Star of the show

When the Holy Roman Emperor, Friedrick II of Hohenstaufen, returned from the Crusades in the 13th century he had a curious item in his baggage. Made in one of the Islamic courts of the time, it was a dome-shape tent pierced with many holes in the shapes of the constellations of the night sky so a person sitting inside during the day would see the sun shining through the holes. It was mounted on a turntable so the stars would turn above the observer.

Was it an aid to teaching navigation by the stars? Nobody knows, and the tent is now lost. In any case, what is certain is that it was the first known example of a planetarium.

Technology has moved on a bit since then. The world’s latest planetarium opened in Greenwich this May. Bisected by the Zero Meridian, the 11.5m-wide dome of the National Maritime Museum’s Peter Harrison Planetarium is patterned by light from a specialised laser projector — the first of its type in Europe.

The DigiStar 3 is made by Evans & Sutherland (E&S), a specialist in computer graphics displays in Salt Lake City, US. Designed for extremely high resolution and delivering images of great clarity, the projector can provide all the images needed in today’s planetarium shows, said Jan Bjernfalk, E&S director of sales and marketing.

Traditionally, he said, planetariums showed starfields — images of the constellations — and the various heavenly bodies. This was done using a type of projector known as a Starball, made by the German optics specialist Zeiss. Anyone who has been to a planetarium will recall these massive, insectoid machines with their multi-lensed heads squatting in the centre of the planetarium dome.

‘Starballs give the very highest resolution for star shows but today, in order to get people to visit planetariums and to spend their money and time, you have to offer them something more interesting, so a lot of the content these days is video,’ said Bjernfalk.

Starballs can only provide images of the stars and planets as seen from the Earth; they cannot provide moving images. ‘With a digital projector, you can have images that travel in space and in time, show what the universe looked like three billion years ago from any point — all things that you can’t do with a Starball.’

In the past, Bjernfalk said, a high-end planetarium would have a Starball backed up by six small video projectors around the perimeter of the dome. ‘They would be edge-blended, so between them, they’d paint the whole dome. They would be masked off cleverly so you couldn’t see the joins in the projections — but you’d waste a lot of pixels.’

The main advance with the DigiStar is its resolution which, at 20 million pixels (5,000x4,000), is 20 times higher than a standard digital projector. Coupled with wide-angle optics, this means a single projector, set in the middle of the dome, can provide enough resolution to produce both video and starfields, with no need for the million-pound-plus Starball. ‘I wouldn’t claim it has as much resolution as the Starball; that’s really the reference standard if you want to pinpoint stars with the best resolutions,’ said Bjernfalk.

‘But we’ve crossed over into the territory of “good enough”. The only person who could tell the difference is a discriminating astronomer. The schoolchildren you bring in can’t tell the difference, and wouldn’t care.’

The DigiStar’s high resolution comes from its imaging device, which is a micro-electro-mechanical system called a grating light valve (GLV).

Developed at Stanford University in Connecticut, US, in the 1990s, GLVs are silicon chips that carry an array of reflective ribbon-like structures, each a few tens of nanometres across, that can be moved up and down by a distance of a fraction of the wavelength of light using electrostatic forces.

If the ribbons are set in one position, they reflect all the light that falls on them. In other positions, they allow varying amounts through to a collector, which then sends it to a projection lens.

Other digital projectors use minute mirrors to manipulate light but in these, said Bjernfalk, each mirror produces a single pixel of light. ‘The GLV is a 1D array of 4,000 pixels and, rather than shining a beam of light on the chip, we shine a wide ribbon of light, and all of these ribbons move in parallel, so in effect we paint an entire column of the image of 4,000 pixels simultaneously. So instead of sweeping one poor little dot of laser light back and forth, we’re slowly sweeping a bar of light. Sweep 4,000 pixels across the screen 60 times a second, and that’s where the resolution comes from.’

The GLVs are so small and need such a precise and controlled focus of light that lasers were the only option to illuminate them. ‘We need to focus the light on an almost microscopic area on the chip, so we need light that doesn’t converge,’ said Bjernfalk. ‘We have a red , a green and a blue laser, and a GLV is attached to each one of them. By mixing these colours we can make whatever colour image we need.

‘We had to employ laser scientists to make lasers of our own to get the exact shades we needed because the quality of laser at the power levels necessary for this application just weren’t available.’

For the team designing the Greenwich planetarium, it was as much the quality of laser light as the resolution that swung the decision to purchase an E&S system.

When using a video projector rather than a Starball to provide the image, there is a problem with contrast, said senior design engineer Rob Sneddon.

‘Starballs are purely light hitting a black dome,’ he said, ‘but with a video projector, you’re trying to create the blacks as well as the light images. The contrast ratio with laser light is fantastic, and you get a magical quality to it that’s hard to quantify in terms of contrast ratio, resolution and brightness. You get a sort of sparkle with laser light that you don’t get from any other projection, and that’s ideal for planetariums.’

The projector is linked to a bank of PCs that houses a 3D representation of all the constellations and astronomical objects displayed in the planetarium show, so the image can fly through space. ‘The bad news is that, as with any projector technology it is, at its core, a planar imaging device and all it knows how to do is to paint a 5k x 4k flat image,’ said Bjernfalk.

That is a problem when you want to project a round image on the inside of a hemispherical dome.

The IT portion of the system is therefore crucial. The computers, connected via 16 DVI cables, process and distort the image so that, when it is projected through the fish-eye lens on top of the DigiStar, it appears realistic on the curved surface of the dome.

‘It’s a very exotic lens, actually, with a 160° field of view,’ said Bjernfalk. ‘If you put it just below the horizon line, that field of view fills up the 180° hemisphere. We have optical engineers here who design the lenses — that’s a big part of what we do — but we don’t have a lens-grinding shop on site, so we sub-contract to a specialist.’

Sneddon and his team were designing the Peter Harrison Planetarium at the same time as E&S was developing the DigiStar 3. ‘We were hoping that E&S would come good, because their specification seemed ideal for what we needed. It’s very rare to be in a position like that with new technology — either you don’t touch it with a bargepole because it doesn’t have any track record and you don’t want to be the guinea pig, or you go in too late and you’re left behind. But we think we came in at pretty much the right time. The feedback from the audience has been great.’

Sneddon admits to an admiration for the wow-factor of the Starball. ‘I remember seeing one in Amsterdam maybe 15 years ago, a huge thing like an insect that came out of the floor, and before they’d even turned it on it took my breath away.’

But the DigiStar allows the splendour of the night sky to be the star of the show — and that, as the inventors of Emperor Friedrick’s astronomical tent would have understood, is what planetariums are all about.