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. Which query interface or tool should I use?
6. Where in the sky do these images come from?
7. How can I get all this information?
8. What help is available?
9. How can I use SkyServer to learn about astronomy?
10. What is the difference between Target and Best?
11. What is the difference between SpecObj and SpecObjAll? What does sciencePrimary mean?
12. What are the differences between PhotoObj, PhotoTag, and PhotoObjAll?
13. How do I get photometry for spectroscopic objects?
    What is the SpecPhotoAll table?
14. What is the difference between SpecPhoto and SpecPhotoAll?
15. Why do z and zErr (in SpecObj) have different numerical precision?
16. What is the difference between specClass and objType for spectroscopic objects, and which one should I use?
17. Why does SDSS use the long (64-bit) objID fields, and what is the composition of the PhotoObj objID and SpecObj specObjID fields?
18. I want to mirror the SDSS archive - how can I get a copy of all the data?
19. Where can I get a copy of the HTM (Hierarchical Triangular Mesh) spatial index library?
20. Who is responsible for all this?!

1. What is SkyServer?   Top

   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.

2. What is the Sloan Digital Sky Survey?   Top

   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.

3. How do I get around the site?   Top

   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.

4. What can I see on SkyServer?   Top

   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.

5. Which query interface or tool should I use?   Top

   There are several levels of query interface available in SkyServer, and it can be quite confusing to choose the one that is appropriate for you. Your choice should depend on what exactly you want to do, and how comfortable you are with SQL (Structured Query Language) - the standard language used to query relational databases.

   If you are new to SQL and would rather not use it, you should restrict yourself to the form-based interfaces available on the SkyServer Search page. Use the radial search to do a simple radial (cone) search, and the rectangular search to search a rectangular area of the sky. For more sophisticated searches but still without using SQL, you should use the Imaging Query form and the Spectro Query form. These will let you build quite complex SQL queries by filling out a query form. Each of these form-based interfaces will help you learn SQL, since they display the actual SQL that is sent to the CAS DB server when you hit the Submit button. You should also look at the Introduction to SQL page for help on SQL.

   Once you become more familiar and comfortable with SQL, you should take a look at the sample SQL queries to see examples of real SDSS science queries. Then you can use the SQL-based interfaces and tools. To run short, not too complex SQL queries, you can use the SkyServer SQL Search page. You may also use the Emacs interface or the command-line sqlCl interface. To submit longer, more complex SQL queries that may take more than a few minutes to complete, you should use either the CasJobs batch query workbench (recommended), or the downloadable sdssQA Java client. However, note that the sdssQA query tool has not been actively supported beyond DR4.

6. Where in the sky do these images come from?   Top

   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 DR4 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.

7. How can I get all this information?   Top

   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.

8. What help is available?   Top

   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.

9. How can I use SkyServer to learn about astronomy?   Top

   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.

10. What is the difference between Target and Best ?   Top

    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 TARGDR4 in the DR4 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 BESTDR4 in the DR4 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.

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

    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
      483	174
      471	136
      500	125
      470	123
      418	120
      550	108

    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.

12. 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 that 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 of PhotoObjAll that contains only those objects that 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 (because we want to keep it as thin as possible). 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.

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

    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.

14. What is the difference between SpecPhoto andSpecPhotoAll?   Top

    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.

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

    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.

16. What is the difference between specClass and objType for spectroscopic objects, and which one should I use?   Top

    The objType in the SpecObj and other tables is set when the objects are targeted for spectroscopy, i.e. when the spectroscopic plates are prepared. The specClass is set by the spectroscopic pipeline after the spectrum is observed. For science, you should use the specClass attribute. The objType field is included for studies of the targeting algorithm.

17. Why does SDSS use the long (64-bit) objID fields, and what is the composition of the PhotoObj objID and SpecObj specObjID fields?  Top

    The 64-bit ID fields are required as primary keys (unique identifiers) in the SDSS database tables. They are used to uniquely identify each record in the database indices for enhanced performance. Each of them are bit-encoded with information about the observational origin, i.e., the run,rerun, camera column, etc. for photometric data, and the plate, MJD, fiberID etc. for spectroscopic objects. Please see the entry for SDSS ObjID Encoding in the Algorithms page.

18. I want to mirror the SDSS archive - how can I get a copy of all the data?   Top

    A copy of the current publicly available SDSS data release is usually available from UIC (University of Illinois, Chicago) for worldwide distribution over fast links. Please see the SkyServer support site at skyserver.org for further details onhow to host a mirror site and where to get the data. Click on the Mirrors link on this site.

19. Where can I get a copy of the HTM (Hierarchical Triangular Mesh) spatial index library?   Top

    Freely downloadable copies of the HTM library (in C++, Java and now C#) are available at the SkyServer support site at skyserver.org. Click on the HTM link on this site.

20. Who is responsible for all this?!   Top

    Please see our Credits page.

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