One of the images that had gotten temporarily concealed in our Gallery was
DSS2 Red: 2nd Digitized Sky Survey (Red)
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There are a couple of weird things going on here. There’s a galaxy which seems to be cut off with a sharp edge. And the quality of the image seems to change abruptly along two lines. What’s going on here?
This is a DSS2 image. The DSS2 survey data are Schmidt plates, about 6 degrees on a side. Especially in the southern hemisphere there’s a fair bit of overlap on the plates, and a single region may be covered by multiple images. For each pixel SkyView uses the image where that pixel is the furthest from the edge of the plate. It looks like this region has data from at least 3 plates. What we are seeing is the boundaries between the plates. The image in the top right corner is deeper than the one on the center, so that it shows the outer region of the galaxy while the central image only shows the core.
Usually SkyView does a better job of joining DSS2 images. Since the 0 point of the plates is pretty arbitrary, SkyView normally looks at the image boundaries and adds an offset to each image so that as we go from one image to another the median shift is 0. This ‘Edge reduction’ option can be turned off. For most surveys that’s the default, but it is turned on by default for some of the optical surveys including the DSS. If we turn it off the image becomes
The boundaries are even more pronounced now. In this case maybe that’s better, since it makes it clearer what’s going on. Most of the time, though, the Edge reduction does a pretty good job of matching the images. The problems tend to arise when there are strong intensity gradients over one or more of the images. We’re only matching a 0 point and not a slope so even if these intensity gradients are real rather than some background effect, the slopes in the different images may not match.