SkyView Blog

The GALEX release was accompanied by a few other changes. The one that some users may notice is that for surveys where the data is gzip compressed, SkyView should be a little faster. SkyView was sometimes reading the entire image when it only needed to read the FITS headers.

To support the GALEX survey the image finder code in SkyView was updated. In SkyView an image finder is used to find the input image that should be sampled for each pixel in the output image. A new abstract class RectRecurse encapsulates a basic underlying algorithm which says that if we can find a rectangle in the output image where all of the pixels on the edges of the rectangle should be sampled from the same input image, then we can sample the entire rectangle from that input. This class supports two new settings: MinEdge and MaxRad. The MinEdge setting specifies that input images should not be sampled less than MinEdge pixels from their edges. Similarly MaxRad specifies that they may not be sampled at a distance greater than MaxRad from their centers. Note that the units used are pixels in the input image.

The RectRecurse class is extended by non-abstract classes that define criteria for how to pick the best input image for a single pixel in the output image. Current classes include Border which returns the distance from the edge of the image. This is the default for all but the GALEX survey. ScaledBorder is similar except that it scales the pixel distance by the size of the image. It can be useful if the images in the survey are very different in size. Radius returns essentially the inverse of the distance between the output pixel and the center of the input image. MaxExposure returns the exposure of the input image. That is the default for GALEX (where a MaxRad value is also specified).

When putting these in place a change to the main SkyView class was also made so that the ImageFinder and Mosaicker can be specified in the survey specific settings.

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.