Home / Homeschool science! The five most important numbers that will tell you everything about a telescope and binoculars
Feb 26th, 2020
When buying a telescope one of the least important factors to consider is the power (magnification).
The key to observing fine detail is not power, but aperture and on a telescope it’s the diameter of the main mirror or objective lens.
A telescope with a mirror of 200 mm collects four times as much light as one with a 100 mm, because the light collecting ability is directly proportional to the area of the mirror (or lens).
Once the light falls on the mirror, it gets directed by curing the mirror in a certain way so that the light comes together into focus. The focal length is the distance between the lens or primary mirror and the point where the light rays come together into focus:
f/number = mirror’s focal length divided by its aperture
The focal ratio, or f/stop, or f/number of any lens system (including telescopes), is found by dividing the focal length by the aperture (usually expressed in millimeters).
A long focal ratio means higher magnification and a narrow field of view for any given eyepiece. If you want to see wide views of the Milky Way or star clusters, use a lower f/number. For detail of the moon and double stars, choose a higher focal ratio.
The f/ratio also changes the brightness of objects that you can see. For example, a telescope with a focal ratio of f/5 will show an image 4X brighter as an f/10 scope, but the image of the f/5 will only be half as large.
There are times when you might want to know the magnification you’re using with your telescope.
The magnification of an astronomical telescope changes with the eyepiece used.
You can find the magnification by dividing the focal length of the telescope (usually marked on the optical tube) by the focal length of the eyepiece (both in millimeters), also printed on the eyepiece.
Typically the most useful magnification you’re going to get out of a telescope is 50 times the aperture in inches (or twice the aperture in mm) – more than that and all you’ll get is a fuzzy or dim image.
If you have binoculars, you can figure out the power or magnification the same way – you divide the focal length of the binocular tube by the focal length of the eyepiece, but you need the instruction manual to tell you the focal length of the binoculars and also for the eyepiece (mine says 150mm for the binocular length and 15mm for the eyepiece focal length = 150/15 = 10). There’s an easier way, though.
Most binoculars have numbers on them. Mine are 10x50. The first number is binocular power, the second is the size of the objective in mm.
Resolution is directly proportional to the size of your telescope – the aperture. A 200 mm scope can resolve details twice as well as a 100mm scope (if everything else is the same).
True Field of View: The circle of sky that you see when you look through a telescope or binoculars. Generally, the lower the magnification, the wider the field of view.
The exit pupil is the diameter of the light that comes out of the eyepiece. For most people, their eyes can dilate to about 7 mm.
Any larger than that and it’s too much light for your eye to accept, any less and things look dimmer and you won’t be able to see well in the dark.
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