Types of Data: SDSS Photometric and Spectroscopic Data

The only information we get from stars, galaxies, or collections of stars in the sky comes to us as some form of light. We are all familiar with the rainbow of colors that make up the spectrum of visible light, but light is much more than that. Some forms of light, or electromagnetic radiation, are invisible to our eyes but can be measured by instruments.

When we refer to “two types of data” in the SDSS, we are referring to the light measurements we gather when two different instruments are attached to the telescope.

Photometric Data

When you are looking at beautiful images of galaxies in Navigate you are looking at the results of light that is captured when the SDSS camera is attached to the telescope. The camera records 5 images at a time using 5 different colored filters. Measurements of the amount of light in each filter are recorded. These data are called photometric data (“photo” = light; “metric” = measurement). Photometric data can be gathered from any place in an image, but that does not mean that SkyServer reports measurements for every possible location.

To see where photometric data have been recorded:

Zoom in on any galaxy in Navigate.
Select the Photometric objects checkbox in the Drawing options box.

You can select any one of the circles in the image to find out more about the measurements. What parts of this galaxy appear to be of most interest to astronomers?

Clicking on any one of the circles indicated on the images displays the photometric measurements of magnitude in the selected object box. See Pre-Flight Training – What is Magnitude? for help understanding these numbers. All you need to know for now is that they are measurements taken from the image itself.)

Next we look at the measurements taken with a different instrument, the spectrograph.

Spectroscopic Data

Return to Navigate to view the objects that have spectra recorded. Let’s start with a galaxy.

Zoom in a few clicks so you can see some detail.
Select the “Objects with spectra” checkbox under the “Drawing options” menu. Navigate should reload automatically.
A red box will mark any objects that have spectra. You should notice a huge difference in the number of spectroscopic objects verses the number of photometric objects. Can you think of an explanation?

With “Objects with spectra” still selected:

Select “Plates” from the “Advanced options” menu. This shows you the boundaries of the aluminum plates that hold the optical fibers which allow the light from individual objects to travel to the spectrograph. See Pre-Flight SDSS Plates for more details.
Zoom out until you see one or more plate boundaries in your field of view.
You may want to use the direction buttons (N,S, W, E) to see if you can get an entire plate in the viewing window.

Finally, let’s find the spectrum that is one of the final products of the light recorded by the SDSS spectrograph.

Select the “Quick Look” radio button on the right-hand side of the page. This takes you to a page that contains a summary of some of the data available for that object. You will learn more about using “Quick Look” in other activities. (For immediate help, go to the Quick Look Main Window and Sidebar pages in the Help section.) Observe the spectrum, and note that you can see a larger image by clicking on the plot with the spectrum.

There is a lot to learn about spectra. For now, it is enough for you to know that:

A spectrum is the final data product of the SDSS spectrograph.
You can access the spectrum for an object through the “Quick Look” link in Navigate.
All the pieces of information that went into making the graph exist in tables of numbers in the SDSS database.

Just for Fun!

From your previous location in Navigate, check the Photometric objects box once again. You should be completely amazed by the amount of data recorded for a patch of sky no bigger than the width of three full moons!