SDSS data and how to access it

The SDSS surveys are described at http://www.sdss.org

The latest data from SDSS-I and SDSS-II plus new imaging from SDSS-III are available at http://www.sdss3.org/dr8

SDSS-III is the current survey http://www.sdss3.org/

The data has 2 components images/photometry from the camera and spectra/redshifts form the fiber fed spectrographs
Available data summary page

Results from photometry and spectra are available from the DR8 database. There are also links to the "flat files" (fits files), raw and reduced images and spectra.

Each data release version may have been rereduced using improved software.

The SDSS database is a relational database and access is via SQL.
The SDSS database is typical of most big databases in the world.

Think of a database (DB) as a number of tables, where each row has a unique key, and the columns in the table are attributes of the key. The entries for matching objects in tables are linked.
SDSS database intro page

For example:
PhotoObjAll Table contains the full photometric catalog quantities for SDSS imaging.
key,ra,dec,u,g,r,i,z,flats,......foreign_keys

SpecObjAll Table contains the measured parameters for a spectrum.
key,ra,dec,redshift, object_type.....foreign_keys

Foreign keys are link to entries for matching objects in other tables. A link from a row of the current table to a row in another table

There are also indexes that make it possible to efficiently to select a subset of the data. For example, find objects that are close to an given RA,DEC without having to test the RA and DEC of every object in the DB.

Data access the easy way

Most web interface to DBs have GUIs that allow you to do typical queries.
Navigate Window is a good first look at a location in the sky. You can also highlight which objects have photometry (almost all) and spectra.

Default Search Form allows you to get the photometry for all objects in a region of the sky. Alternately you can get photometry and links to the spectra for objects that have spectra, a small subset of the objects with photometry.
The is a guide to the Search Form
To match a list of your own objects based on their RA and Dec you can use the Cross ID Form

SQL data access

SQL tutorial
Lots of SQL query examples

SDSS nomenclature

Imaging Basics
Spectroscopy Basics
The Schema Browser list what whats available in what table, plus lost more.

What type of magnitudes to use

The different ways of measuring the brightness of objects are described on the magnitudes page.
PSF mag best for stars
Model mag is the best for galaxies
The optimal measure of the flux of a galaxy, uses a matched galaxy model for each color.
This is what is returned by most GUI based searches.

A description of the details of home most things are calculated can be found on the algorithms page.

The SQL way

If you read the documentation the hard work may have already been done for you.
Instead of selecting from tables PHOTOOBJ and SPECTOBJ and using a join.

It's faster to use the GALAXY view of the PHOTOOBJ table that only contains only (primary) galaxies, but still have to join with SPECTOBJ

There is an easy way to do the search for velocities and colors on a table where the join has been precomputed for you.
SPECPHOTO
table

What about saturated galaxy cores, cosmic rays, galaxies at the edge of the CCD.
CLEAN
photometry

Use this GUI for project 3

Sky Server Search Form

The use the pull down menus and fill in the boxes for the search coodinates and radius.

Then use the "SQL Query: Generate Query" button to see the SQL code for the search.

Select the output in CSV format so SM or a spread sheet can read the file you generate.

Then the "Submit query to server" button

If you change any of the boxes once you have generated the query you will need to use the "Update Query", then "Submit"

You can edit the query yourself, after using the "Edit query" button. However once you have edited the query by hand you cannot change the query by the buttons above.

When using "Objects with spectra only" select "redshift" "spectrum ID" "plate/mjd/fiber"

Plate, mjd, fiber make it easy to find and look at a spectrum of a cluster galaxy.

The full SEARCH F0RM user guide is here.

Cross matching other catalogs with SDSS

There is a special SDSS GUI where you can input your coordinate list and match them to SDSS photometry and spectra.

The GUI to use is SDSS CrossID

Somewhat helpful information about how to use the form is provided in the bottom half of the SDSS CrossID web page.
However the GUI is fussy is about how you provide coordinates. The coordinate must be formatted as follows:

RA and DEC have to be the last 2 columns in the file.
No # comments lines.
If you want column headings they have to be SPACE delimited and first line only
No # comments lines anywhere in the file

The buttons to select are shown in this CrossID example.
using cut and paste coords so that a correct coordinate format can be seen.

Select:

Search type: SPECTRA
Search scope: Nearest Primary Spectrum
Upload type : Ra Dec
Search radius: 0.1 arc min, or whatever is sensible
Number of preceding non-data columns: N columns before RA column
You can also use JOIN with: Images but it is not necessary
Either the Cut and Paste or the UPLOAD window must be blank
Format: HTML This allow you to view the spectra automatically
When all is correct press: Submit You will get a new window with a list of matching spectra

Use the UPLOAD TEXT FILE once you have a correct format.

Cross match results page. Select the SUBMIT Upload list of spectra to SAS button.

This returns the "Get spectra - Bulk Search" window (example)
In the "Get spectra - Bulk Search" window select Plot to see the spectra of the star.
A small image is also shown as well as a table with the redshift and spectra classification.

Press Cas to bring up a navigate window to view the image.