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 The system has been replaced by its full-dome descendant, the Digistar 3.
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 graphics 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.
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.
While Digistar ran off large VAX 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.
Despite its limits, Digistar II was well received by many planetarians, and has been distributed worldwide. Though it lacks the pin-point stars of opto-mechanical projectors, and the full rendering abilities of full-dome video, many planetarians consider it a good balance between the two. Its reliability and range or projection means many planetaria can still present stunning, quality shows, allowing the many Digistar II planetaria to safely wait for full-dome video technology to improve and resolve its remaining issues before upgrading.