Types of Data

Types of Data: Photometric and Spectroscopic

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 stars and galaxies in the SDSS SkyServer 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 explore the recorded photometric data:

  • Zoom in on any object (star, galaxy, etc.,) you can see
  • Select the “Photometric objects” checkbox from the drawing options

Stars are often confined within a single circular region; while larger “extended” objects like galaxies often have photometric measurements sampling multiple parts of their structures. In the galaxy shown on the right, dozens of bright regions within the galaxy’s spiral arms have been measured and cataloged.

You can select any one of the circles highlighting photometric measurements in a SkyServer image to find out more about the regions of space they encircle. Clicking on any one of the circles displays the photometric magnitude (brightness) measurements in each of the 5 SDSS filters. See the pre-flight description of SDSS magnitudes for more information about what these numbers mean.

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

Spectroscopic Data

As detailed in the pre-flight section on spectra, we can learn much more about astronomical objects — including what they are made of and how far away they are — if we observe their light with a spectrograph. The resulting spectroscopic data (i.e., spectra) provide measurements of an object’s flux (i.e., brightness), sampled in many smaller bins of wavelength. The SDSS collected spectra for up to 1000 different objects at once using specially drilled aluminum plates. You can learn more about those plates here.

SkyServer Navigate also provides a portal to the spectroscopic data of the SDSS. Let’s return to the galaxy in our example above to explore its spectroscopic properties.

  • Select “Objects with spectra” from the “Drawing Options menu.
  • Red squares will mark any objects that have been observed with the SDSS spectrograph. (Note that there are many fewer spectroscopic targets compared to the photometric objects in any area of the sky. This is because it is too expensive and time-consuming to obtain spectra for every bright object in the survey, and so only a fraction are observed this way.)
  • Click on the red square at the center of an object you’re interested in to explore its spectroscopic data in more detail.
  • The black and white plot in the bottom right-hand corner provides a snapshot of the spectrum obtained for the object. Click on the plot to expand it.

You can also click on “Analyze Spectrum” to see the plot in more detail and interact with the data:

To better appreciate how the spectroscopic data for this object was collected, return to the SkyServer Navigate window:

  • Select “SDSS Plates” from the “Advanced Options” menu. This will overlay the boundaries of the aluminum plates that hold the optical fibers that carry light from individual objects from the telescope to the spectrograph.
  • Zoom out until you see one or more spectroscopic plate boundaries in your field of view
  • You may want to use the direction buttons (N,S,W,E) to see if you can see an entire plate in the viewing window.
  • Each circular plate boundary is 3 degrees in diameter, which is equivalent in width to 6 full moons placed side by side.
  • To learn more about how thousands of SDSS plates were designed and used to tile the sky, visit the SDSS website.

Just for Fun!

From your previous location in SkyServer Navigate, check the “Photometric objects” box once again. You may be completely amazed by the amount of data recorded within the boundary of a single plate.