Downloading Sloan Digital Sky Surver (SDSS) data

We're going to download SDSS data using SQL ("Structured Query Language") queries, saving the data to a file for us to analyse.

To download the data for a cluster do this: but only AFTER you have read through the documentation below:

Go to the main SDSS skyserver site here: skyserver.sdss.org and then click on the "SQL Search" tool.
Delete the sample query that shows up in the box and cut-n-paste the query I describe in the documentation below. Make sure the Output Format checkbox is set to HTML and hit "submit query". This will show you a sample dataset of the first 100 objects it finds. Look at it and see if it is what you expected (that it returned a table with the values you asked for).
Assuming all looks good, then change the query in two spots:

Change TOP 100 to TOP 500000 (that's "five hundred thousand" -- which will give you all the data).
Change Output Format to CSV ("comma separated values")

Rerun the query and save the file that gets returned. Move the file to a sensible spot on your computer and give it the name "Abell2065_SDSS.csv"
When you do this for the other clusters, make sure to change the ra and dec range to be centered on the correct cluster coordinates:

Abell 2065: RA = 230.62156, dec = 27.70763
Abell 2063: RA = 230.77116, dec = 8.60859
Abell 1795: RA = 207.21886, dec = 26.59160

DOCUMENTATION:

Here is an SQL search which will get data for the cluster Abell 2065:

SELECT TOP 100
   p.ra,p.dec,p.type,
   p.u,p.g,p.r,p.i,p.z,
   p.modelMagErr_u,p.modelMagErr_g,p.modelMagErr_r,
   p.modelMagErr_i,p.modelMagErr_z,
   p.lnLDeV_g,p.lnLExp_g,
   ISNULL(s.z, -999) AS redshift, ISNULL(s.zErr,-999) AS redshiftErr
FROM PhotoObj AS p
   LEFT OUTER JOIN SpecObj AS s ON s.bestobjid = p.objid
WHERE
   p.ra>230.1 and p.ra<231.1
   AND p.dec>27.2 and p.dec<28.2

Let's parse this out slowly.

Red lines: this says that I want to get the first 100 entries from the SDSS "PhotoObj" table (objects with photometry), which I'm going to label "p", and I'm only going to get them for objects "WHERE" some criteria are true. In this case the criteria are that I want objects that only lie in some small range of right ascension and declination. In this case, it's a one degree box centered on the position of Abell 2065.

Blue lines: these are the properties I'm going to pull: position (ra, dec), magnitudes, etc. I'll explain the specific values in a second. "p." means get the properties from the PhotoObj table (which, remember, I labelled "p").

Purple lines: I want to also check the SDSS spectroscopy to see if they have spectra. So I am JOINing two catalogs (PhotObj and SpecObj, where SpecObj is being labelled as 's') and I want to make sure that SDSS identifications match, i.e., that when I grab data from SpecObj, I'm matching it to the proper PhotObj object. So I am JOINing ON the condition that the object id's in the two catalogs match each other (they go by slightly different names in each catalog).

Green lines: The tricky part of the join is that most of the photometric objects WONT have spectroscopy, and I want objects even if they dont exist in the spectroscopy catalog. So I do something called a "LEFT OUTER" JOIN, which means join the two catalogs even if they dont have entries in both catalogs. And so where they don't have spectroscopy (so s is NULL), I need to assign a value to tell me there was no data. For example ISNULL(s.z, -999) AS redshift means if there is no SpecObj redshift value (what SpecObj calls z), just give it a redshift value of -999, and I'll know to ignore those values.

So the properties I am getting for sources are the following:

Position: ra and dec
Photometric type: 3 = extended/resolved source (galaxy), 6 = unresolved source (star or unresolved galaxy)
ugriz magnitudes and their uncertainties (called "modelMagErr"s)
the log-likelihood that the object's surface brightness profile is well-fit by an exponential model (ie a disk) or a deVaucouleur model (ie an elliptical). Obviously this only makes sense for resolved sources.
the redshift and redshiftErr (if available from the spectroscopic catalog).

What do these magnitudes refer to? The SDSS data pipeline takes every photometric object detected and fits its light profile in one of three ways: as a point source, as an exponential profile, or as a deVaucouleur profile (a Sersic profile with n=4). It figures out which of these profiles does the best job of fitting the source, and then assigns a magnitude for the source based on the total integrated magnitude of whichever profile gave it the best fit. The error is just the uncertainty in the magnitude due to the uncertainty in the fit. For bright sources, this uncertainty can be quite small, but remember that uncertainty doesnt include a lot of the systematic errors we've talked about.

If you want, you can browse the PhotoObj and SpecObj tables to see if there are other photometric or spectrscopic properties you might be interested in looking at, and alter the SQL query to add those properties to your download request as well.