SkyView Blog

This image from 2009-04-27 22:10:22 seems to show a lot of disks in the sky. What’s going on here?

The image is taken from the ROSAT pointed observations. The ROSAT mission comprised two phases. In an initial phase the satellite scanned the entire sky producing the data SkyView shows in the RASS surveys. After the all-sky survey was done, ROSAT made much deeper observations of particular points in the sky. Each observation looked at a circle roughly a degree in radius. These observations are the source for the ROSAT PSPC surveys in SkyView. We took all of the observations and added them up to create all sky tiles. Only about 20% of the sky was covered in total, so a very large scale image, like this one, shows the observations as little disks in the black, unobserved, background.

The GALEX survey is very similar, though the overall sky coverage is substantially higher. However while we created a set of static tiles for ROSAT, with GALEX we add the observations together dynamically when the user makes a request. A big part of the reason is that computers have gotten much faster in the 10 or so years since we built the ROSAT surveys, so its much more feasible to do this kind of dynamic addition of data today.

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.

A new survey including seven bands of data from the ROSAT All Sky Survey data has been added to SkyView. The RASS Background 1-7 data show the entire sky in diffuse X-ray emission. Point sources have been subtracted out. This new survey replaces the RASS 1/4, RASS 3/4 and RASS 1.5 KeV maps. Those data are still available but we no longer display the survey names on the Web page. You can still access them through the batch interfaces or using the Jar file. The new data sets have better spatial and energy resolution and were built by the Steve Snowden who also created the earlier maps.