Frequently Asked Questions
- What is the Sloan Digital Sky Survey?
- How can I get SDSS data?
- What is the Catalog Archive Server (CAS)? What is the Data Archive Server (DAS)?
- What is SkyServer?
- Where in the sky do the data come from?
- How can I search for data?
- What help is available?
- I am a teacher. How can I use the data in my classes?
- How can I see if the SDSS has an image of my favorite object?
- How can I match a list of objects to see what the SDSS knows about them?
- Why doesn't SDSS have data for well-known visible stars (Sirius, Vega, etc.)?
- Why are some bright stars classified as galaxies?
- What does the long SDSS ID number mean, and how do I work with it?
More Technical Questions
- What interfaces are available to SDSS catalog data?
- How do I convert from the SDSS's ugriz magnitudes to UBVRI magnitudes?
- What is the difference between Target and Best?
- What is the difference between
SpecObj and SpecObjAll?
What does sciencePrimary mean?
- What are the differences between PhotoObj, PhotoTag, and PhotoObjAll?
- How do I get photometry for spectroscopic objects?
What is the SpecPhotoAll table?
- What is the difference between SpecPhoto
- Why do z and zErr (in SpecObj) have different
- How do I change the precision of values in the output of my query?
- What is the difference between specClass and objType for
spectroscopic objects, and which one should I use?
- Why does SDSS use the long (64-bit) objID fields, and what is the composition of the PhotoObj
objID and SpecObj specObjID
- I want to mirror the SDSS archive - how can I get a copy of all the data?
- Where can I get a copy of the HTM (Hierarchical Triangular Mesh) spatial index library?
What is the Sloan Digital Sky Survey?
The Sloan Digital Sky Survey (SDSS) was one of the most ambitious scientific projects of all time. Its
primary science goal was to make a high-quality three-dimensional map of the universe. The survey
used a specially-built 2.5-meter telescope in New Mexico and a CCD camera to
take images of about one-quarter of the night sky. It entire dataset is now available online to astronomers and the
The SDSS has taken images of more than 300 million objects. The survey has already completed a preliminary map
of the universe, and has made many scientific discoveries from our Solar System to the edge of the Universe. The SDSS's discoveries are
detailed in the Science section of the sdss.org website (link opens in a new window).
How can I get SDSS data?
SDSS data is available from this Data Release 7 website.
What is the Catalog Archive Server (CAS)? What is the Data Archive Server (DAS)?Top
SDSS data can be accessed in two ways. It can be viewed as catalog data (measured parameters for sky objects) 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 Data Archive Server.
What is SkyServer?
SkyServer is a web site where students and members of the general public can get data from the Sloan Digital Sky Survey (SDSS). SkyServer shares
many data access tools with the Catalog Archive Server, but also includes some tools designed specifically for the public.
SkyServer's Projects are excellent resources for astronomy educators who want to include real data in
Where in the sky do the data come from?
The SDSS takes data in long, narrow "stripes." See the Sky Coverage page (link opens
in a new window) for maps and tables that show where in
they sky the current data come from. The CAS Navigate tool (new window)
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 (new window).
How can I search for data?
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 Inaging Query Form
(new window). For simple searches of spectroscopic data, use the Spectroscopic Query Form
(new window). 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
(new window), the SDSS's batch query interface.
The CAS has many other tools too. See Getting Started
(new window) for more information on all the tools.
What help is available?
This site has an extensive Help section, including
a Glossary of SDSS terms and a set of
Tutorials, which are guides for doing common research
tasks with SDSS data. Each tool also has its own Help section.
I am a teacher. How can I use the data in my classes?
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
How can I see if the SDSS has an image of my favorite object?
Find the coordinates of the object using a name resolver like SIMBAD (link opens
in a new window) or NED (new window).
Then, go to the CAS's Navigate tool (new window) 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.
How can I match a list of objects to see what the SDSS knows about them?
Go to the CAS's Imaging Upload
tool (opens in new window). 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.
Why doesn't the SDSS have data for well-known visible stars (Sirius, Vega, etc.)?
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 - link opens in a new window), but their images are unreliable, and the SDSS gets no catalog data.
Why are some bright stars classified as galaxies?
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.
What does the long SDSS ID number mean, and how do I work with it?
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
More Technical Answers
What interfaces are available to SDSS catalog data?
The Catalog Archive Server (CAS) is the database that contains the SDSS's catalog data. There are multiple
interfaces to the data, including (all links open in a new window):
- Catalog Archive Server: a web-based, synchronous interface for browsing and searching
- SkyServer: a special interface to the CAS designed for the general public and students
- CasJobs: a batch (asynchronous) system for querying the database and storing results
- sqlcl: a command-line interface
- Emacs buffer: an Emacs interface for running queries
Alternatively, you can retrieve SDSS FITS images with the Data Archive Server.
How do I convert from the SDSS's ugriz magnitudes to UBVRI magnitudes?
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.
What is the difference between Target and Best?
Because the survey imaging and processing was improved over the course of the survey, the SDSS
often obtained improved photometric measurements of objects AFTER they have
been chosen ("targeted") for spectroscopy. However, it is important
to keep a record of the photometric measurements at the time objects were targeted. We therefore maintain two versions of the photometric catalog:
Known as TARGDR7 in the DR7 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
image quality may be worse, and 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
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
Known as BESTDR7 in the DR7 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.
What is the difference between SpecObj and SpecObjAll? What does
SpecObjAll table contains ALL spectroscopic objects, regardless
of their status in the survey. Queries on this table can produce unusual or undesired objects. Thus, we have created the
view, which contains data for ONLY those fibers defined as SciencePrimary. To be defined as SciencePrimary, an object must meet all of the following criteria:
- It was targeted in the target skyVersion
- The plate on which the spectrum was taken is the primary observation of that
- The plate was a main survey plate (not part of the Southern survey or a special project)
- The objType is not QA, SKY, or SPECTROPHOTO_STD (these object types are repeatedly observed)
- The fiber was mapped correctly (this is the
As a result, some plates may have many (or even all) of their fibers excluded from SpecObj. Some instances where this occurs are:
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 hundreds of parameters for each of 340 million objects, 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 that are requested most often (a "thin table"). 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
Given the above, you should consider:
- Querying from PhotoTag if it contains everything you are looking for
- 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.
- 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. If you were looking for objects that had
been detected multiple times, the fastest approach would be to perform a join on PhotoTag with itself,
requiring that one object be Primary and the other Secondary.
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.
Note that all spectro-photo matches are not included in SpecPhotoAll, since there are additional JOIN conditions imposed on tiling and
targeting information. The actual SQL fragment from the SpecPhotoAll construction is shown below to indicate all the JOINs involved:
FROM SpecObjAll s
- JOIN TileAll w ON s.tile=w.tile
- LEFT OUTER JOIN TilingInfo t ON s.targetid=t.targetid and w.tileRun=t.tileRun
- JOIN TargetInfo i on s.targetObjid=i.targetObjid
- LEFT OUTER JOIN PhotoObjAll p WITH (nolock) ON s.bestObjid=p.objid
- LEFT OUTER JOIN PhotoTag q ON p.objid=q.objid
SpecPhotoAll is very useful for viewing and comparing objects' photometric and spectroscopic properties.
What is the difference between SpecPhoto and SpecPhotoAll?
As described above, SpecPhotoAll is a precomputed join between the Best
PhotoObjAll and SpecObjAll tables. This includes 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 object.
Why do z and zErr (in SpecObj) have different numerical precisions?
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.
How do I change the default precision of values in the output of my query?
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 Selected neighbors in run or
the Uniform Quasar Sample sample queries (both open in
new windows) for examples of how to use STR.
What is the difference between specClass and objType for
spectroscopic objects, and which one should I use?
The objType parameter in SpecObj and other tables is set when the objects are targeted
for spectroscopy, when the spectroscopic plates are prepared. The specClass parameter 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.
Why does SDSS use the long (64-bit) objID fields, and what is the composition of the PhotoObj
objID and SpecObj specObjID fields?
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 (link opens in a new window) 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 (new window) in the Algorithms page.
I want to mirror the SDSS archive - how can I get a copy of all the data?
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
(new window) 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.
Where can I get a copy of the HTM (Hierarchical Triangular Mesh) spatial index library?
Freely downloadable copies of the HTM library (in C++, Java and now C#) are available at the SkyServer
support site at skyserver.org (new window). Click on the HTM link on that site.
Do you have other questions that aren't answered here? Ask the SDSS helpdesk!
Last modified: Mon Jun 25 22:32:37 CEST 2007