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.

Included in the skyview.util package are two simple utilities that find the coordinate of a pixel in the image or the pixel corresponding to given coordinates. You can use them as:

java -cp skyview.jar skyview.util.CoordsPixel filename.fits 10.3 11.4

to find the pixel in filename.fits that is at coordinates 10.3, 11.4 (in the coordinate system used in the image) or

java -cp skyview.jar skyview.util.PixelCoords filename.fits 512 512

to find the coordinates for a given pixel (using Java counting where pixels begin at 0,0).

One of the major functions of SkyView is mosaicking multiple images together. Occasionally you may be interested in finding out which source image was used for a given pixel. The FITS header indicates which images were used somewhere, but normally that’s all the information you can get.

Recently we added a new class skyview.process.IDMosaic. You can use this class to get exact pixel source information when using SkyView (in local mode). To any request add the setting mosaicker=skyview.process.IDMosaic. This will generate an output FITS file where each pixel is the index of the image used to create it. You can compare this file with the file generated using the normal mosaicker to understand exactly how your image was generated.

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.

SkyView treats a number of projections (Aitoff, Cartesian, Sansom-Flamsteed/Sinusoidal) as fixed projections. Regardless of the position you specify, the sky is projected to the plane in the same way. All that your coordinate entry changes is the center of the image you get. Thus if you ask for an image near the pole in a Cartesian projection, you’ll find that there’s a large distortion since you are near a singularity in the projection. By contrast, projections like the Tangent and Sine projections are centered at the position you specify: that’s the point of minimum distortion.

If you wanted to make an Aitoff or Cartesian map really centered on some point other than the coordinate origin, older versions of SkyView couldn’t help you. With the version we’re releasing today, you can specify the new RefCoords setting. It takes a pair of decimal coordinates and uses that as the coordinate center for fixed projections. E.g., use RefCoords=0.,90. Position=0.,90. projection=Ait to ask for an Aitoff projection centered around the pole. Note that you still need to specify a position.