Tube and Mount Construction
The tube itself may
be made of anything from cardboard tubes to plywood (as shown here). One may even forgo a
solid tube and connect the secondary mirror with a few metal trusses. The
purpose of the tube is to hold the primary mirror, secondary mirror and
eyepiece all in alignment. The arrangement of these parts focuses a wide
section of light into a small image (see
diagram). At first glance, the secondary mirror appears to obstruct light
from the center of the view, but this disturbance is actually very small. This
is because the center of the mirror does not correspond to the center of the
image. In fact, only a small section of the mirror is needed to produce an
image. The rest of the mirror is there to collect more light. Many a hopeful prankster has placed his or her
head in front of a telescope to make a funny face, but this only results in the
darkening of the image (see
diagram).
The two mirrors need
to be held stiffly to the tube, but still must be adjustable. For the primary,
one can make a wing-nut adjustable mirror
cell that rests on springs. Ideally, there should be plenty of room for air
ventilation as thermal disparities can cause disturbances to the mirror. The
secondary mirror is held by a few pieces of metal (called the spider). The eyepiece is placed opposite the
secondary with room for focus adjustments.
There
are two main ways to mount a telescope. For imaging and large observatories, an
equatorial mount is most ideal. This type of mount can be aligned with the
celestial poles so that the telescope follows the rotation of the earth. With a
small motor, the telescope can stay on an object as it rotates all the way
across the sky. A simpler mount will work for most visual observation (without
a camera). One of the easiest types to make is the Dobsonian.
Dobsonians can move
along two axes. Up and down (altitude)
movement is made possible with a simple cylinder resting on Teflon while left
and right (azimuth) movement
occurs on a flat base of Formica. (The base also doubles as a nice table when
the stars refuse to come out). Controlling these movements is easy and requires
no set up, but unfortunately they cannot automatically track the stars across
the sky unless a special base is added. The operator must continually nudge the
telescope so the stars do not rotate out of view.
When all parts are
complete, the tube can be set in the Dobsonian mount and the telescope is ready
for a deep voyage into the night.
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