Typical
numbers are 8 and 10. One might think that the bigger the magnification the better - this is
generally not the case. The larger the magnification the smaller the field of view (see
below) but, perhaps even more important, the more the image will appear to jump about.
Unless the binoculars are to be mounted on a tripod or are of the new image stabilising
type, a magnification greater than 10 is not to be recommended.
Objective Size
This is simply the diameter of the objective (front) lens measured in mm and is the second
number in the specification - for example a pair of 8 x 40 binoculars has a magnification of
8 times and an objective diameter of 40mm. The larger the diameter is, the more light that
is collected and so, in principle, the dimmer the objects that can be seen - but see also the
section about "Exit Pupil". Thus large objective lenses will be an asset for astronomical
use, but as a consequence the size and weight of the binoculars will increase. The most
common objective sizes are 20, 35-40 and 50 mm. The light collected increases as the
square of the diameter, so a pair of 50mm binoculars will collect over 6 times as much
light as a 20mm pair.
Field of view
This depends on firstly the magnification - the larger the magnification the smaller the field
of view, and secondly the design of the eyepiece. In the specification it may say "wide
field" or "extra wide field". This implies that, for a given magnification, you will see a larger
field of view. The field of view depends on the complexity - and hence cost - of the
eyepieces used in the binoculars. A simple eyepiece may have a field of view of 5 degrees
whilst a very complex one might have a field of view of up to 9 degrees across. In the latter
case you will actually see over 3 times more sky at one time! The field of view is often
given, as for example, 135m at 1000m. The larger the first number, the greater the field of
view. For almost all uses, including astronomical, it’s better to go for a wide field of view,
but do expect to pay a little more.
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Eye Relief
This is almost never discussed, but can be important if, as many people, you wear
glasses. With many binoculars, if you have your glasses on, even if the eye caps are
folded back you will find that you cannot see the whole of the nominal field of view.
Obviously you can take your glasses off, but that can be a nuisance and, if you suffer from
significant astigmatism, may not give you as good an image as you could see with the help
of your glasses. The solution is to get binoculars with good eye-relief. This tends to be the
case with more expensive binoculars - often you will need to try the binoculars out as it is
often not mentioned in the specifications. You know when you are seeing the whole field of
view when the edge of the field is a sharp transition from image to black.
Lens coatings
The size of the objective determines how much light enters the binoculars. Not all will
leave it. Some will be reflected at the various glass/air interfaces in the binoculars - prisms
as well as lenses. This light is scattered and will reduce the contrast of the image as the
darker parts of the image become apparently lighter. Good binoculars will have lenses and
prisms which are multi-coated to minimise reflections thus reducing the scattered light and
giving much better images. Naturally this makes them cost more.
Exit Pupil
This is the diameter of the "cone" (more like a cylinder) of light that leaves the eyepiece
and enters the eye. Its diameter is given by simply dividing the size of the objective by the
magnification. Let’s take two examples: pairs of 8 x 20 and 8 x 40 binoculars. The first will
have an exit diameter of 20mm/8 = 2.5mm and the second 40mm/8 = 5mm. The light that
can enter the eye is determined by the iris diameter which in daytime is about 2.5mm
diameter - so all the light from the 8 x 20 pair will enter it but only a quarter of the light
collected by the 8 x 40 will be able to enter it - there is thus no point is using large
diameter objectives in broad daylight! A lightweight pair of 8 x 20 binoculars cannot
really be beaten. When dark, the iris opens up to allow in more light into the eye and
reaches about 5mm in diameter. Then all the light from the 8 x 40 (or 10 x 50) binoculars
can enter the eye. So for astronomical use it is worth having a pair of binoculars with
larger objectives.
Prisms
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Porro and Roof Prisms
In the conventional design of binoculars, Porro prisms are used. These can be made of
different glass types. The BAK-4 glass prisms give better performance than the cheaper
BAK-7 types. More recently, roof prisms have begun to be used to give very compact
designs, but to work well they must be manufactured with very high precision - hence good
quality binoculars using this prism type are expensive.
Astronomy
A large objective is obviously an advantage so 8 x 40 or 10 x 50 is ideal. A wide field of
view is important so that you see more sky. If you want to wear glasses when observing
then they should provide good eye relief. Top quality pairs will cost over 500 pounds. The
galaxy M31 is another astronomical object that can best be seen with binoculars. 20 x 70
and/or 20 x 80 binoculars are also available for better light collection purposes.
M31 - The Andromeda Nebula
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Things to See
The Milky Way
Constellations
Orion – The Hunter
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Horse shoe nebula, Great Nebula M42, Witch Head Nebula
North Star Polaris
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Ursa major and minor, including polaris
Casseopia
Taurus and Pleiades
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Pleiades
The Moon
Apollo 11 – first moon landing
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Planets
Saturn
Jupiter
Mars
Pluto - Hubble Space Telescope
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Pluto’s Surface from Nasa probe - A Nasa probe has sent back photographs of what
appear to be trees on the planet's surface. But it is an optical illusion - they are sand dunes
coated with a thin layer of frozen CO2. The "trees" are trails of debris caused by landslides
as ice melts
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Isle of white: this image taken by NASA's Terra satellite shows the extent of ice and snow
covering Britain this last winter 2009/2010
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Earth
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