Circular images, GALEX and Image Finders.

In the past week we’ve begun the process of adding the GALEX near and far UV data into SkyView. Assuming we don’t run into unexpected problems it should be available sometime next week. One issue that did come up is that GALEX images are circular not rectangular. Normally when we look for which image to sample at a given pixel we use the candiate source image that we would sample furthest from the edge of the image. That’s the Border image finder. For GALEX a more appropriate choice is to take image whose center is nearest the pixel. There’s a new Radius image finder for that. Since the exposure and characteristics of the observation don’t vary very much within the observed circle, a still better approach would be to find the image where the pixel is within some fiducial radius of the center, but which has the longest exposure. That way we get the best image over the largest field of view. That’s a combination of the Radius and Exposure image finders in the current release. By design it’s very easy to add in an image finder with exactly these characteristics and that’s what we’ll be doing.

You may wonder why this didn’t come up in the much older SkyView ROSAT PSPC surveys — they also have circular images. If we were to build images from the PSPC the same way we do from GALEX, by dynamically combining observations in response to a user request, that’s exactly what would have happened. However SkyView ran through all of the PSPC data and created a set of rectangular tiles that added the exposure from all observations that overlapped the tile. It’s these pre-coadded tiles that are used for the PSPC surveys. An advantage of this approach is that in regions where more than one observation was made, data from multiple tiles is added together. We’ll want to make that possible for GALEX data someday too.

This entry was posted in Discussion and tagged , , , . Bookmark the permalink.

2 Responses to Circular images, GALEX and Image Finders.

  1. David Black says:

    I was very glad to discover this blog. I recently discovered an interest in galex when a friend of mine showed me an article on the Galaxy Evolution Explorer.

    In particular the part that said During its nominal 29-months mission it makes observations at ultraviolet wavelengths measuring the history of star formation. An incredible 80% of the way back to the big bang. This is an incredible 10 billion years of cosmic history.

    After reading this I was blown away and realised how incredible the universe really is. Your blog will allow me to broaden my horizons further.

    Incredible blog. Many thanks

  2. Tom McGlynn says:

    Over the next month or two we should be putting the WMAP all sky images into SkyView. WMAP shows effects of the first few moments of the universe’s existence, though the light comes primarily from the time when radiation and matter decoupled about half a million years after the big bang. Even if we use that time WMAP looks back across more than 99.99% of the age of the universe.

Leave a Reply

Your email address will not be published. Required fields are marked *