How is a planetarium different from a telescope/dome?|
A telescope dome is round, and has something to do with astronomy; and a planetarium is round, and has something to do with astronomy. As a result, many people confuse the two; but they are very different. A telescope dome is used to hold telescopes, which can be used to observe the heavens, either for aesthetic or scientific pursuits. The purpose of the telescope dome is to shield the telescopes from rain, wind and other weather effects. A hemisphere is the traditional dome shape, but any shape could be used, and modern telescope domes are often not "domes", at all.
What Is A Planetarium?
A planetarium dome is a screen for viewing images created by a planetarium projector. A hemispherical shape is preferred, because it mimics the appearance of the sky, but modified hemispheres are often used in large public installations, particularly those which are more often used for entertainment than astronomical instruction.
The critical difference between the two "domes" is that for a telescope to be of any use, its enclosure must open to the heavens, which are the source of the images to be observed; while a planetarium never opens to the heavens, as its images are created at or near its center, by the projection system. So a telescope dome opens, to allow light to enter the dome, while a planetarium dome is a fixed screen, which has nothing to do with the sky outside, unless the projected content happens to be about astronomy.
(old discussion, in considerable need of updating)
(1) The planetarium dome:
The most obvious feature of a planetarium is the dome, usually but not always hemispherical, which serves as a projection screen for the planetarium projector and auxiliary devices. A thirty-five foot diameter hemispherical dome was installed in the planetarium classroom near the end of the D building remodel, several years ago. The base of the dome sits about 8 feet above the classroom floor, and with a radius of nearly 18 feet, the top of the dome extends more than 25 feet above the floor. To reduce the size of the concrete box which holds the planetarium, a truncated pyramid was centered above the room, and the dome and its support extend into that pyramid. In addition, a patinated copper dome, complete with lightning rod, was added to the top of the pyramid, albeit only as an advertisement of the location of the planetarium, as the planetarium dome does not extend into the exterior dome.
The dome and cove lighting cost the best part of $200,000 to purchase and install, but that is only a down-payment on the overall cost of the planetarium, which is why fund-raising is ongoing, and permanent.
(2) A star projector:
The heart of a planetarium is the star projector. Traditionally, this was also the most expensive part of the installation; but fortunately, the cost of top-quality star projection systems has decreased dramatically in recent years. Otherwise, we might still merely be wishing for one, years from now.
There are two kinds of star projectors:
(a) Opto-mechanical star projection systems date back to the 1930's. One or two ball-shaped structures containing a complex system of light bulbs, glass plates and lenses is used to project images of the stars onto the planetarium dome. Depending upon the quality (and cost) of the projector, the star images can be relatively large and fuzzy, or almost as sharp and brilliant as the real sky. Aside from their relatively large size, even the best opto-mechanical star projectors have important limitations, as their fixed star plates cannot show anything save a fixed, unchanging sky. Over long periods of time, even the "fixed" stars change their positions; and the Sun, Moon and planets move noticeably over periods ranging from a few hours, to a few decades. None of these motions can be directly shown by a traditional star projector. Instead, numerous auxiliary projectors are used, mounted inside the projector or at its base, considerably increasing the cost and complexity of the system. Additional effects, such as close-ups of planets, nebulae or galaxies, require still more projectors, usually mounted around the periphery of the dome.
A typical traditional (1960's) opto-mechanical star projector, at the Montreal Planetarium, in Montreal, Canada. The two spherical balls at either end of the structure project fixed images of the stars onto the hemispherical dome (partly shown, behind the projector), while auxiliary projectors -- the complex systems located inside the axis between the ends of the projector -- show the positions of the Sun, Moon and planets. Additional slide, film and video projectors at the base of the projector, and scattered around the dome, complete the projection system. The myriad pieces of equipment are controlled from a console, off to the side of the room.
Until very recently, the only method of planetarium projection was the opto-mechanical projector. But in recent years, another method has become practical and affordable:
(b) Digital (computer-driven) projectors, such as the Digistar 3 SP2 which we purchased from Evans and Sutherland, can now produce a detailed, reasonably sharp image of the sky and, for that matter, anything which can be created with a graphics program. Like a traditional planetarium projector, a digital projector is mounted beneath the center of the dome, but since anything, including the Sun, Moon and planets, can be included in the digital image, the dozens of auxiliary projectors usually added to the main projector are not needed. We will use an additional projector for Power Point and web-enhanced lectures which do not require full-dome projection, but during full-dome projection, only the Digistar projector is needed.
Early digital projectors had fairly low resolution, and were very expensive; if we'd had the money to buy such a projector when the D building remodel was completed, it would have run between $700,000 and $2,500,000 to purchase and install it; and the images produced would be vastly inferior to the two megapixel image produced by the current model, which was also vastly less expensive (approximately $150,000 including installation). So the long delay in acquiring a projection system, although unfortunate in some ways, has been a blessing in disguise.
The Digistar 3 SP2 is a DLP (Digital Light Projection) system, and like DLP televisions, provides a high-quality image; but just as plasma displays provide even sharper, brighter television images, there is another technology now appearing on the planetarium market, albeit at ten times the price of DLP systems. Laser projection systems, such as the one now being installed at Griffith Planetarium, provide even better images, with twenty megapixel resolution, and considerably enhanced brightness. Given their current cost (several million dollars), they are not practical for a "small" installation, such as ours; but given the rapid march of technology, when our current system needs replacement (perhaps a decade from now), it is quite possible that it will be replaced with a laser version. But just as the programming is the same no matter which television technology you purchase, the appearance of the hardware, and of the display it provides on our dome, will be very little changed if and when such an upgrade is made. Save for minor improvements, digital projection technology has reached a plateau which we can be very happy to have reached at relatively little cost, and a moderately early date.
(3) A sound system
Although the use of the planetarium for lectures can dispense with anything but the least sophisticated sound systems, public presentations require (and even lecture presentations benefit from) a high-quality sound system. To provide adequate sound for a huge space, such as the planetarium classroom, this means a high-cost system, as well. The 5.1 surround sound system to be installed in September cost $45,000, and even that large expense is low, compared to what it would have cost at the time of the D building remodel.
(4) Seating for the audience
Even for lectures, comfortable seating which tilts back for viewing of full-dome images would be greatly appreciated by the students; and for public presentations, it is a must. About 90 seats will be installed in the planetarium, once the seating arrangement is approved by the State, sometime in 2007. The far more comfortable view this will provide students and planetarium visitors will undoubtedly feel cheap to those who have to use the temporary plastic and wood chair-desks between now and then, despite its close to $50,000 cost.
(5) Control mechanisms (hardware and software), and presentation graphics
Basic instrumentation control is provided by the manufacturer, but to do even minimal scripting and preparation for shows, additional workstations and software are required, at a cost of $7000 per user. One such system has been purchased already, and depending upon the time required to prepare presentations, a second one may be needed. That will not allow the production of full-dome, virtual-reality graphics, which require $30000 workstations, and take several thousand man-hours per hour of production video. As a result, we will need to purchase a few astronomy-related planetarium videos (that may sound redundant, but most planetarium presentations nowadays are more related to thrill rides, than astronomy). Each such program costs upwards of $5000, so to provide adequate variation in presentations, the long-term cost required for public outreach is tens of thousand of dollars. Of course, if we had additional staff, we could produce our own presentations, but the salary for such staff would be tens of times larger yet, and can only be justified by having most of their time spent in the classroom, and a small portion on planetarium preparation and presentation.
(6) Maintenance and Upgrades
The equipment above is not only expensive to purchase, but requires relatively expensive maintenance and, eventually, replacement or upgrades (such as the laser projection system discussed above). To keep the planetarium up-to-date and in working order will require ongoing funding of a few thousand dollars a year, and long-term funding of tens of thousands of dollars a year.