Photometric Objects are all of the objects in the SDSS database that have information associated with them that was obtained directly from the image. These data points are marked with blue circles.
When we look at an SDSS image in the SkyServer, we are seeing the total light captured in multiple color filters which have been combined to make a color image. The SDSS makes photometric measurements for each object in each of its five filters. Images from three of these filters, “g,” “r,” and “i,” are combined as proxies for “blue,” “green,” and “red,” to make the color images shown in the SkyServer.
Using the photometric data available from each filter, astronomers are able to make calculations about the brightness of an object in select colors along the spectrum. A measurement of brightness that astronomers use to compare one object to another is magnitude. The magnitudes calculated for the object that is selected (as indicated by the green box) are reported in the Selected object box to the right of the Navigate window. See the Selected object for more information about the information in this box.
Observe! If you look at the image of the galaxy on this page, you notice that it is covered with blue circles. Because the galaxy is so large, the SDSS telescope was able to gather photometric data from many different points. Do you notice a pattern? What kinds of questions could you ask about galaxies if you knew this kind of data were available?
