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Frequently Asked Questions

This page gives a list of frequently asked questions about the SDSS and Data Release 9.

Don't see your question answered here, or elsewhere on the DR9 website? Email the SDSS-III Helpdesk!

About the SDSS

  1. What is the Sloan Digital Sky Survey?
  2. What is SDSS-III? Why the "three"?
  3. What data does SDSS have? What can SDSS tell me about the sky?
  4. I am a teacher. How can I use SDSS data in my classes?
  5. Why doesn't SDSS have data for well-known visible stars (Sirius, Vega, etc.)?
  6. Why are some bright stars classified as galaxies?

About Data Release 9

  1. What help is available for DR9?
  2. What is the sky coverage of DR9? Where in the sky do the data come from?
  3. How can I see if DR9 has an image or spectrum of my favorite object?
  4. How can I search for data?
  5. How can I match a list of objects to see what data are available for them in DR9?
  6. How do I convert from the SDSS's ugriz magnitudes to UBVRI magnitudes?

About DR9 catalog data

  1. What interfaces are available to SDSS catalog data?
  2. My query no longer works in DR9 - what happened?
  3. What is the difference between SpecObj and SpecObjAll? What do sciencePrimary and bossPrimary mean?
  4. What are the differences between PhotoObj, PhotoTag, and PhotoObjAll?
  5. How do I get photometry for spectroscopic objects? What are the SpecPhotoAll table and SpecPhoto view?
  6. Why do some values (such as z and zErr in SpecObj) have different numerical precision?
  7. What is the difference between class and type for spectroscopic objects, and which one should I use?
  8. I want to mirror the SDSS archive - how can I get a copy of all the data?
  9. Why can't I log in to CasJobs?
  10. Why doesn't my query work on SDSS-III CasJobs? It works on other SDSS-III sites.
  11. How do I recover my CasJobs password?

About the SDSS

  1. What is the Sloan Digital Sky Survey? Top

    The Sloan Digital Sky Survey is an ongoing astronomy project that has been working since 1998 to make a detailed map of the universe, including our Milky Way galaxy. Since 2008, the survey has been in operating its third phase. Results and from the first two phases are available at www.sdss.org. Results and data from the current (third) phase are available on this website, www.sdss3.org.

  2. What is SDSS-III? Why the "three"? Top

    The Sloan Digital Sky Survey has been funded to operate in three phases. Phase one, from 1998 to 2005, was the original Sloan Digital Sky Survey.

    Phase two, from 2005 to 2008, was the Sloan Digital Sky Survey II (SDSS-II), which consisted of three interrelated surveys. The SDSS Legacy Survey completed observations on the full coverage area of the SDSS as originally planned. The SDSS-II Supernova Survey scanned the same area of sky each clear night for three years, and found hundreds of type Ia supernovae. The SDSS-II SEGUE survey (Sloan Extension for Galactic Understanding and Exploration) mapped the structure and composition of the Milky way.

    Phase three, the Sloan Digital Sky Survey III (SDSS-III), is the current phase of operations. This website, accessible from the SDSS-III home page, describes SDSS-III, its four component surveys, and its science results. This site also includes data collected by the SDSS-III. The latest data is Data Release 9, made public on July 31, 2012.

  3. What data does SDSS have? What can SDSS tell me about the sky? Top

    The most recent SDSS data release, the ninth overall, is SDSS Data Release 9 (DR9), made public on July 31, 2012. It includes most data from prior data releases; so to get the most recent, up-to-date data for anywhere in the sky, use Data Release 9.

    Data Release 9 includes imaging data for about one-third of the night sky. The imaging data include images from the camera on the SDSS's 2.5-meter telescope as preview images (JPGs) and as FITS files. The data also include catalogs of detected objects, with parameters measured from imaging, including positions and magnitudes. DR9 also includes spectroscopic data for more than two million stars, galaxies, and quasars (800,000 are new with this data release). Spectra are available as preview images (PNGs) and FITS files. Spectroscopic catalog data include object types and redshifts.

    All DR9 data are available from this site. See DR9 Data Access to learn how to view and download DR9 data.

    Prior SDSS-III data releases are also available on this site. Data Release 8 (DR8) was made public on January 11, 2011.

    The final data release for SDSS-I and SDSS-II is Data Release 7.

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

    The SDSS-III Education page describes what educational resources are available from the SDSS-III project.

    The SDSS-III SkyServer website includes a variety of educational projects that 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.

  5. 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 too bright for the SDSS's camera; they are saturated, and the light from them spills all over the detector. The SDSS still gets an image of those stars (for example, here is an image of Pollux from SkyServer), but the image processing software is unable to handle these images, and there are no entries in the SDSS catalog.

  6. 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 saturates the camera, and thus the light spills over the image, 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.

About Data Release 9

  1. What help is available for DR9? 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-III helpdesk is there to help.

  2. What is the sky coverage of DR9? Where in the sky do DR9 data come from? Top

    The SDSS takes imaging data in long, narrow "stripes", 2.5 degrees wide. Part of the processing of SDSS images is to select some objects as targets for follow-up spectroscopy. The SDSS then collects spectra later. Most spectra were collected using the original SDSS-I/-II spectrograph. DR9 includes, for the first time, data collected using the BOSS spectrograph.

    The Scope of DR9 page shows what areas of sky (sky coverage) are included in DR9. The Scope page also includes a Coverage Check form that allows you to see what data DR9 has for a given point on the sky.

  3. How can I see if DR9 has an image or spectrum of my favorite sky object? Top

    Go to the SkyServer Navigate tool. Enter the name of your object in the "Name" box, then press Enter. The RA/dec coordinates of the object will appear in the boxes below. Then, click Get Image.

    If a sky image appears in the main Navigate frame, then DR9 has an image for your object.

    If there is an image for your object, you can also check to see whether DR9 includes a spectrum. Under the RA/Dec boxes, find the Drawing options. Click the checkbox next to Objects with spectra. If a red square appears around your object, then your object has a measured spectrum in DR9.

    Whether or not your object has a spectrum in DR9, the object's magnitude data will appear in the right-hand pane. Click Quick Look to go see additional basic data about that object. Click Explore for more detailed data.

  4. 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 SkyServer'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.

    Many other search tools are available. See the DR9 Data Access pages for more options.

    An important note if you are using Excel with the SkyServer educational projects: SDSS 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.

  5. How can I match a list of objects to see what data are available for them in DR9? Top

    If you have a fairly small list of objects to match - a few hundred or so - use the SkyServer 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.

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

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

About DR9 catalog data

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

    SkyServer (also known as the Catalog Archive Server [CAS]) is the website that hosts 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 SDSS-III catalog data, including:

    • SkyServer: an interface to the Catalog Archive Server designed for astronomers, students and the general public
    • CasJobs: a batch (asynchronous) system for querying the database and storing results, designed to allow astronomers to search SDSS-III catalog data flexibly and efficiently
    • sqlcl: a command-line Python interface to SkyServer
    • Emacs buffer: an Emacs interface to SkyServer for running queries
    • Science Archive Server (SAS): simple queries for retrieval of individual spectra and field binary FITS images, plus direct download of flat files

  2. My query no longer works in DR9 - what happened?

    Data Release 9 includes several small schema changes to handle the new data. A list of the most important changes is available on this site in Significant Changes to Data from DR8 and DR7.

    The SkyServer DR9 Schema Browser (offsite) documents the DR9 catalog data schema in its entirety. Compare this documentation to the DR8 Schema Browser (offsite) and the DR7 Schema Browser (offsite).

  3. What is the difference between SpecObj and SpecObjAll? What do sciencePrimary and bossPrimary 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 - essentially, the best spectrum that the SDSS has obtained for that object. That is, specObj contains no duplicate observations.

    Similar to sciencePrimary, another parameter called bossPrimary flags the best spectrum that BOSS has obtained for an object. Exact definitions of sciencePrimary and bossPrimary can be found in the spectroscopic catalogs documentation.

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

    PhotoObjAll is a table that contains all of the measured photometric quantities for all of the imaging detections for every object. 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, PhotoObjAll includes duplicate and "special" detections such as parent or bright detections, which are normally not of interest to science users.

    In an effort to speed up catalog data queries, we have created PhotoTag, a view (virtual or logical table) of PhotoObjAll with only a subset of the photometric parameters that are most often requested. Queries on this "thin" version of PhotoObjAll run much faster usually than queries on the larger table. If you have a query that uses and returns only values stored in PhotoTag, then you should definitely use PhotoTag.

    In addition, we have created PhotoObj, a view of PhotoObjAll that contains only those objects that are survey primary or survey secondary. PhotoObj excludes special objects such as parent or bright detections. Because PhotoObj contains the same parameters as PhotoObjAll but effectively contains fewer objects, any queries for survey primary or survey secondary objects will run faster on PhotoObj.

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

    To summarize, the PhotoTag view is a vertical partition of PhotoObjAll (fewer columns), and PhotoObj, PhotoPrimary and PhotoSecondary are horizontal partitions of PhotoObjAll (fewer rows).

    Given the above, you should consider:

    1. Querying from PhotoTag if it contains all the parameters you are using to search and all the parameters you want returned
    2. Querying from PhotoObj otherwise, UNLESS you are interested in data for objects that are neither PRIMARY nor SECONDARY. In that case, you will need to use the full PhotoObjAll table.
    3. Importantly, the "shorthand" quantities u,g,r,i,z do not exist in PhotoTag (because we want to keep the view as thin as possible). Instead, you must use the modelMag_[ugriz] columns. HOWEVER, in PhotoObjAll and its views, only the u,g,r,i,z are indexed, and NOT the ModelMags! This difference in indexed columns is to save space (since indexes on a huge table like PhotoObjAll are expensive), not to confuse you!

    Because PhotoTag has many fewer parameters, larger portions of it can be cached, improving performance. We have found that for almost all queries that contain parameters fully in PhotoTag, it is faster.

  5. How do I get photometry for spectroscopic objects? What are the SpecPhotoAll table and SpecPhoto view?  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. The SpecPhoto view contains the same parameters as SpecPhotoAll, but only for objects that are survey primary or survey secondary.

  6. Why do some values (such as 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() (offsite) function to change the string format to whatever you like.

    To get z to 6 decimals, for example, change your query to search for 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 function str(col,length,dec) 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). The str() function rounds the result to the number of decimals you request.

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

    For any object in DR9, the type parameter in SpecObj and other tables was set when that object was targeted for spectroscopy (i.e., it is based on its morphology in the imaging data). The class parameter is set by the spectroscopic pipeline after the spectrum is observed (i.e., it is based on features in the spectrum).

    In nearly all cases, you should believe the classification in class, since it tells you what the measured spectrum actually was. The type field tells you about why it was targeted. (However, there is more information about the object's target selection history in the target bits; see the target bitmasks pages for more information.

  8. 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.

  9. Why can't I log in to CasJobs? Top

    There are actually two CasJobs sites. SDSS-I/-II CasJobs hosts data from the original SDSS-I and SDSS-II, up through Data Release 7. SDSS-III CasJobs hosts data from Data Releases 8 and 9. If you have a login on SDSS-I/-II CasJobs, you will need to create a new account on SDSS-III CasJobs to work with SDSS-III data.

  10. Why doesn't my query work on SDSS-III CasJobs? It works on other SDSS-III sites.  Top

    CasJobs uses the idea of query contexts, allowing you to specify what data sources you can query. The default context is to search your MyDB personal database space. To search DR9, the context should be set to DR9. You can choose from the Context drop-down menu above the area where you run queries. Before you run a CasJobs query, check what context you are searching. See the CasJobs user guide for more information.

  11. How do I reset my CasJobs password?  Top

    CasJobs does not include a password recovery utility. To reset your CasJobs password, email the SDSS-III helpdesk. We will create a temporary password that you should change when you log in.

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