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Basic Astronomy Series: Telescopes

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by Andrew Burton (subscribe)
I am a Freelance Writer-Photographer and Novelist. I travel to find inspiration, wherever the distant horizons lead.
Published April 6th 2015
How to choose your first telescope and how to use it
For those reading and finding astronomy a fascinating subject, now may be a good time to discuss what all those strange looking telescopes actually do!

The Newtonian Reflector Telescope.
The Newtonian Reflector telescope, a good 'all-rounder'.

Firstly, does anyone in the house have a good set of binoculars? To be of use, I mean the sort that have optics rated at 8 x 30mm or even better, 7 x 50mm. These figures indicate the magnification power at 8 and 7 times respectively and the diameter of the lenses is 30 or 50 mm. If you can find some binoculars, go outside and spend some time slowly gazing around, and see if that fires-up your enthusiasm!

Even if you are on a tight budget, or the telescope is for a child, the lower costs relative to the average wage today means your buying power is better compared to past years. It is possible for most people to afford a useful, and capable instrument with good quality accessories and manufacture.
 


Buying your first telescope.
A typical Refractor telescope on an Alt-azimuth mount.


There are two main types of telescope, the Refractor and Newtonian Reflector. The basis of these two telescopes, is how the light is transmitted to the eye. Put simply, the Refractor has the main lens at the entry point of light at the front end of the tube, and the light is bent (refracted) to a focus point. The eyepiece is the lens at the other end and can be moved in and out using a focusing knob, to give a sharp clear image to the observer. Because of the simple lens system and length of the tube, the Refractor gives a better image at higher magnification than a Reflector of the same size. Due to the simple optical system, the Refractor suffers a lower loss of light in the process, the image is therefore brighter too. For general observation, it is great for the moon and planets and for seeing smaller galaxies and nebula.
 

Telescope optical systems.
Refractor and Refelector telescope optical systems.

Now it becomes complicated, the Reflector is generally lower cost since it does not have an objective lens. It has an open tube with a mirror at the far end, which is concave and directs the light onto a smaller mirror mounted towards the front of the telescope. Finally, this small mirror reflects the light out at a right angle through an opening into the eyepiece. The focal length or tube length is shorter relative to the diameter of the tube, giving rise to lower magnification and light gathering ability. But the lower cost means you can afford a larger diameter tube. The greater the diameter of the objective lens or mirror, the greater the light gathering ability. As a comparison, my first reasonable telescope was a 60 mm diameter Refractor, and it would have had the same ability as a 75 or 80mm Reflector.
 
An inexpensive and powerful telescope.
The Newtonian Reflector on a Dobson Mount.

The Reflector telescope gives better all-round performance, considering they better display larger open star clusters and rich star fields or nebula. If you start to specialize, making planetary observations, or splitting closely spaced 'double stars' or investigating lunar features, then the better resolving power and higher magnification of the Refractor telescope may make more sense. Other alternatives include other types of Reflectors, including the Scmidt-Cassegrain, which is a compact model and they can be bought with large diameter objectives mirros and having shorter tubes if space is an issue. They can be very expensive, but have fully automated star tracking capability built into the mountings. If you make astronomy a major hobby, a fine instrument will give you great rewards and 'wow factor' viewing.

Next we can consider tripods and mountings and what they mean and do. The average telescope has an alt-azimuth mounting, essentially it moves up and down and side to side. This sounds ridiculous, but it becomes complicated when you consider the model representing the celestial sphere. The earth rotates 360 degrees in 23 hours and 56 minutes, or 1 degree every 4 minutes. So every minute, you will move the average telescope twice to keep the object in view (the average telescope has a ˝ degree field of view). To move the telescope by hand is a bit 'hit and miss' and fiddly. So an alt-azimuth mount with slow motion controls helps. Slow motion controls are a worm gear with hand controls to move the telescope up and down or side to side in small increments.
 
An Equatorial telescope mount.
Setting circles on an Equatorial telescope mount.

The next step up is an equatorial mount, which has an adjustable axis that you align with the rotational axis of the earth. Then a motor drive or slow motion control can operate on that 1 axis only, to keep the object in view. So, at the equator, the celestial axis is horizontal and aligned north to south. At the poles it is vertical and at mid latitudes it is inclined at angle equal to your latitude! I am imagining the entire blurb from previous articles is coming together and you are nodding your head thinking it makes a lot of sense now, if so, great!

In order to fully understand Equatorial mounts, they have setting circles with marks corresponding to the lines of declination and right ascension. On a given night, at a given times, the right ascension will have to be set by finding a bright star of know co-ordinates. Then objects can be found after the celestial axis has been aligned with the earths axis and the right ascension or 'R.A.' is set. Now it is a matter of rotating the telescope on each axis and using the pointer on the setting circles to indicate the R.A. and declination, or 'Dec' of the object to be found. It is like using a GPS when you input latitude and longitude, except the co-ordinates are 'entered' on the setting circles. The object will be in view if everything is setup correctly. If you have a clock drive motor, engage that and the object remains in view as the telescope tracks the object. Refer to the first astronomy article if you do not understand R.A. and Dec.

A telescope with auto tracking capabilities.
The Schmidt-Cassegrain Telescope.

For those with a budget that has less constraints, the Schmidt –Cassegrains come on a alt-azimuth mount, which will have a solid double yoke and the 'big deal' is auto-star-tracking. The motor drives will track any star automatically, without having to align a celestial axis, or adjust 'setting circles' to match the position of a known object. It does cost more, but will find any object you have co-ordinates for, without you moving the telescope around to find it. Auto-tracking requires an initial setup after purchase and will not require further set-up after that is done.


More simple and less expensive telescopes will require you to use a finder telescope or use the 'point the tube and guess' method to find faint and small objects. The 'finder scope' you will see as the little telescope mounted on the main tube of the telescopes pictured. Setting circles on equatorial mounts and auto-tracking telescopes make life easier. Of course, 'auto-tracking' is the ultimate for simple observations.

Other species; a very simple telescope with a basic mount that is large and has great viewing power for its price is the Dobson mounted Newton Reflector, and they are available in 150mm diameter or greater. The 200mm model available locally, is great value if you can live with the simply mount, and manual tracking of objects. It is quite a powerful and capable instrument you might just 'go ape' when you find out how good it really is!

Schmidt-Cassegrain telescope optical system.
Schmidt-Cassegrain telescope optical system.

If you are lucky enough to know a friend with a telescope, then I would ask if they would give you a demonstration before buying your own. As for helpful and knowledgeable dealers, I have found that Optics Central in Mitcham, Victoria were really good. To see there website, click here. I bought a Newtonian Reflector for my son, which is his first telescope. He now owns a 120mm Newtonian Reflector on an Equatorial mount with a motor drive. It really is a good all-round performer with good features and in my books, it is good value too.
 
Have you ever seen the moon through a telescope?
Spend some time outside at night and discover something new.

The next article will address practical astronomy concepts such as brightness, seeing conditions, the planets, what to see and expect, star charts and catalogues. If astronomy is becoming interesting, then you will be armed to witness great wonders and know when and how to find them! Happy observing!
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Why? Fascination!
When: All year round.
Where: Anywhere.
Cost: Good telescopes start from about $120.00
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