At the heart of any modern planetarium is the projection equipment – without that, there is only the physical placement and movement of objects. Whatever the show, chances are it will call for a variety of celestial objects to appear on the dome – and the projector is how they get there. It isn’t restricted to the Sun and the Moon; the Earth will often be shown as the program varies its point of origin in space, and so will the Milky Way – but there can also be other galaxies, black holes (don’t get sucked in!), asteroids and all sorts of other entities.
The question for the planetarium owner is: which of the three commonest forms of projection to use? And the answer will depend on two factors:
* How much money is in the kitty?
* Is the planetarium a fixed building, or will it be transported from place to place?
For fixed building planetariums, the optical-mechanical projection solution is the best; it is also the most expensive. The very first modern (that is, projection) planetarium used a projector made in Germany by the Carl Zeiss Company with stars represented by light passing through very small holes in metal plates inside lenses projecting from an incandescent light bulb. Rotating the apparatus simulated earthly rotation. Things have advanced somewhat since then, though Zeiss is still one of the leading players.
Digital Video Projection Planetariums
The advent of powerful computers operating in real time stimulated the development of digital video projection. The images are created using computer graphics on an exceptionally bright cathode ray tube and then projected through wide-angle lenses.
Coupling this system to the extensive database of star positions now available allows projection to be from other places in the solar system – so, for example, the Earth and its relationship with the Moon, the Sun and other celestial objects can be clearly shown; a big educational advance.
Planetarium Projection By Laser
As with digital projection, this system couples the vast range of data available to a brilliant light source – in this case, the laser.