AVM: Placing images in the Sky in WorldWide Telescope


WorldWide Telescope Web Client showing results of clicking View in WWT link in Flickr Astrometry group


One of the cool things about WorldWide Telescope is it is not just about the images and data that are built in to the WWT catalog, but about brining in data from where ever it lives. Images are especially interesting as they may be pictures of the sky and belong in a specific alignment in the sky. For images in the file format used by professional astronomers call "FITS", there is built in support for a standard called WCS, or World Coordinate System. WWT can load these images and place them in the sky. For random PNG and JPEGS there is not a build in way of storing sky coordinates, so that is where AVM comes to the rescue.

AVM, or Astronomy Visualization Metadata, is a standard created by the Virtual Astronomy Multimedia Project (or VAMP for short). AVM uses specially designed XML tags to tag, the Who, How, Why and When of the image to allow astronomy press release images to retain all the information relevant to their use and interpretation no matter where the images might roam. The AVM data also will tell you where you can get the original image at full resolution and what rights you have to use it.

Adding AVM data to an image is not trivial, but with the right tools it's not hard either. The tagging tools on the AVM website (http://www.virtualastronomy.org) and the tagging tools in Photoshop when used with the AVM plug-in and optionally FITS liberator, can made the process pain free, but this process is usually the domain of the image publisher, not an ordinary user. The process also requires the publisher to know all the WCS data and other metadata. If you don't know this data, then how do you get it?

There at least two other ways to get the image aligned in the sky. One is Astronomy.net. Astronometry.net has a convenient service in that if you post a image to Flickr and tag it with "Astrometry" it will grovel the images with those tags and solve the alignment and add metadata to the tag list, including a link to view the image in the WWT web client. The alignment data is *NOT* added to the image, like AVM, but instead is encoded in the viewing link. Since the viewing link actually creates a WTML file on the fly, you can use that part of the link to create a WTML file to view that images directly in the desktop version of WWT. To do this you need to add the text "&wtml=true" to the end of the Flickr WWT link and paste it into your browser. When loaded, it will pop up that image and allow show it to you in the sky in WWT.

At this point I should mention that using a huge image as the source of data in WWT is not the preferred way of distributing images. WWT is mind blowingly fast because it in fact almost exclusively deals with images that have been pre-transformed into tiled multi-resolution image pyramids. In other words when a astronomical image first comes into view, all you need is a very small thumbnail sized image of it to view it, and when you zoom deeply into the image, you only need a few small chunks of the highest resolution data of that images, but you never can look at more than a screen-full at a time, so why force the computer to download the whole image before the user can see anything? Using this trick WWT can completely hide any network latency from the users notice. When you prepare you images for mass use you should tile them with the WWT image SDK, rather than use monolithic images. It's kinder to your viewers!

The second way to view an image on the sky that does not have WCS or AVM data embedded into it is to use the Explore...Load... Astronomical Image menu. First you need to zoom in to the area of the sky where the image is located. Then select the menu item to load the image, and find your file. It will display a message about not finding alignment metadata, and then load the image into position on the Sky where you are pointing. Now Click Ctrl+E and it will bring up an alignment menu. You can either use the keys and mouse to align the image by hand or drag the image so that one star in the new image aligns directly with a star in the background image. Then right click directly on the corresponding stars. This activates the two star align mode. You can now drag any other star and align it to its representation in the background image. A couple of iterations of this might be needed, but WWT will rotate and scale the images so the stars correspond in each image. With just two stars aligned, you should have a very close match almost everywhere else. You can use the B" key to blink the images and compare the location, then click Ctrl-E to exit the edit mode.

When you are done with the alignment you can right click on the image thumbnail in the explore tab and save it you a new or existing collection. The WTML file for that image collection will be automatically saved in your Documents folder under WWT Collections.

If you want to share this images with friends, you can put the original image on the web and edit the image location in the WTML file (using a text editor or XML editor) and put the URL to your image on the web and save the file. Now that WMTL file points to an image everyone can see, and your alignment metadata is saves. Images can also be used in tours, but remember Tiled images load much faster that monolithic ones, so use this method sparingly, or when instant loading is not as important.



Flickr astrometry group http://www.flickr.com/groups/astrometry/

AVM/VAMP Website (http://www.virtualastronomy.org)