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Methods of Observation
A telescope is a “light bucket.”
Large telescopes work better
than smaller ones because they
collect more light.
Optical telescopes gather visible
wavelengths.
Two basic types:
Reflectors –
curved mirrors.
Refractors –
curved lenses.
Both of them focus light
to a point called the
prime focus.
The distance from the
mirror or lens to this
point is the focal length.
There are five basic
problems, unique to
lenses, that make refractor
telescopes inferior to
reflectors.
(Especially very large
telescopes. Small refractor
telescopes work just fine.)
Lenses in refractors
bend different colors
of light different
amounts. It splits light
into its spectrum to
some degree.
This undesirable
splitting of light is called
chromatic aberration.
It is a problem with
lenses that increases
with the size of the lens.
Lenses also block much
IR and UV radiation.
In addition, lenses are
heavy and must be at
the top of the telescope.
Since the lens can
only be supported
around the edges, the
lens tends to deform
under its own weight.
Finally, a lens
has two surfaces
that must be
polished.
Mirrors, on the other hand,
(1) do not cause chromatic
aberration,
(2) do not block any type
of light,
(3) are put at the bottom
of the telescope,
(4) can be supported
across the entire back
so the mirror does
not deform, and
(5) only have one
surface to be polished.
Because of these facts,
all large telescopes are
reflectors. The largest
refractor is at the Yerkes
Observatory and is 1 meter
in diameter. Most large
reflectors are between
4 and 6 meters across.
There are four different
reflector telescope
designs.
All have a primary mirror;
the difference is where
the light rays go after
this mirror.
In a prime focus reflector the
instruments are placed at the
prime focus. This is not
a good system for bulky
instruments.
In a Newtonian focus, the light
is deflected 90° to an eyepiece
before it reaches the prime
focus. This is the normal
design for smaller reflectors.
A Cassegrain Focus telescope
reflects the light back through a
hole in the primary mirror. This
allows heavy equipment
to be used easily .
In a Coudé Focus, a third mirror intercepts
the light before it returns to the primary
mirror and deflects it to the side. The light
then goes to a laboratory called a coudé
room. This is used for very large or
precise equipment.
One telescope that fits most of
these types is the Hale 5-meter
diameter telescope on Mount
Palomar in California. Construction
was completed in 1948, and it was
the largest telescope in the world
for almost 30 years. Observations
can be made at the prime, the
Cassegrain, or the coudé focus.