Digistar II
Encyclopedia
Digistar II is a planetarium projection system by Evans & Sutherland
- Digistar Users Group
. It was released in the early 1990s as a descendant to the earlier Digistar, developed in the late 1970s. The system has been replaced by its full-dome descendant, the Digistar 3
.
based, rather than raster
based. As such, neither of these systems can provide full-color, rendered effects. Evans & Sutherland's newer system, the Digistar 3
, can do so.
Unlike modern full-dome systems, which use LCD, DLP, SXRD
or Laser
projection technology, the heart of the Digistar II is a large cathode ray tube
. A phosphor plate is mounted atop the tube, and light is then dispersed by a large lens to cover the planetarium dome. The use of a CRT means the Digistar II has a darker black-level than full-dome projectors, but, as it is only one tube, the D2 is monochromatic. The Digistar II projects a bright, phosphorescent green—though many (including both visitors and planetarians) report they cannot distinguish between this green and white.
Due to the use of vector graphics, as opposed to raster imaging, the Digistar II does not have the resolution issues that many full-dome systems have. Thanks to this, and the brightness of the CRT, only one D2 projector is needed to project on the entire dome (most full-dome systems require between 1-6 raster projectors, depending on dome size).
As Digistar II was being developed, many planetaria were sold Digistar LEA projectors. The LEA, called Digistar 1.5 by many users, is effectively a prototype of the D2 projector, compatible with Digistar and upgradable to Digistar II. There are no significant differences in performance between the LEA and the true D2.
The D2 also suffers focus limitations. Because it uses a single lens to cover the entire dome, it is difficult to gain perfect focus across the dome. Coupled with this, stars greater than a certain brightness are "multihit" points, meaning the projector draws two dots at the given position to accommodate for the brightness of the star. As in the Digistar, errors in the projector can lead for the second dot to be slightly out-of-place with the first one. These two issues together, along with other issues that can occur within the projectors focus system, give the stars a blobby look. Many planetarians, used to the pinpoint opto-mechanical projector stars ubiquitous in the day, rejected the Digistar and D2 because of this, ignoring the other advantages of the system.
The CRT in the Digistar and D2 begins to burn out and lose brightness after roughly 1000 hours of use. This means most planetariums must change out the tube after every year or year-and-a-half.
computers, Digistar II runs off the much more compact and advanced Sun Microsystems
SPARCstation 5
. D2 uses two primary file types, .vl and .sf. .vl files are binary models files, while .sf files are binary show data files. Model files contain vector, line and dot data, as well as parametric changes to data within the file, show files contain commands to the system, regarding the manipulation of the observer and models declard within the file. Several show files are often strung together underneath each other in show production. Both .vl and .sf have ASCII
equivalents for editing--.vla and .sfa respectively. These are converted to their binary equivalents by a utility built into the Digistar system, which also checks for errors within the file. Digistar II show files are programmed in a language related to Pascal
.
Further, Digistar II can run animation files, .af, with the ASCII format .afa. An animation file consists of several model files, grouped together and loaded as one object. The Digistar II can either select frames individually, or animate the entire file.
Digistar II is able to convert Digistar show and model files. Similarly, Digistar 3 is can convert Digistar II model files, though it cannot, at this time, convert show files.
Evans & Sutherland
Evans & Sutherland is a computer firm involved in the computer graphics field. Their products are used primarily by the military and large industrial firms for training and simulation, and in digital projection environments like planetariums.-History:...
- Digistar Users Group
Digistar Users Group
- History :The Digistar Users Group began in the mid-1980s as an informal gathering of planetarians. This first gathering took place in St. Louis, Missouri . At the time there were only five Digistar systems worldwide. Today there are more than 100 Digistar equipped planetaria on four continents...
. It was released in the early 1990s as a descendant to the earlier Digistar, developed in the late 1970s. The system has been replaced by its full-dome descendant, the Digistar 3
Digistar 3
Digistar 3 is a dome-based projection technology created by Evans & Sutherland - to offer audiences immersive entertainment and education experiences that integrate fulldome video, real time 3D computer graphics, and a digital planetarium facility...
.
Projector
The Digistar II which is, in many ways, a direct upgrade of the original Digistar projection system, was an early attempt at full-dome video. Unlike true full-dome video, Digistar and Digistar II are vector graphicsVector graphics
Vector graphics is the use of geometrical primitives such as points, lines, curves, and shapes or polygon, which are all based on mathematical expressions, to represent images in computer graphics...
based, rather than raster
Raster graphics
In computer graphics, a raster graphics image, or bitmap, is a data structure representing a generally rectangular grid of pixels, or points of color, viewable via a monitor, paper, or other display medium...
based. As such, neither of these systems can provide full-color, rendered effects. Evans & Sutherland's newer system, the Digistar 3
Digistar 3
Digistar 3 is a dome-based projection technology created by Evans & Sutherland - to offer audiences immersive entertainment and education experiences that integrate fulldome video, real time 3D computer graphics, and a digital planetarium facility...
, can do so.
Unlike modern full-dome systems, which use LCD, DLP, SXRD
SXRD
SXRD is Sony's proprietary variant of liquid crystal on silicon, a technology used mainly in projection televisions and video projectors. In the front and rear-projection television market, it competes directly with JVC's D-ILA and Texas Instruments' DLP...
or Laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
projection technology, the heart of the Digistar II is a large cathode ray tube
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
. A phosphor plate is mounted atop the tube, and light is then dispersed by a large lens to cover the planetarium dome. The use of a CRT means the Digistar II has a darker black-level than full-dome projectors, but, as it is only one tube, the D2 is monochromatic. The Digistar II projects a bright, phosphorescent green—though many (including both visitors and planetarians) report they cannot distinguish between this green and white.
Due to the use of vector graphics, as opposed to raster imaging, the Digistar II does not have the resolution issues that many full-dome systems have. Thanks to this, and the brightness of the CRT, only one D2 projector is needed to project on the entire dome (most full-dome systems require between 1-6 raster projectors, depending on dome size).
As Digistar II was being developed, many planetaria were sold Digistar LEA projectors. The LEA, called Digistar 1.5 by many users, is effectively a prototype of the D2 projector, compatible with Digistar and upgradable to Digistar II. There are no significant differences in performance between the LEA and the true D2.
System limits
Though technologically advanced in its day, and the closest system to true full-dome at the time of its release, the D2 is a limited system. The D2 can only project dots and lines—meaning only wireframe models can be projected. To compensate for this, the projector is capable of "defocusing" specific models, blurring lines and dots together. An example of this is in the D2's built-in milky way model. The model is a circle of parallel lines that, when defocused, appear as the continuous band of the milky way across the sky. On more complex models, especially three-dimensional ones, brightness and details may be lost in this process, so it is not useful in all situations.The D2 also suffers focus limitations. Because it uses a single lens to cover the entire dome, it is difficult to gain perfect focus across the dome. Coupled with this, stars greater than a certain brightness are "multihit" points, meaning the projector draws two dots at the given position to accommodate for the brightness of the star. As in the Digistar, errors in the projector can lead for the second dot to be slightly out-of-place with the first one. These two issues together, along with other issues that can occur within the projectors focus system, give the stars a blobby look. Many planetarians, used to the pinpoint opto-mechanical projector stars ubiquitous in the day, rejected the Digistar and D2 because of this, ignoring the other advantages of the system.
The CRT in the Digistar and D2 begins to burn out and lose brightness after roughly 1000 hours of use. This means most planetariums must change out the tube after every year or year-and-a-half.
File types
While Digistar ran off large VAXVAX
VAX was an instruction set architecture developed by Digital Equipment Corporation in the mid-1970s. A 32-bit complex instruction set computer ISA, it was designed to extend or replace DEC's various Programmed Data Processor ISAs...
computers, Digistar II runs off the much more compact and advanced Sun Microsystems
Sun Microsystems
Sun Microsystems, Inc. was a company that sold :computers, computer components, :computer software, and :information technology services. Sun was founded on February 24, 1982...
SPARCstation 5
SPARCstation 5
SPARCstation 5 or SS5 is a workstation sold by Sun Microsystems. It is based on the sun4m architecture, and is enclosed in a pizza-box chassis. A simplified, cheaper version of the SS5 was later released as the SPARCstation 4...
. D2 uses two primary file types, .vl and .sf. .vl files are binary models files, while .sf files are binary show data files. Model files contain vector, line and dot data, as well as parametric changes to data within the file, show files contain commands to the system, regarding the manipulation of the observer and models declard within the file. Several show files are often strung together underneath each other in show production. Both .vl and .sf have ASCII
ASCII
The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
equivalents for editing--.vla and .sfa respectively. These are converted to their binary equivalents by a utility built into the Digistar system, which also checks for errors within the file. Digistar II show files are programmed in a language related to Pascal
Pascal (programming language)
Pascal is an influential imperative and procedural programming language, designed in 1968/9 and published in 1970 by Niklaus Wirth as a small and efficient language intended to encourage good programming practices using structured programming and data structuring.A derivative known as Object Pascal...
.
Further, Digistar II can run animation files, .af, with the ASCII format .afa. An animation file consists of several model files, grouped together and loaded as one object. The Digistar II can either select frames individually, or animate the entire file.
Digistar II is able to convert Digistar show and model files. Similarly, Digistar 3 is can convert Digistar II model files, though it cannot, at this time, convert show files.