Galileo invented the telescope in 1609. His telescope was a crude refractor, giving around 20x magnification, smaller and optically far inferior to even children's telescopes of today, yet the views revealed through it were as wondrous to him as those of the Hubble Space Telescope are to us today, nearly 400 years later. Maybe even more so!
A telescope allows you to explore the skies and see well beyond the capacity of the human eye. Even a pair binoculars can give great moon views.
Choosing a Telescope
This guide is intended to help you understand the basics and get you to the point where you are able to choose your telescope.
A telescope is an optical instrument which brings distant objects closer for examination. It usually consists of a tube containing the optics, and a mounting with a tripod to point at its target.
There are two applications for a telescope: spotting terrestrial objects during the day and studying astronomical objects at night. Although any telescope may be used for either, there are certain features desirable for each.
Image Orientation. (Everything is upside-down)
Astronomical telescopes do not require upright views as there is no 'Up' or 'Down' in space. this is normal for astronomical use, however, Terrestrial telescopes require corrected upright images.
Intended Use and Portability
What will the telescope be used for? Astronomical observing? Terrestrial observing? Both?
take ease of use into consideration.
There is little point having a large telescope if it is too hard (or heavy) to use. The right telescope for you is the one you would use and enjoy.
You must also consider where your telescope will be used.
light pollution will make full use of a larger telescope difficult whereas in a good dark sky location you can reach further than a small telescope will allow. If you'll be travelling to dark sky locations then ease of transport will become an important factor.
A telescope can be transported in 2 main parts - telescope tube and mount. Loosen the thumbscrews on the tube rings and remove the telescope tube from the mount. Remove the accessories (finderscope and bracket, and the eyepiece) from the optical tube. Cover the telescope tube and the eyepiece with their caps. Remove the fine-adjustment control cables and counterweights. The accessory tray should be removed in order to transport with the 3 tripod legs closed.
A telescope can easily be transported in a car but mirrors may require collimation after transportation.
Refractor telescopes have the lens on one end and the eye piece at the other
They have a slight advantage in clarity when viewing planets in our Solar system
refracting telescopes require little or no maintenance because lenses are set in the factory and rarely, if ever, need resetting
Refractor telescopes produce an image that is upside down however a 'Prism
' may be used to give upright, mirror image views.
There are also 'Erect Image' Prism diagonals which will correct the 'mirror image' for land use.
|Mirror reversed image seen through a refractor|
|Sharp Images||Chromatic aberation is visible in cheaper models|
|Good in Light Polluted Areas||Large apertures are expensive|
|Can be used for Terrestrial viewing as well as Astronomy|
The main lens is actually a curved mirror which collects the light and is then reflected into the focuser.
Reflectors can be knocked out of alignment - although this does not happen very often and it only takes a bit of practice to reset them.
A reflector is by far the cheaper option. This is because a large mirror is easier to make than a large glass lens
Reflectors are good for viewing planets.
Newtonian Reflectors produce an image that is upside down and there are no way to correct this. They are not recommended for land use.
|Inverted (upside down) image seen through a reflector|
|No Chromatic aberation||Some loss of contrast so images are not as sharp|
|Less bulky than a refractor of the same aperture||Need to be maintained and collimated (mirrors aligned)|
|Most aperture for your money (especially Dobsonians)|
DobsonianThe term " Dobsonian " or "Dob" refers to any telescope with an alt-azimuth mount (lazy susan) and a Newtonian telescope tube assembly. With large aperture and fast focal ratio make these telescopes ideal for deep sky objects.
The Catadioptric telescope uses compound optical systems. Mirrors and lenses collect the light. The eye piece is at the end.
Maksutov-Cassegrain, Klevtsov-Cassegrain and Schmidt-Cassegrain are all catadioptric telescopes They provide high powers and a narrower field of view. This makes them unsuitable for wide-field imaging but superb at lunar and planetary imaging and astrophotography , as the images viewed through them are very clear. They are also very good at imaging tightly packed formations such as globular clusters and at splitting double stars.
Cassegrain telescopes produce an image that is upside down however a 'Prism Diagonal' may be used to give upright, mirror image views.
There are also 'Erect Image' Prism diagonals which will correct the 'mirror image' for land use.
|Mirror reversed image seen through a Cassegrain|
|Compact and portable||Some loss of image brightness|
|Can be computer driven for easy navigation||can be prone to dew formation which hinders viewing|
|Less expensive than equivalent refractor||more expensive than equivalent reflector|
Any telescope will be rendered useless unless it's attached to some form of stable mount that permits it to be directed to any desired part of the sky and to follow a celestial object smoothly and precisely.
There are two common methods:-
A Alt-azimuth mount allows movement left and right using slow motion controls across the sky (azimuth), and the raising and lowering of the barrel (altitude).
With this mount the telescope has to be moved in two directions, simultaneously, to follow the stars as they move across the sky in an arc.
Go To Mount
The most recent generation of telescopes use electric motors on an alt-azimuth mount. Attached to these is a computerised handset. The telescope then not only tracks an object with a simpler mount, but it can also be made to 'Go-To' a planet or constellation.
Equatorial mounts can be lined up with the Earth's axis, as the earth spins on this axis, an opposite spin on the telescope will keep a planet or constellation in continuous view and greatly helps the tracking of all celestial objects simply by turning a single control knob or cable.
It's even easier when an internal battery-powered motor drive is attached to the mount as the telescope will track the planets automatically.A motor drive is necessary for many types of astrophotography , but it is more than just a convenience for visual observation as well. At 200x magnification, the Earth's rotation will move an object out of your field of view in about two seconds. A Right Ascension motor drive will keep an object in the centre of the field where the image is the best without producing the objectionable vibrations experienced with manual tracking. Adding a Declination motor drive and a hand controller allows you to guide for astrophotography.
A telescope has three types of power and they are measured against the performance of a normal human eye. They are magnifying power, light gathering power and resolving power. All three are important but the most important is resolving power. The longer the focal length of a telescope, the more a particular eyepiece will magnify the image.
Since the astronomical telescope is used at night to observe faint objects, it is advantageous to have a larger diameter objective to ensure greater light transmission. The objective can be a lens (refractor) or a mirror (reflector) or a combination of both (catadioptric). The larger the objective is the greater the brightness and resolution will be, so telescopes with larger objectives can be used at higher magnifications usually with much better results.
Be wary of telescopes advertised by magnification alone. Magnifying power is NOT the most important consideration when choosing a telescope
Choose a telescope with a large aperture. A larger aperture allows in more light, and allows you to see more and further.
A 6" reflector is the smallest really useful, but a 4" reflector will be perfectly adequate for observing the moon and in good conditions you'll be able to see a ring around Saturn, maybe the red disk on Mars and a band of colour around Jupiter.
For a first telescope, we would suggest a refractor of 60-90mm aperture, or a reflector of 6" aperture, unless you are really serious. After you've learned the basics of observing and developed an appreciation for the hobby, then you can move up to a bigger telescope.
An intermediate-level observer, who wishes to purchase one telescope for a lifetime, should consider an 8" reflector telescope.
If you're after the most aperture for your money then the Dobsonian type telescopes offer large apertures at the lowest prices. They are mounted on simple turntable mounts and can produce highly magnified images with good clarity.
A useful guide on magnification is to think of the maximum magnification as
being twice the aperture size in millimetres (i.e. a 100mm telescope can
magnify up to 200x however the brightest and sharpest images are obtained at 25x to 50x)
Magnifications above 300X will only be usable under excellent atmospheric conditions.
The distance from the lens or mirror to the point where the image comes to focus is called its focal length and a telescope's focal ratio (f-ratio) is the focal length divided by the diameter of the main lens or mirror.
An f-ratio of 6 and lower would be considered fast
An f-ratio of 4 or 5 is preferred by amateurs observing deep sky objects such as galaxies and nebulae as the observing field is wide and bright.
An f-ratio of 11 and up would be considered slow, but ideal for planetary work or studying binaries.
Planetary observers prefer long focal lengths, with corresponding narrower fields of view; this allows more detail on viewing solar systems.
Eyepieces have different focal lengths and change the magnifying power of any telescope. The shorter the focal length of an eyepiece, the higher the magnification. The magnifying abilities are dependent upon the objective lens or mirror. So an eyepiece that provides a 20X Magnification on one telescope, may provide a 60X Magnification on a different telescope. To calculate the magnifying power of a telescope - divide the focal length of the objective lens or mirror by the focal length of the eyepiece.
Magnification=Focal length of the Objective/Focal length of the Eyepiece
use the same measuring units (Don't divide centimeters by inches). eyepieces are generally sold by their focal length in millimeters while telescopes are often sold by their focal lengths in centimeters or inches
types of eyepiece.
For standard eyepieces with a 50 degree apparent field of view, Plossl eyepieces are a good choice if your telescope's focal ratio is f7 or less. For longer focal ratio instruments, simpler eyepieces such as Kellners will be sufficient.
A 2x Barlow will double the magnification of the eyepiece. For example, a 20mm eyepiece on a telescope with a 100mm focal length would give a 50x magnification. attaching a 2x Barlow lens to that eyepiece will double the effective magnification of that eyepiece to 100x.
So using a barlow lens with a 20mm eyepiece will be the same as using a 10mm eyepiece.
When choosing a Barlow lens it is important that the barrel diameter size fits the eyepiece. The barrel size is the diameter of the eyepiece tube that fits into the focuser. The standard eyepiece barrel size is 1-1/4 (1.25) Inches. Some eyepieces use the larger 2-inch format and some inexpensive telescopes use the smaller 0.965" format. It is important that the Barlow lens has the same barrel size as the eyepiece it connects to.
Barlow lenses are offered in different magnifications. The most common is 2x. This means it will double the magnification of any eyepiece it is connected to. There are also 3x or higher Barlow lenses
A Barlow lens is simple to use, first drop in the Barlow lens into the focuser, then the eyepiece will connect to the Barlow lens.
The greater the diameter of your telescope the more magnification you can achieve, go for the largest aperture your budget allows if you want to see as much as possible. And try to buy a telescope you won't quickly outgrow.
Inch for inch, a reflector is less effective than a refractor, but it is also cheaper.
Remember that there is more to a telescope than the tube assembly, mount, and tripod. so save some of your budget for additional eyepieces to expand the telescope's magnification and filters for enhancing nebulae and planetary images.
Colour filters, which usually thread into the eyepiece barrel, are almost a necessity for viewing planetary detail. By using an appropriate colour, you can highlight a specific planetary feature. This often allows you see two to three times as much detail as in an unfiltered view.
It is unnecessary to separate the optical tube and the mount when storing the telescope. It can be stored in one unit in a clean, dry, and dust-free environment. If it has to be stored outdoors, cover it with a heavy-duty plastic cover to prevent it from getting wet. Make sure that the dust cap for the front of the telescope and the cover for the rear opening are on. Accessories should be stored separately in a box, with all their caps on. Some people do store the reflecting telescope in two parts, leaving the telescope tube up side down on the ground to prevent dust from settling down on the primary mirror.
Jupiter is a shimmering disk.
Astronomers must be patient; you must optimize your observing site and times, as well as your equipment. When you observe the Moon and the planets, and they appear as though water is running over them, you probably have bad "seeing" because you are observing through turbulent air. Always observe objects as high in the sky as possible. Don't observe immediately after sunset and avoid viewing across heat-radiating ground objects such as buildings and parking lots. Let your telescope come to temperature with the surrounding air; sometimes the shimmering is due to "tube currents" within the telescope tube. Try to enhance planetary detail by using colour filters. Optimize all that you can then be patient because good seeing comes and goes. All those beautiful highly processed posters are pictures from the Hubble Telescope. We see beginners who think they can reproduce those images by spending a few pounds. Be realistic in your expectations.
Permanent eye damage can result from looking anywhere near the sun directly, or through a camera viewfinder, or with binoculars or a telescope. Under some circumstances even the moon can damage your eyes without the right equipment and knowledge of how to use it.
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