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SkyServer Frequently Asked Questions
Frequently Asked Questions
  1. What is SkyServer?
  2. What is the Sloan Digital Sky Survey?
  3. How do I get around the site?
  4. What can I see on SkyServer?
  5. Where in the sky do these images come from?
  6. How can I get all this information?
  7. What help is available?
  8. How can I use SkyServer to learn about astronomy?
  9. What is the difference between Target and Best?
  10. What is the difference between SpecObj and SpecObjAll? What does sciencePrimary mean?
  11. What are the differences between PhotoObj, PhotoTag, and PhotoObjAll?
  12. How do I get photometry for spectroscopic objects?
    What is the SpecPhotoAll table?
  13. What is the difference between SpecPhoto and SpecPhotoAll?
  14. Why do z and zErr (in SpecObj) have different numerical precision?
  15. Who is responsible for all this?!

  1. What is SkyServer?
  2. SkyServer is the education and outreach web site of the Sloan Digital Sky Survey (SDSS). SkyServer makes the entire survey available, free of charge, to the public. With SkyServer, you can study exactly the same stars and galaxies that professional astronomers are studying right now.

  3. What is the Sloan Digital Sky Survey?
  4. The SDSS is one of the most ambitious scientific projects of all time. Its goal is to make a high-quality three-dimensional map of the universe. The survey uses a specially-built 2.5-meter telescope in New Mexico, a CCD camera, and sophisticated software to store and analyze its data.

    The SDSS began in June 1998. By the time it ends, the SDSS will have mapped 25% of the night sky, taking images of over 100 million objects. The survey has already completed a preliminary map of the universe: you can see the map in our First Discoveries section.

  5. How do I get around the site?
  6. The main menu bar near the top of the screen (from Home to Help) is always available to navigate the site. In addition, when you browse using the menubar, a side menu comes up to help you navigate different parts of the site.

  7. What can I see on SkyServer?
  8. SkyServer offers two types of data: images and spectra. Images are pictures of the night sky taken by our digital camera. Spectra are measurements of the amount of light a star or galaxy gives off at different wavelengths. SkyServer has images of more than 80 million stars and galaxies, and spectra for more than 180,000. For more on our data, see the Getting Started pages.

  9. Where in the sky do these images come from?
  10. The SDSS takes data in long, narrow "stripes." The current data includes several stripes near the celestial equator (the Earth's equator projected into the sky), and several in a C-shaped band of the northern sky. See the DR1 Sky Coverage page for maps and tables of our sky coverage. To see if a specific area is part of the SDSS, enter its coordinates into the Finding Chart.

    Over the next few years, we will add more data to SkyServer until we have about one-quarter of the sky covered.

  11. How can I get all this information?
  12. Use one of SkyServer's tools. Each tool is designed to access a single type of information - see the Getting Started pages for more details.

    Here is a quick summary of what the most commonly-used tools do. Famous Places is a gallery of beautiful SDSS images. The Navigation tool lets you point and click through the sky. The Object Explorer gives you access to complete data on a single star, galaxy, or quasar. The Search tools let you see data on all objects in a certain part of sky. Or, if you know SQL, you can use the search tools to return all objects that meet whatever criteria you can think of.

    Did you get all that? The best places to start are with the Famous Places and Navigation tools. And don't worry; all the tools have Help pages.

  13. What help is available?
  14. SkyServer has an extensive Help section, including a Glossary and How-To Tutorials. The Schema Browser is essential for using the Search tool. SkyServer's About Astronomy and About SDSS sections also contain readings that help explain concepts from astronomy and the SDSS.

  15. How can I use SkyServer to learn about astronomy?
  16. SkyServer's Projects use SDSS data to interactively teach astronomy. With our Projects, you can learn about the evolution of stars, the types of galaxies, the history of the universe, and much more.

    Teachers, we welcome you to use and adapt any of our projects in your classes, free of charge. For more information on what you can do with SkyServer in the classroom, see our Teacher FAQ.

  17. What is the difference between Target and Best ?

    Because the survey imaging and processing is always being improved, the SDSS often obtains improved photometric measurements of objects AFTER they have been chosen ("targeted") for spectroscopy. However, it is important to keep a record of the photomeric measurements at the time objects were targeted. We therefore maintain two versions of the photometric catalog:

    1. Target:

      Known as TARGDR1 in the DR1 CAS, this database contains the photometric catalogs AS THEY WERE WHEN OBJECTS WERE CHOSEN FOR SPECTROSCOPY. This database contains the union of all target chunks. It may cover a slightly different area than Best; blended objects may be deblended differently; image quality may be worse; photometric calibration may be less accurate. However, if you want to see what the SDSS thought the magnitudes and other properties of an object were when it was chosen for spectroscopy, this is the place to look. Note that this database DOES NOT contain links from the photometric objects to the spectroscopy (you can always get the Target data for spectroscopic objects using the TargObjID field in the SpecObjAll table), nor does it contain the tiling information. This is because the Target database is intended to be a snapshot of the survey before any spectroscopy is done.

    2. Best:

      Known as BESTDR1 in the DR1 CAS, this database contains the latest, best versions of the imaging data, processed with the latest version of the photometric processing software, and with the most recent understanding of the photometric calibration applied. For any science based on object photometry, you will want to use the Best data. In addition, only the Best database contains all of the spectroscopy and tiling information.

  18. What is the difference between SpecObj and SpecObjAll? What does sciencePrimary mean?

    The SpecObjAll table contains ALL spectroscopic objects, regardless of their status in the survey. Queries on this table can produces unusual or undesired objects. Thus, we have created the SpecObj view, which contains data for ONLY those fibers defined as SciencePrimary. To be SciencePrimary, an object must meet all of the following criteria:

    1. It was targeted in the target skyVersion;
    2. The plate on which the spectrum was taken is the primary observation of that tile;
    3. The plate was a main survey plate (not part of the Southern survey or a special project);
    4. The objType is not QA, SKY, or SPECTROPHOTO_STD (these object types are repeatedly observed);
    5. The fiber was mapped correctly (this is the zWarning check)

    As a result, some plates may have many (or even all) of their fibers excluded from SpecObj. Some of these instances are:

    • They were targeted but are outside the official survey boundaries. These objects cannot be matched to a PhotoPrimary in the target. Plates 344-346, 348, and 364 (tileRun 6) have 0 SpecObjs. Other plates from tile runs 4 and 6 have reduced numbers of objects, especially 315 and 342. This could affect plates 266-315, 363, 361 from tileRun 4 and plates 342-348, 364 from tileRun 6.
    • Fibers where objType = 'SKY' do not show up in the SpecObj view. Some plates (for undetermined reasons) have large numbers of sky fibers: Plate 417 has 214 sky fibers, and they all seem to be on one half of the plate; Plate 595 has 91 SKYs and Plate 359 has 84 SKYs.
    • Fibers where objType = 'QA' also do not show up in the SpecObj view. Below is a table of the plates with more than 100 QA fibers and the number of such fibers:
      Plate# of QA fibers
    • Note: The definition of SciencePrimary relies purely on spectroscopic and geometric considerations. There are objects in SpecObj which do not have a corresponding Best photometric object (406 in DR1).

  19. What are the differences between PhotoObj, PhotoTag, and PhotoObjAll?

    PhotoObjAll is a table in the Best and Target databases which contains all of the measured photometric quantities for all of the imaging objects. Because we measure so many different parameters for each object, this is a very large table, and queries can take very long to run.
    In an effort to speed up queries, we have created a table with only a subset of the parameters (a "thin table") that are requested most often. This table is called PhotoTag. If you have a query which uses and returns only values stored in PhotoTag, it will execute much faster than if you used PhotoObjAll.

    In addition, we have created a view ofPhotoObjAll which contains only those objects which are Primary or Secondary. This view is called PhotoObj. Because this view effectively contains fewer objects than PhotoObjAll, but all the measured quantities for these objects, queries will execute faster.

    Given the above, a user should:

    1. Query from PhotoTag if it contains everything you are looking for;
    2. Query from PhotoObj otherwise, UNLESS you are interested in data for objects which are neither PRIMARY nor SECONDARY. In that case, you will need to use PhotoObjAll.
    3. Importantly, the "shorthand" quantities u,g,r,i,z do not exist in the PhotoTag table. Instead, you must use ModelMag_[ugriz], which is indexed to make queries faster. HOWEVER, in PhotoObjAll and its views, only the u,g,r,i,z are indexed, and not the ModelMags!

    Because PhotoTag has many fewer parameters, larger portions of it can be cached, improving performance. We have found that for almost all queries which contain parameters fully in PhotoTag, it is faster. In the case of looking for objects that are detected multiple times, one could perform a join on PhotoTag with itself, requiring that one object be Primary and the other Secondary.

  20. How do I get photometry for spectroscopic objects? What is the SpecPhotoAll table?

    The SpecPhotoAll table is a precomputed join between the Best PhotoObjAll and SpecObjAll tables. It includes the most requested parameters from these two tables, as well as a few pieces of information about tiling. It also includes the TargetObjID, which allows user to retrieve the Target version of the photometry.

  21. What is the difference between SpecPhoto andSpecPhotoAll?

    As described above, The SpecPhotoAll is a precomputed join between the Best PhotoObjAll and SpecObjAll tables. This includes all non-science objects, and a variety of objects many users will not be interested in. The SpecPhoto view includes only those pairs where the SpecObj is a sciencePrimary (see the definition above), and the BEST PhotoObj is a PRIMARY.

  22. Why do z and zErr (in SpecObj) have different numerical precision?

    Internally, these numbers are stored to their full precision as they come out of the spectroscopic pipeline. When you perform a query, they have some default string format applied that cuts them to what you see. But you can use the str() function to change the string format to whatever you like.

    To get z to 6 decimals, e.g., change your query to 'select str(z,8,6) as z' instead of just z, and analogously for zErr. This applies the function str() to the values in column z and returns the result with column label z (without the "as", the result of a function has no column label). The str(col,length,dec) function takes the numerical value in 'col' and formats it as a string of length 'length' and with 'dec' significant digits, i.e. it's the SQL equivalent to the C function printf("%8.6f",z). str() rounds the result to the number of decimals you request.

  23. Who is responsible for all this?!
  24. See our Credits page.

Do you have other questions that aren't answered here? E-mail them to us!