This article featured in the February 2004 Beginners Magazine


A Dobsonian is a type of telescope designed by an American named John Dobson. It is generally based on a Newtonian reflecting telescope tube supported on a very simple mounting. Shown below is the layout of the optics of a Newtonian tube assembly. Light from a distant object enters the open tube and is reflected back up the tube by a parabolic (concave) mirror. Because the mirror is curved the light focused into a point where an image is formed. To enable the observer to study the image without blocking the light entering the tube, a second ‘flat' mirror, is mounted at the top of the tube to direct the light out through a hole in the tube. A focusing unit is fitted to the hole to hold and adjust the eyepiece. The eyepiece is a simple microscope used to magnify the image formed by the mirror. Using a variety of eyepieces with different focal lengths, higher or lower magnifications can be obtained.


The Skyliner shown above is a typical 150mm (6 inch) shop bought Dobsonian telescope retailing at about £230. It has a Nylon disc mount on either side of the tube that sit in Nylon cups to provide a trunnion type bearing. This allows the tube to be aimed up and down. The cups are fitted to the tops of two supporting ‘Cheek' panels. These panels are mounted on to a turntable that allows the telescope to be rotated through a full circle. The telescope can therefore be aimed at any point in the sky. Drives are not normally fitted to this type of telescope. To help the observer to find and object a smaller telescope is fitted to the tube of main telescope this is called a Finder. An astronomical telescope has a very narrow field of view, this means that only a very small area of sky can be seen. This makes it very difficult to locate a specific object. The finder has a wider field of view so can be used to find the object first. Once centred the object should be visible in the main scope.

Using a Dobsonian is very simple design making it an ideal telescope for the beginner. It is also a large powerful telescope made available at a modest cost. To enable a beginner to see astronomically interesting objects other than The Moon and the larger planets, a telescope of at least 150mm is required and most Dobsonians are of at least this size.

A 150mm Dobsonian is very quick to set up, in most cases all that is needed is to carry it from its place of storage to the observing position. It is almost instantly ready for use. Better images will be obtained however if the telescope is allowed to cool for about half an hour before serious observing is attempted. A seat may prove useful for comfort during long observing sessions. A small table (or box) will be useful for finding eyepieces and charts in the dark. A red torch will also be handy but never use a white light as this will spoil night vision instantly.

Make sure that the telescope is on level ground and away from walls if possible. Heat from the Sun may have warmed the wall and the heat will cause air currents in the cold night air. While the telescope is acclimatising to the cold air make a plan listing the things to observe and where to find them. Make sure you have every thing you need and you are dressed for the cold. Fit the lowest power eyepiece into the focuser, this will be the one with the larges dimension in mm written on it. A 25mm will be ideal to start with. Allow a few minutes for your eyes to get used to the dark then it is time to begin.

Look around the sky for the constellation in which your first object is located. Pin point the position within the constellation where the object is or find the object if it can be seen with the naked eye. Aim the telescope in the direction of the object. Look along the telescope or the edge of the finder to point it a accurately as possible by eye. Look into the finder and see if the object can be found. If you can't then while looking through the finder, move the telescope slightly up and down the side to side to see if the object comes into view. When the object is found move the telescope until the object is in the centre of the field of view of the finder. Some finders have a cross hair to enable the centre of view to be found easier. A hint here if this is the first time the telescope is used. Pick a bright object such as a planet, bright star, bright messier object or even the Moon.

Try not to move the telescope and look through the eyepiece of the main telescope. Hopefully there will be a bright object visible in the eyepiece. This object may appear as a splodge of light because it is out of focus. Gently take hold of the telescope while looking through the eyepiece and keeping the object in the field of view. Adjust the focus while steadying the telescope with the other hand. Some telescopes have a knob that adjusts a rack and pinion drive on the focuser unit. Others may have a simple push / pull sliding focuser. Move the focuser one direction until the object comes into sharp focus. If the object appears to be getting bigger then adjust the focuser in the other direction.

A star will always appear as a small point of light. If a it appears as a disc then it is either out of focus or it is a planet. Groups of stars may be best seen through a low power eyepiece but if there is a compact cluster a higher power may be used to increase the magnification to see the individual stars. To do this, ensure the object is in the centre of view, gently remove the eyepiece and replace it with an eyepiece with a shorter focal length perhaps a 10mm then adjust the focus.

Stars or other objects will move across the field of view due to the rotation of Earth. The object will appear to move much faster when using higher magnification powers. Most Dobsonian telescopes are not power driven and need to be guided by hand. With practice this becomes a very simple and almost automatic task. The technique used by most astronomers is to move the telescope so that the object to be observed is almost on the edge of the field of view. It then can be left to drift across the field without touching the telescope. When the object reaches the opposite side of the field of view the telescope is again move to place the object back to the other side. Because all object move in an arc across the sky, it is necessary to move the telescope in both directions (up or down and east to west).


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