Download Telescopes – How They work – Basics

Telescopes – How They work – Basics
Lets start with a lens – the heart of a telescope
A lens can focus light:
The lens can do this because of its ability to bend light … called refraction.
A point source at “infinity” results in “Parallel” light – example a star.
Now suppose you look at some object which is not simply a point of light and is not
at necessarily at infinity:
Shazam! It will form an image as shown in the picture below
Now that we are super experts on lenses, how could we
use them to make a telescope?
The answer is simple (its blowing in the wind):
The pix above shows that not only could a lens form an image of an object, but it
can also be use to magnify … the eyepiece. We now getting extra mileage from
the lens!
The above simpleminded picture represents what is called a Refracting Telescope
– a lens on the business end and an eyepiece on the other end
Examples are a spotting scope (“spy glass”) or binoculars. Note that the image is
upside down – but no big deal! … we all get used to looking at stuff in a mirror
which is reversed from left to right.
Chances are that you won’t see the rings of Saturn by holding
up two magnifying glasses!
Lets take a look at the real refracting telescope

It follows the same basic concept as in the above pix – but is more
complicated … the devil is in the details!
 To view astronomical object like Saturn, you will need powers from 50x to
400x – with a “jury-rigged” setup like the magnifying glasses the slightest
movement would cause the object to leave the field of view – you need a
mount.
o
o
o
o
Equatorial mount – can track the stars – moves north-south & east-west
Altazimuth mount – moves up-down and left and right
Dobsonian mount (similar to Altazimuth) – simplest – this is for us!
More on this later.
 In a real telescope the objective lens and eyepiece must be precisely aligned
(collimated).
 A simple magnifier suffers from defects which at high powers would
produce a blurry image with a lot of color halos around it.
o You need lenses made up of multiple components to correct the defects.
o Also real eyepieces are of much higher power that a reading glass and are made of
multiple components also.
 At 100x you will never find Saturn in the sky – you will need a small low
power finder scope mounded piggy-back on the main scope to find the
object.
 Need a way of focusing the eye-piece
 Need counterweights to balance the scope – especially for equatorial mounts.
Here is what a real refractor scope looks like:
OK – we know what a refractor is – but what is a
reflector scope?
The second basic class of astronomical telescopes are Reflectors – image is formed
by a concave mirror rather than a convex lens.
As with the lens, the mirror can also focus light:
Note that the light doubles back on itself to get to the focal point !
We just saw how a lens forms an image, but how does a
mirror do the same?
Note that the image is in front of the mirror
Question: how to you get to look at the image without blocking the light coming
in???? …. ==>
Answer: The standard Newtonian reflector solves this problem:
Note that a simple plane mirror (the secondary) is positioned at a point before the
focus and deflects the focal point outside of the tube thru a hole to be viewed by
the eye piece
A small but negligible amount of light is blocked by the secondary.
The secondary is mounted on a gadget called a spider and athe main mirror is
mounded on an adjustable “cell”
Because of two mirrors the collimation is more difficult.
The reflector – fully mounted (equatorial):
Finally some thoughts on mounts:
Equatorial mount: has one axis aligned with the earth’s axis and thus need to be
turned in only one direction to track an object as the earth turns.
Altazimuth : moves up and down from the horizon and left and right – need to
move in two dimension to track a star
Dobsonian is a special case of an Altazimuth and is easy to build from wood and
scrap parts – this is what we will go with for the 6” and 4.25“ scopes:
Example of a Dobsonian scope:
The scope in the foreground is a Dobsonian reflector and the background scope is an antique
refractor on an equatorial mount