How the WorldWide Telescope works

On my Perspectives show last week, Curtis Wong and Roy Gould relate the history and educational mission of the WorldWide Telescope. On this week’s show, principal developer Jonathan Fay describes how the underlying technologies enable the WWT’s seamless view of the sky.

There were a bunch of things I wanted to know, including:

How does the WWT project build on, and extend, the SkyServer project to which Jim Gray made fundamental contributions?

What standards and protocols enable the various sky surveys to be woven together?

What’s the relationship between Deep Zoom and the WWT’s own scheme for managing and viewing tiled multi-resolution imagery?

How much of the data is stored on Microsoft servers, how much is stored elsewhere, and in what ways do the supporting data services cooperate?

Jonathan answers all these questions, and he also answers one I didn’t think to ask:

What technique is used to project the stars onto an imaginary sphere at near-infinite distance?

The answer to that last question is that a new kind of spherical projection had to be invented:

Imagine taking a round room, and trying to put a bunch of bathroom tiles on it, and grout it. The tiles seem to come together and have parallel lines for a while, but eventually it stops working well. Maybe you can take one line around the equator, but as you go up you have fewer tiles, and weird-shaped tiles, and nothing lines up.

That’s the problem we have. We’re looking at spherical data, so we had to come up with a new spherical transform that preserves the poles. In previous projects, like Virtual Earth or TerraServer or Google Earth, the poles weren’t important, because nobody lives there and nobody needs map directions for driving around there.

So we had to come up with something called TOAST: tesselated octahedral adaptive subdivision transform. It creates a 360-degree wraparound view that’s either a planet surface or the infinite sphere of the sky, and lets you represent it using a 3D graphics accelerator, very rapidly and efficiently. So we can have an image pyramid the way Deep Zoom does, and TerraServer before it, but we don’t have to give up the poles.

This transform isn’t proprietary, and in fact it’s being applied to the 50-odd full-sky surveys hosted at NASA’s SkyView virtual observatory. The implications are pretty astounding. This imagery is stored in astronomical databases using what’s called tangential projection, which suffers from polar distortion when combined into large mosaics. Now the imagery can be combined into large mosaics — or indeed a complete view of the sky — and seen without distortion. What’s more, multiple surveys can be aligned to that spherical projection. That’s why, in WorldWide Telescope, you can cross-fade between a view of the Milky Way in visible light and views in infrared or ultraviolet light.

What the WorldWide Telescope really is, Jonathan says, is a browser, like a web browser but for an information space defined in astronomical terms. Here’s how he sums up the work that was necessary to make that possible:

The vision of getting everybody access to all this astronomy data required systematic changes at every single level. We built on some things that Jim pioneered with NVO, and worked from there, but it was very systematic. How people process the data. The client to access the data. The protocols over the wire. Educating people, providing the context for it.

We put a lot of things together, but we also created a systematic model for how to do everything end to end, top to bottom, left to right. Now there may be other people who use the pieces that we’ve created, and then change them to use different data sources, different visualizations. Say someone creates a Mac client, or an iPhone client, that’s possible. Or a mobile phone version of it, or a web-based version. Over time we or others can replace various components, but as a reference model for solving all the problems in order to get the data into people’s homes and into their eyeballs — you had to solve for all of those problems, otherwise people are still blocked from being able to really explore.

For Curtis Wong, the WWT is an extension of John Dobson’s sidewalk astromony — a way to bring telescopes to the public and to enable astronomers to share their knowledge of the sky with everybody. For Jonathan Fay, it’s the perfect application of earth and sky visualization technologies he’s been developing throughout his career. Their interests and talents combined, as Jonathan says, like peanut butter and chocolate:

Curtis had been exploring how to create an educational environment with rich tools for exploring space, and he’d been collaborating with Jim Gray on TerraServer, and now he was looking for the technology to make it possible.

Here I had this technology, and was looking for somebody who was enthusiastic about having a purpose for it. So it was the peanut butter and chocolate moment.


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