29-03-2012, 02:27 PM
Telescopes
Telescopes.pdf (Size: 3.83 MB / Downloads: 147)
Why do we need telescopes?
There are two main reasons for needing telescopes:
• Mirrors are much cheaper, per unit area, than detectors. So if you want to gather a
lot of radiation, it makes sense to build a big mirror to gather all the radiation into
a small spot, and put a detector there. Note that this is now always true. The
MWA and LOFAR radio telescope designs use dipoles as detectors, and don’t
have any mirrors/dishes/reflectors. Some gamma-ray detectors similarly lack a
mirror.
• Directionality: telescopes sort out the radiation coming from different directions,
and deposit it into different parts of the detector.
How do you focus the light?
Radio telescopes use chicken-wire or metal sheeting. X-ray telescopes use mirrors, but
the X-rays must bounce off the mirrors at an extreme angle, or they would pass straight
through.
Optical telescopes can use either mirrors or lenses. Both have advantages:
Focal Stations.
The purpose of a telescope is to bring the light to a focus somewhere. There are many
possible locations for this purpose, each with its own advantages and disadvantages.
Prime Focus:
The light bounces off the primary
mirror and is brought straight to a
focus, high up on some support
structure.
It is simple, and because only one
reflection is involved, minimal
light is lost. It can also give a
very wide field of view.
The disadvantage is that you have
to have the detector and
instrument high up on a wobbly
support structure, blocking some
of the light as it comes down to
the primary mirror.
Field of View.
The correct shape for a primary mirror that
brings all the light coming straight down
the optical axis to a focus at a point is a
parabola.