The earlier DR2 version of this page.
The DR3 imaging data cover 5282 square degrees, and include
information on roughly 141 million objects. The DR3 spectroscopic data
include data from 826 plates of 640 spectra each, and cover 4188
square degrees. The DR3 footprint is defined by all non-repeating survey-quality
imaging runs within the a priori defined elliptical survey area in the
Nothern Galactic Cap, and three stripes in the Southern Galactic Cap
obtained prior to 1 July 2003, and the spectroscopy associated with
that area obtained before that date. In fact, 34 square degrees of
imaging data in the Nothern Galactic Cap lie outside this ellipse.
While the DR3 scans do not repeat a given area of sky, they do overlap
to some extent, and the data in the overlaps are included in earlier
releases as well. The sky coverage of the imaging and spectroscopic
data that make up DR3 are given on the coverage page. The natural unit of
imaging data is a run; the DR3 contains data from 162 runs in the best
database, and 164 runs in the target database. A total of 183 square degrees of sky are different runs between target
and best, the majority along the Equatorial Stripe in the Fall sky. Except for the sky coverage, the pipelines and databases are
identical in DR3 and DR2. Thus, DR3 is a proper superset of DR2. The
DR2 included reprocessing of all data included in DR1, and those data
in EDR that pass our data-quality criteria for the official
survey. For details about what changed from DR1 to DR2, please refer
to About DR2
on the DR2 web site.
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 DR3 WWW page is found at http://www.sdss.org/dr3 while the project's WWW site is located at http://www.sdss.org. The official DR3 publication (Abazajian et al., 2005) can be found in AJ 129, 1755. The official DR2 publication (Abazajian at al., 2004) can be found in preprint form at: astrp-ph/0403325. A lot of useful information is also in the DR1 and EDR publication at: AJ, 126, 2081 and AJ, Vol. 123, Issue 1, p. 485., respectively.
There are several ways to access DR3 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:
Similarly SDSS spectra are indexed by 4 parameters:
You can also get individual object images (or see if a given RA/Dec has been released in DR3) and finder charts based on object coordinates. (One of the DR1 finder chart tools has gone away; this is the only available tool, now.)
The Image and Spectroscopic Query Server
The DAS is only good if you know what images or spectra you want, but since DR3 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 DR3 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 DR3 data called the skyserver. (Actually, as opposed to DR1, the DR3 version of the IQS and SQS referenced above are now part of the skyserver instead of separate products and databases.) 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 is now improved over the DR2 version in that is no longer limits you to choosing from a set of possible return parameters for all catalog objects with a set of user-entered coordinates. You can now also cross-reference via run, rerun, etc. and write arbitrary SQL code for your return information. The spectroscopic tool allows 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. There is also a good set of sample SQL queries on the Introduction to SQL page.