Scot's Page for Public SDSS Data Access

DR1 is now out.

What is DR1? From the SDSS DR1 WWW site:

DR1 includes reprocessed data from the Early Data Release [and more]. The photometric data in DR1 are based on five-band imaging observations of 2099 square degrees of sky, and include measures of 53 million unique objects. Based on these photometric data, objects were selected for spectroscopy to a surface density of about 120 extragalactic candidates per square degree. DR1 includes these spectra, with derived spectroscopic parameters, for over 150,000 galaxies and quasars.

The SDSS spectra cover a wavelength range of 3850 - 9200 Angstroms in two channels with a wavelength scale of 1.14 Angstroms/pixel for a resolution of ~1800.

The official project DR1 WWW page is found at while the project's WWW site is located at The official DR1 publication (Abazajian, 2003) can be found in AJ, 126, 2081. A lot of useful information is also in the EDR publication at: AJ, Vol. 123, Issue 1, p. 485.

The data access tool you need

The Data Archive Server

There are several ways to access DR1 data. If you know what spectra/images you want already, you can use the SDSS DAS (Data Archive Server). The standard things to get there are the reduced images, or fpC (corrected frames) files, the postage-stamp object images, or fpAtlas files, and the reduced spectra, or spSpec files. All imaging files are indexed by some set of 6 parameters:

Everything on the DAS is organized at least by field - the returned FITs files, then, having several entries, one for each object (indexed by id) in that field. The SDSS imaging filters are discussed on the Imager information page on the DR1 WWW site.

Similarly SDSS spectra are indexed by 4 parameters:

For DR1, spRerun is always 20, since only one reduction is used.

You can also get individual object images and finder charts based on object coordinates.

The Image and Spectroscopic Query Server

The DAS is only good if you know what images or spectra you want, but since DR1 contains so many objects and spectra, you will probably need to sort things to get only the relevant objects for your project. There are several ways to do this (the most versatile is described later), but one is through the Imaging and Spectroscopic Query Servers, the IQS, and SQS. These tools let you enter position, SDSS magnitude, and QA-flag constraints and can return a variety of photometric and spectroscopic outputs. If you choose the "minimal" set of parameters to be returned, you will get the required magic parameters mentioned above to retrieve your objects' images or spectra from the DAS.

A note here on SDSS "sky versions" is probably warranted. The photometric catalogs always have at least two versions: one is the "target" version which is the observation and the reduction that were done to produce the spectroscopic tiling information used to assign objects fibers for observation. This sky version is useful if you want to analyze why an object was targeted and/or you are investigating completeness of a given sample. Otherwise, you will most often want the "best" version which may be the same or latter observation from the "target" observation, but will be reduced with the latest, best version of the reduction pipeline.

The Tutorials Section of the SDSS DR1 WWW site has some nice examples to help you work through common tasks with the IQS, SQS, and DAS.

The Catalog Archive Server

There is another database of SDSS DR1 data called the skyserver. It contains many other ways of accessing the data, but basically allows you to enter SQL queries to find the information you need from the database. Typically, these queries return two types of parameters (but of course you are not limited to these): reduced photometric or spectroscopic quantities, or coordinates and the indexing parameters needed to retrieve images and spectra from the DAS. These tools are located in the CAS (catalog archive server) section of the skyserver WWW site.

You can enter your SQL queries directly from a WWW page, via a downloaded Java applet called sdssQA, via a custom emacs interface, or via a custom python interface. For beginners, I recommend either the WWW page or sdssQA. The sdssQA product contains lots of sample queries which can be used to help learn the SQL syntax and the SDSS schema via any of these interfaces.

The online skyserver schema browser will allow you to figure out what quantities you need to query on or return to get the information you need. There are also two crossID tools including one for imaging which allows you to choose from a set of possible return parameters for all catalog objects with a set of user-entered coordinates. The spectroscopic tool is more flexible, allowing you to enter free-form SQL code and return information for all objects indexed by user-entered plate, MJD, and fiber.

If you do not know SQL, but need more sophisticated data-sorting tools than the IQS and SQS provide, you will simply need to learn SQL. For most tasks, however, it is fairly straightforward to form a satisfactory SQL query. The examples that come with sdssQA, even if you don't use sdssQA to input them, are a great place to start.

Last update: 10Mar04.


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