Site Map

Help!

Please look at the FAQ and tutorials below to see if your question has an answer. Much of what you need to know is already available:

If the FAQ and tutorials below, as well as the documentation here or on SkyServer, fail to answer your question, you may email us at the helpdesk, and your question will be directed to a survey expert.

Jump to:

[FAQs] [Tutorials]

FAQs

General Questions

  1. What is the Sloan Digital Sky Survey-III?
  2. How can I get SDSS data?
  3. What is the Catalog Archive Server (CAS)? What is the Science Archive Server (SAS)?
  4. What is SkyServer?
  5. Where in the sky do the data come from?
  6. How can I search for data?
  7. What help is available?
  8. I am a teacher. How can I use the data in my classes?
  9. How can I see if the SDSS has an image of my favorite object?
  10. How can I match a list of objects to see what the SDSS knows about them?
  11. Why doesn't SDSS have data for well-known visible stars (Sirius, Vega, etc.)?
  12. Why are some bright stars classified as galaxies?
  13. What does the long SDSS ID number mean, and how do I work with it?

More Technical Questions

  1. What interfaces are available to SDSS catalog data?
  2. How do I convert from the SDSS's ugriz magnitudes to UBVRI magnitudes?
  3. What is the difference between SpecObj and SpecObjAll? What does sciencePrimary mean?
  4. What are the differences between PhotoObj, PhotoTag, and PhotoObjAll?
  5. How do I get photometry for spectroscopic objects? What is the SpecPhotoAll table?
  6. What is the difference between SpecPhoto and SpecPhotoAll?
  7. Why do z and zErr (in SpecObj) have different numerical precision?
  8. How do I change the precision of values in the output of my query?
  9. What is the difference between specClass and objType for spectroscopic objects, and which one should I use?
  10. Why does SDSS use the long (64-bit) objID fields, and what is the composition of the PhotoObj objID and SpecObj specObjID fields?
  11. I want to mirror the SDSS archive - how can I get a copy of all the data?
  12. Where can I get a copy of the HTM (Hierarchical Triangular Mesh) spatial index library?
  13. In SDSS spectroscopic data, why do some objects show up as bestObjID = 0?

Answers

  1. What is the Sloan Digital Sky Survey-III? Top

    A terrific new survey mapping the Universe on the largest scales, mapping our galaxy, and finding planets around other stars. A full description of SDSS-III is available on our front page.

  2. How can I get SDSS data? Top

    The most recent, 8th data release (DR8), is available here. The previous SDSS data release, DR7, can be obtained from this website.

  3. What is the Catalog Archive Server (CAS)? What is the Science Archive Server (SAS)? Top

    The SDSS-III data can be accessed in two ways. It can be viewed as catalog data (such as redshifts, metallicity, etc.) or as FITS images and data files. Catalog data, along with preview images and spectra, are available from the Catalog Archive Server. FITS images and data are available from the Science Archive Server. For specific detail on how to access the data please refer to this page for the imaging data and this page for the spectroscopic data.

  4. What is SkyServer? Top

    SkyServer is a web site where professional astronomers, students and members of the general public can get data from the SDSS. SkyServer's Projects are excellent resources for astronomy educators who want to include real data in their courses.

  5. Where in the sky do the data come from? Top

    The SDSS takes data in long, narrow "stripes." See the scope page showing the DR8 imaging and spectroscopic coverage. The CAS Navigate tool also has an interactive sky globe that shows where the SDSS has data. To see if a specific area has been seen by the SDSS, enter its coordinates into the Finding Chart.

  6. How can I search for data? Top

    When you search for data in the SDSS, you are going through the SDSS database and looking for objects that match criteria you choose. For simple searches of photometric data, use the Imaging Query Form. For simple searches of spectroscopic data, use the Spectroscopic Query Form. For more complex searches, use Structured Query Language (SQL). See the CAS's guide on Searching for Data to learn more about SQL. To see thumbnail results of objects that meet your criteria, use the Image List tool.

    For large, complex queries that will take a long time to run, use CasJobs, the SDSS's batch query interface.

    The CAS has many other tools too. See Getting Started for more information on all the tools, as well as the imaging data access page and the the spectro data access page.

  7. What help is available? Top

    Apart from this page there is a Glossary of SDSS-III terms. Each SkyServer tool also has its own Help section. Finally, the SDSS helpdesk is there to help.

  8. I am a teacher. How can I use the data in my classes? Top

    SkyServer's Projects use SDSS data to teach topics in astronomy and other sciences, using guided and open inquiry. With our Projects, you and your students can learn about spectra and colors of stars, galaxy types, the history of the universe, and much more.

    You are welcome 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.

  9. How can I see if the SDSS has an image of my favorite object? Top

    Find the coordinates of the object using a name resolver like SIMBAD or NED.

    Then, go to the CAS's Navigate tool and enter the object's coordinates. You can enter the coordinates as decimal degrees or as sexagesimal in the format HH:MM:SS and (+/-)DD:MM:SS. Click "Get Image" to see the object, and click on the object for its SDSS data. See the Help link in the Navigate tool for more information.

  10. How can I match a list of objects to see what the SDSS knows about them? Top

    If you have a fairly small list of objects to match - a few hundred or so - use the Cross-ID tool. Paste your list of objects, or upload a file containing data with the last two columns as (ra, dec) in decimal degrees. Click Submit. The next page will show only those objects that appear in the SDSS, with SDSS Object IDs that link to the Explore tool.

    To see a thumbnail SDSS image of each matching object, use the Image List tool. Enter your list in the textbox on the upper left and click "Get Image". Click on one of the thumbnails to go to that position in the Navigate tool, or on one of the object names to go to that object's Explore tool entry.

    For longer lists, use the Neighbors Search feature of CasJobs. See the CasJobs FAQ entry on "How can I cross-identify (find matching objects in SDSS) for my list of sources for which I have RA,decs?" for more information.

  11. Why doesn't the SDSS have data for well-known visible stars (Sirius, Vega, etc.)? Top

    The SDSS has a very sensitive camera. Stars that you can see with your unaided eyes are a little too bright for the SDSS's camera, so they show up as washed out. The SDSS still gets an image of those stars (for example, here is an image of Pollux from SkyServer), but their images are unreliable, and the SDSS gets no catalog data.

  12. Why are some bright stars classified as galaxies? Top

    The SDSS distinguishes between stars and galaxies based on their shapes: single points of light are stars, and fuzzy patches of light are galaxies. Some stars are bright enough that their light washes out the camera, so to the SDSS's camera, they look like fuzzy disks instead of single points of light. Their appearance fools the SDSS's software into classifying them as galaxies.

  13. What does the long SDSS ID number mean, and how do I work with it? Top

    The SDSS needs a way to uniquely identify every object in the database, so it generates ID numbers. The ID numbers are code numbers that include information about how the object was observed.

    One very important point when working with SkyServer is that the object IDs are so long that they get cut off in Excel, and show up with 000 as the last three digits. This means you won't be able to find your objects anymore! To get around this problem, see this workaround.

More Technical Answers

  1. What interfaces are available to SDSS-III catalog data? Top

    The Catalog Archive Server (CAS) is the database that contains the SDSS-III's catalog data. The Science Archive Server (SAS) serves flat files and provides searches for FITS spectra, images of fields, and mosaics. There are multiple interfaces to the data, including:

    1. SkyServer: an interface to the Catalog Archive Server designed for astronomers, students and the general public
    2. CasJobs: a batch (asynchronous) system for querying the database and storing results
    3. sqlcl: a command-line Python interface to SkyServer
    4. Emacs buffer: an Emacs interface to SkyServer for running queries
    5. SAS: simple queries for retrieval of individual spectra and field binary FITS images, plus direct download of flat files

  2. How do I convert from the SDSS's ugriz magnitudes to UBVRI magnitudes? Top

    The SDSS measures magnitudes through ugriz filters, which give ugriz magnitudes. These magnitudes can be converted into UBVRI magnitudes using a set of transformations described on the Algorithms page of this site.

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

    The SpecObjAll table contains ALL spectroscopic objects, even duplicate spectra of the same object. Thus, we have created the SpecObj view, which contains data for ONLY those fibers defined as sciencePrimary. That is, specObj contains no duplicate observations. An exact definition of sciencePrimary can be found in the spectroscopic catalog documentation.

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

    PhotoObjAll is a table which contains all of the measured photometric quantities for all of the imaging detections. Because we measure hundreds of parameters for each of 1.2 billion detections, this is a very large table, and queries can take very long to run. Also, because it includes duplicates and "special" detections such as parent or bright detections, it is not normally what you want.

    In an effort to speed up queries, we have created PhotoTag, a table with only a subset of the parameters that are requested most often (a "thin table"). 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 PhotoObj, a view of PhotoObjAll that contains only those objects that are survey primary or survey secondary. These tables exclude special objects such as parent or bright detections. Because this view effectively contains fewer objects than PhotoObjAll (but all the measured quantities for these objects), queries will execute faster.

    Finally, there is PhotoPrimary, a view of PhotoObjAll which only includes survey primary detections; if you are not interested in duplicate detections at all, this is the table to use.

    Given the above, you should consider:

    1. Querying from PhotoTag if it contains everything you are looking for
    2. Querying 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.

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

    The SpecPhotoAll table is a precomputed join between the PhotoObjAll and SpecObjAll tables. It includes the most requested parameters from these two tables. Because the join has already been performed, it can be faster than computing the join on the fly.

    Be aware that the join was performed using bestobjid, the flux-based match. Consult the spectroscopic-photoometric matching documentation for details. In particular, for spectroscopic objects which are definitely point sources, the flux-based match can be less appropriate than a pure position-based match. To perform a join using the position-based match, join using origobjid.

  6. What is the difference between SpecPhoto and SpecPhotoAll? Top

    The SpecPhoto view includes only those pairs where the SpecObj is a sciencePrimary (see the definition above), and the PhotoObj is a survey primary object.

  7. Why do z and zErr (in SpecObj) have different numerical precisions? 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 SQL's str() function to change the string format to whatever you like.

    To get z to 6 decimals, for example, 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. In other words, str(z,8,6) is the SQL equivalent to the C function printf("%8.6f",z). str() rounds the result to the number of decimals you request.

  8. How do I change the default precision of values in the output of my query? Top

    Use the str(column,n,d) SQL construct (where n is the total number of digits and d is the number of decimal places) to set the precision of the column that your query requests. SkyServer returns values with a default precision that is set for each data type, and this may not be enough precision for some types of science. See the sample query "Selected neighbors in run" for an example of how to use STR.

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

    The objType parameter in SpecObj and other tables was set when the objects are targeted for spectroscopy in the Legacy survey, based only on the photometry. The class parameter is set by the spectroscopic pipeline after the spectrum is observed. Typically, you should use the class attribute, which tells you what the spectrum actually was. The objType field tells you about why it was targeted (however, there is more information in the target bits; see the algorithms pages on target selection.

  10. 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. See the glossary for exactly how objid and specobjid are defined.

  11. 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 available from UIC (University of Illinois at Chicago) for worldwide distribution over fast links. Please see the SkyServer support site at skyserver.org for further details on how to host a mirror site and where to get the data. Click on the SDSS Mirrors link on that site.

  12. 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 that site.

  13. In SDSS spectroscopic data, why do some objects show up as bestObjID = 0?

    These are places where the SDSS spectrograph measured a spectrum, but that spectrum did not match spatially with a specific object in SDSS photometry. Most of these spectra without photometric matches are Sky spectra - but some are spectra of real objects that have no photometric match in DR8. For example, the deblending may have changed, or in the latest reductions the photometric pipeline may have failed on the field containing that object.

[Back to top]

Tutorials

These pages provide detailed worked examples of SDSS data retrieval using the various interfaces provided. In addition, we provide discussions of how to access and read some of the unusual file types used in the survey, and perform certain operations, such as calibration.

[Back to top]

Copyright © 2010-2013 SDSS-III