DARK SKIES: Which telescope should you use?
REFRACT means the bending of light when passed through at an angle. In the case, of a refracting telescope or a long tele lens, this instrument has a two-element lenses up front made of a double convex lens (positive) and a plano-concave (negative) lens.
These two placed together with a tiny airspace is called an Objective lens which is mounted in a metal tube, that collects light to pass through those lenses.
As it does, the light cone forms a small image further down that tube, called the Focal Length (F/L).
Measured in millimetres, the higher this number is, the longer the tube which makes it difficult to handle and mount.
In the case of a refracting telescope, we look up through a tube to see a magnified image.
To do this, we must insert a small set of lenses called an EyePiece (EP, that magnifies that image.
That is when we close one eye and look to see what the telescope is looking at.
The world's largest refracting telescope built in the 1890's has an objective lens 100cm diameter and with a focal length of nearly 20 meters, the worlds' famous Yerkes telescope, based in Williams Bay, Wisconsin, USA stands proud.
In astronomy, we have an old saying: 'There is no substitute for aperture' meaning the larger the lens is, the more light it will collect of fainter objects at night.
So if you want to view faint objects like galaxies, we need larger aperture like a reflector, where it uses mirrors to collect the light. Refracting telescopes are more robust that can stand rougher handling and have less maintenance than compared to reflectors.
Refractors are best for viewing the moon, planets, double and multiple star groups and of course, land viewing.
All telescopes have an F/ratio (F stop) which is the ratio of the telescope's focal length to its aperture.
By multiplying these two gives the telescope's focal length.
For instance, a telescope with an objective of 15cm diameter and an F/ratio of 15 or F15, the focal length is 2286mm.
To work out what it can magnify, all Eyepieces have a number on them, like 10mm, 15mm 25mm and so on. So with a F/L of 2286mm and an EP of 10mm, you divide that number into the F/L, which in this case, it's 228X.
To get a lower magnification we use an EP with a higher number. While the type of the glass used in modern day optics is measured by its price, there is no substitute for quality.
And when it comes to aperture per dollar, the cost of a 20cm diameter lens to a 20cm mirror, is comparing apples to oranges. That is why there are more reflecting telescopes in the world than refractors.
If you mention and bring this article when you book into the Observatory for a night visit, it will attract a discount.