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Conclusion

Take another look at the query you used to get your first 1,000 quasars:

select
w.plate,w.mjd,s.fiberID,s.z as redshift,p.u,p.g,p.r,p.i,p.z,p.ra, p.dec,p.ObjID

from
SpecObj s, PhotoObj p, plate w

where
p.ObjID=s.ObjID and w.plateID=s.plateID and s.zConf>0.95 and s.specClass = 3

The SkyServer SQL Search tool automatically limits all queries to 1,000 objects. The query you used also limits the objects it returns by applying one "filter" - the query returns only objects with zconf > .95. Zconf is a measure of the statistical confidence level in the redshift measurement. The query only returns quasars for which we are more than 95% certain that the measured redshift is correct.

Try modifying the query. A couple of ideas would be to change or eliminate the confidence level zconf. You also could get more than 1000 quasars by playing with the redshift. For example, you could run two separate queries, then combine the results. For the first, set z < 1 by adding "and z < 1" to the end of the query. For the second, set z > 1. Combine results from the two queries into one spreadsheet for analysis.

Launch the SQL search tool

To learn more about how to work with SQL, see SkyServer's Searching for Data how-to tutorial.

When you are dealing with a large database such as the SDDS (over 13 million objects in the Early Data Release, and probably over 100 million when the survey is finished) you need to use tools such as the SQL search tool to find the data you want.

Exercise 6. Modify the query slightly to obtain a different set of data. For example, look at only quasars with poorly measured redshifts (set zconf < some number). Maybe you want to look at very red quasars by setting the i and z magnitudes larger than a certain number. You could also search for very blue quasars using a similar technique.

Think of a question about quasars that you want to answer, and modify the query to get the data that will let you answer that question. Analyze the data using a spreadsheet, then draw conclusions about the quasars. When you finish your analysis, E-mail us your queries, your results, and your conclusions. We'll look over all the results we receive, and we'll put the best of them up on this web site!

If you need help with Structured Query Language, see SkyServer's Searching for Data tutorial, or do a web search for SQL. For another tutorial on SQL, click here.

 

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