Introduction to Optics
With this Article we will begin our investigation of instruments used to study the sky: optical
systems, binoculars and telescopes. Will focus on the information in the text, Nightwatch .
chapter 5 (pg. 60- 81).
Overview
With all optical instruments the component of the telescope which collects light is referred to as
the Primary Objective (or objective), and can be either a lens or a mirror. If a lens is used, the
system is referred to as a refracting telescope. If a mirror is used, the system is referred to as a
reflecting telescope.
Refracting Telescopes
The refracting telescope functions by bending (refracting) light to bring the parallel light entering
the lens to a point of focus. The figure below demonstrates how light enters the telescope from
the right, passes through the lens (is refracted) and comes to a point of focus. This is where an
eyepiece is placed to view the image.
Note that the lens diameter (Do) and focal length (Fo) are specified by the manufacture.
Reflecting Telescopes
The reflecting telescope functions by reflecting parallel light entering the front of the telescope to
a point of focus. The figure below demonstrates how light enters the telescope from the left,
passes through the telescope down to the mirror then is reflected back up. The reflected light
travels back up the telescope to a secondary flat mirror (diagonal mirror). This secondary mirror
reflects the light out the side of the telescope where it comes to a point of focus. Once again, this
is where the eyepiece is placed to view the image.
Note that the mirror diameter (Do) and focal length (Fo) are specified by the manufacture.
There is one more type of telescope (actually many similar types) which are hybrids, that being a
combination of both reflecting and refracting telescopes. On of the more popular types is a
Schmidt-Cassegrain telescope. The figure below demonstrates how “SC” functions.
In the SC design, light enters the front of the telescope through a correcting lens. It then reflects
off the objective mirror in the back, which sends the light forward to a secondary mirror. This
secondary mirror, which is mounted to the back of the corrector plate, sends the light back
toward the objective mirror. The objective mirror has a hole drilled in the center to allow the
light to pass out the back. This is where the light comes to a focal point and where the eyepiece
is placed.
Note once again the manufacture specifies the Do and Fo of the system.
Binoculars
With binoculars, the specifications and magnification calculation have been done for you. When
you examine a pair of binoculars, there is usually a pair of numbers stamped on the body, which
looks like the following:
7x 35
In this case the “7x” gives you the magnification as “7 times” for this particular set of binoculars.
The diameter of EACH monocular lens (the primary objective lens) is 35 millimeters in
diameter.
Telescopes Parameters
The objective of a telescope (lens or mirror) usually has it’s diameter expressed in inches,
centimeters or millimeters and is abbreviated as :
DO = Objective Diameter
We are also provided a Focal Length, which is the length from the objective to the point where
the light is brought to a focus. This is expressed in inches, centimeters or millimeters and is
abbreviated as :
FO = Objective Focal Length
Eyepieces for the telescope have the focal length stamped on them and are expressed as:
FE = Eyepiece Focal Length
To calculate magnification ( abbreviated as “ Mag.= ” ) we use the equation ( pg. 77 of
Nightwatch ):
Mag = FO / FE
REMEMBER: with binoculars, you are GIVEN the magnification from the stamp on the
instrument (see above and page 60-62 of Nightwatch ).
What is the maximum practical magnification (MPM) that should be practically used for a given
optical instrument? Generally it is considered to be 50 times the objective diameter in inches.
MPM = 50x Do (inches)
To calculate focal ratio ( abbreviated “ f / “ ) we use the equation (pg. 78 of Nightwatch ):
f / = FO / D O
Last, there is a figure of merit which is used to compare two optical instruments. This figure of
merit is Light Gathering Power (referred to as LGP) LGP is basically a comparison of the
objective (light collecting) areas of the two instruments.
LGP = (D1/d2)2
Note: (D1) is the diameter of the LARGER instrument’s objective lens or mirror.
(d2) is the diameter of the SMALLER instrument’s objective lens or mirror.
Remember to square the (D1/d2) term !
With this set of telescope equations, you should be able to complete your Assignment.