Sloan Digital Sky Survey's SkyServer: A Vast Collection of Celestial Images and Information

In my last article I wrote about online telescopes that allow users to control telescopes from their personal computers. Most of these allow users to actually control a telescope in real time or to schedule images in a queue. Some of these were not real telescopes at all, though; they are collections of images that users can request as if they are directing a telescope. The Sloan Digital Sky Survey's (SDSS) SkyServer is such a great resource because it can be used as such, but also gives users a large variety of options for requesting data.

The SDSS website states that their goal is to "map one-quarter of the entire sky and perform a redshift survey of galaxies, quasars and stars." In simpler terms, they collected images and information on a large portion of sky and organized it into a database. Information was gathered and compiled in multiple releases, the most recent and final being "Data Release 7" (DR7). SkyServer is a collection of tools allowing users to retrieve images and other data from their publicly accessible catalogue of more than 350 million celestial objects, and spectra of 30,000 galaxies, 120,000 quasars, and 460,000 stars.

The data comes from three surveys, named Legacy, SEGUE (Sloan Extension for Galactic Understanding and Exploration), and Supernova. Legacy is a five-band image survey over 8,423 square degrees of the sky in the Northern and Southern Galactic Caps, as well as spectroscopy of complete samples of galaxies and quasars. SEGUE includes images of additional areas of sky at lower galactic latitudes and spectroscopy of 240,000 stars from both the Legacy and SEGUE footprints. These images are used to study the structure of the Milky Way. Images in Supernova are the equivalent of 80 repeated redshift imaging scans of the Southern Equatorial Stripe, which are used to search for supernovae.

The SDSS is important to space science because it has a large amount of easily accessed images and data for research. Currently, data from the SDSS archive is being used to test a theory about how large galaxies like ours form. If these galaxies grow by absorbing smaller galaxies, as is suspected, there should be black holes roaming through the galaxy that didn't absorb into the central black hole. SDSS data is being used to detect these black holes, if they exist.

Like the websites I wrote about previously, SkyServer can be used as if it is a real-life telescope you can control online, but with faster results than an actual telescope. You can retrieve all publicly available data from the SDSS through SkyServer: spectra, photometric data, and spectroscopic data, and – of most interest to those using it as a telescope – images. Other types of data are also available for more advanced research.

The main way to get images from the SDSS is SkyServer's Get images feature. These images are organized into 24,000 pixels wide “stripes,” which can be retrieved by fields, spectra, or spectral plates.

• The Fields option gives you 2,048 pixels by 1,489 pixels sections of “camcols,” each camcol being an array of CCD detectors.
• Spectra retrieves measurements of energy given off by an object by its wavelength, presented visually.
• Viewing images by Plates gives images of 640 spectra measured simultaneously over a 3 degree wide area from aluminum disks with 640 optical fibers, each pointing to an object being observed.

The Scrolling sky option also allows you to use SkyServer as if it were a telescope. This option appears more like a live telescope than Get images. After picking a specific run/camcol combination, you can watch an image of the sky scroll past at approximately 1 arcseconds per pixel. This animation is the equivalent of what you would see if you were looking though the SDSS camera as it passed through the region of sky taking pictures. Scrolling sky is the closest simulation of looking through a telescope in person that SkyServer offers.

Choosing Famous places presents you with a selection of categorized famous celestial objects divided into six sections: galaxies, spirals, clumps, clusters, interactions, and artifacts. This option has many possible uses. If you know the name of a famous object but not the coordinates or other information to find it yourself, you can look for it there. If you need to research an interesting celestial object but have not chosen one – such as for a class assignment – Famous places offers you many possibilities. This option can also just be used for casual browsing, enjoying the selection of self-described “beautiful images seen by the SDSS telescope.”

SkyServer also offers various visual exploration tools that make it easier for you to find and view astronomical data, including:

• Quick Look: Basic information on an object: position, magnitude, and redshift, searchable by object id, ra/dec, 5-part SDSS Id, and plate/MJD/fiber number.
• Finding Chart Tool: Allows you to create a finding chart of a specific section of sky. This chart can help an amateur astronomer find objects through a home telescope. It can also be used to get a picture of an object or region of sky.
• Navigate: This tool helps find objects to observe. You input right ascension (ra) and declination (dec) coordinates. When the “Get Image” button is pressed, the tool shows that area of sky. The objects within this area are clickable, showing a close-up image of the object and a summary of data on the object, including position, type, and the magnitude in each of the five filters of the SDSS. Objects can be saved in your SkyServer "notebook." Detailed information on the object can be found in the Object Explorer by clicking “Explore.”
• Image List Tool: With this powerful tool you can input hundreds of SDSS imaged object positions at once and quickly look through thumbnails to find objects you find interesting.

More advanced users can search the SDSS database with a query. Even with this more advanced option, there are query submission options with different levels of difficulty. These include:

• Easy
  • Search Form: The easiest option, which translates an English sentence into an SQL query.
  • Rectangular: Searches a rectangular region.
  • Radial: Searches around a given point.
• Hard
  • Query Builder: Builds an SQL query based on chosen menu option. SQL: The hardest option, in which you type an SQL query directly.

For scientists, SkyServer also offers to even more advanced options: Imaging Query for imaging data and Spectro Query for spectroscopic data.

There are more tools that SkyServer offers for using the SDSS archive than what I have described. Enough options are offered that no matter what your interest level is or your astronomical skills are, there is probably an option for you. I have used SkyServer for two months and have yet to exhaust the options or get near viewing all of the images available. Check it out – it may become the last astronomy website you will ever need.