Starship Asterisk*

http://en.wikipedia.org/wiki/Planetarium wrote:
<<Traditional planetarium projection apparatus uses a hollow ball with a light inside, and a pinhole for each star, hence the name "star ball". With some of the brightest stars (e.g. Sirius, Canopus, Vega), the hole must be so big to let enough light through that there must be a small lens in the hole to focus the light to a sharp point on the dome. In later and modern planetarium star balls, the individual bright stars often have individual projectors, shaped like small hand-held torches, with focusing lenses for individual bright stars.

The star ball is usually mounted so it can rotate as a whole to simulate the Earth's daily rotation, and to change the simulated latitude on Earth. Contact breakers prevent the projectors from projecting below the 'horizon'. There is also usually a means of rotating to produce the effect of precession of the equinoxes. Often, one such ball is attached at its south ecliptic pole. In that case, the view cannot go so far south that any of the resulting blank area at the south is projected on the dome. Some star projectors have two balls at opposite ends of the projector like a dumbbell. In that case all stars can be shown and the view can go to either pole or anywhere between. But care must be taken that the projection fields of the two balls match where they meet or overlap.



Despite offering a good viewer experience, traditional star ball projectors suffer several inherent limitations. From a practical point of view, the low light levels require several minutes for the audience to "dark adapt" its eyesight. "Star ball" projection is limited in education terms by its inability to move beyond an earth-bound view of the night sky. Finally, a challenge for most traditional projectors is that the various overlaid projection systems are incapable of proper occultation. This means that a planet image projected on top of a star field (for example) will still show the stars shining through the planet image, degrading the quality of the viewing experience. For related reasons, some planetariums show stars below the horizon projecting on the walls below the dome or on the floor, or (with a bright star or a planet) shining in the eyes of someone in the audience.

An increasing number of planetariums are using digital technology to replace the entire system of interlinked projectors traditionally employed around a star ball to address some of their limitations. Digital planetarium manufacturers claim reduced maintenance costs and increased reliability from such systems compared with traditional "star balls" on the grounds that they employ few moving parts and do not generally require synchronisation of movement across the dome between several separate systems.

In a fully digital planetarium, the dome image is generated by a computer and then projected onto the dome using a variety of technologies including cathode ray tube, LCD, DLP or laser projectors. Sometimes a single projector mounted near the centre of the dome is employed with a fisheye lens to spread the light over the whole dome surface, while in other configurations several projectors around the horizon of the dome are arranged to blend together seamlessly.

Digital projection systems all work by creating the image of the night sky as a large array of pixels. Generally speaking, the more pixels a system can display, the better the viewing experience. While the first generation of digital projectors were unable to generate enough pixels to match the image quality of the best traditional "star ball" projectors, high-end systems now offer a resolution that approaches the limit of human visual acuity, making their images subjectively indistinguishable from the very best "star balls" to most eyes.>>

Orca wrote:I had a home planetarium when I was a kid, it looked like this:

• <<Enjoy an amazing starry night view in the comfort of your own home with Uncle Milton's pro series planetarium. Watch as a crystal-clear, rotating field of 10,000 stars is projected on your ceiling or wall with random streaking comets. Features: * Optical Star Projection System * Futuristic sphere design * Two interchangeable hi-res image disks (star field with and without constellations) * Optical-quality lens system with adjustable focus * Ultra-bright white LED light source * Automatic timer, motorized image rotation and random streaking comet modes * Adjustable projection angles AC adapter and external portable battery pack (requires 4 C batteries - not included) Sale Price:$139.00>>

http://space.about.com/od/fungamesandhumor/tp/toyplntrm.htm wrote:


<<* As seen on the Today Show! * Project 88 constellations; 12 celestial objects, including 8 planets, Pluto and its moon, Charon * Uses super bright bulbs to project over 600 stars * Searchable database with over 600 star facts and... See more » * As seen on the Today Show! * Project 88 constellations; 12 celestial objects, including 8 planets, Pluto and its moon, Charon * Uses super bright bulbs to project over 600 stars * Searchable database with over 600 star facts and myths * Interactive talking computer and backlit navigational screen * Features five modes for a variety of astronomy tours and settings * Features a one-hour timer so you can fall asleep under the stars * Includes AC power adaptor and 24-page, full color instruction manualThis accurate, computer-aided, motorized planetarium transforms an ordinary room into a truly stellar display. $99.95>>

neufer wrote:

Orca wrote:I had a home planetarium when I was a kid, it looked like this:

As I recall it was almost impossible to get a room dark enough to see the stars;
and it certainly didn't have 10,000 stars: