Scot's Page for Public SDSS Data Access

The earlier DR1 version of this page.

DR2 was released 15Mar2004.

What is DR2? From the Press Release:

This public data release provides digital images and measured properties of more than 88 million celestial objects, and spectra and redshifts of almost 400,000 objects. The data are available from the SDSS DR2 Web site or from the SkyServer Web site more attuned to the general public.

DR2 consists of images from 3,324 square degrees of the Northern sky and more than 88 million galaxies, stars, and quasars. The dimmest objects detected by the survey are [r=] 22.2 magnitude, three million times fainter than the faintest star that can be seen with the naked eye on a dark night.

In addition to images from the SDSS telescope, the DR2 includes the spectra, and therefore redshifts, of 260,000 galaxies, 36,000 quasars, and 48,000 stars, according to consortium member Mark Subbarao of the University of Chicago. The galaxy and quasar catalogs are the largest ever produced.

There are 574 spectroscopic plates containint a total of 367,360 spectra covering 2,627 sq. degrees of the imaging area. Astrometric calibrations are good to 2% rms and coordinates are accurate to 100 mili-arcsec rms per coordinate. Compared to DR1, the spectra now feature better spectrophotometric calibrations, but are no longer Galactic reddening-corrected. Model magnitudes have also been improved so that they now serve as a better proxy for PSF magnitudes for point sources and Petrosian magnitudes for extended sources.

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 DR2 WWW page is found at while the project's WWW site is located at The official DR2 publication (Abazajian, 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.

The data access tool you need

The Data Archive Server

There are several ways to access DR2 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 DR2 WWW site.

Similarly SDSS spectra are indexed by 4 parameters:

For DR2, spRerun is always 23, since only one reduction is used.

You can also get individual object images (or see if a given RA/Dec has been released in DR2) 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 DR2 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 DR2 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 DR2 data called the skyserver. (Actually, as opposed to DR1, the DR2 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 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: 08Apr04.


At least hits since 20Jun02.