Southern Standard Stars for the u'g'r'i'z' System:

Frequently Asked Questions

  1. What is a standard star?
  2. What is a filter, and why are there so many of them?
  3. What is photometry?
  4. Do filter sets differ?
  5. Why do we need southern hemisphere standard stars if the SDSS is a northern hemisphere survey?
  6. Where can I get more information on standard stars?
  7. Do you have any good pictures that illustrate how a star changes brightness in different parts of the spectrum?
  8. Where can I get more information on the Sloan Digital Sky Survey (SDSS)?

1. What is a standard star?

... A standard star is a star that has been observed for a long enough period to know that it does not vary in brightness or color. After this has been determined and enough observations have been made we can determine its precise brightness in each of the filters (colored glass) of interest to us. We can then use these stars to compare observations of other stars (or galaxies, quasars...) to in order to determine the brightness of the "unknown" object.

2. What is a filter, and why are there so many of them?

... A filter is essentially colored glass that is manufactured to allow a specific part of the spectrum to pass through it. We measure the intensity of this part of the spectrum to get the brightness of the object in that filter. By comparing the magnitude of the object in different filters we are examining that object in different parts of the spectrum and these comparison allow us to derive information about the object such as temperature.

... There are several different filters in use in the astronomical community today. Most "systems" are comprised of a few (3-8 generally) filters that are related in some manner. For instance, the SDSS u'g'r'i'z' filter set that we use for our survey, or the UBVRI set that was one of the first filter systems developed for photometry.

3. What is photometry?

... Photometry is the science of determining how bright an object is.

4. Do filter sets differ?

... Yes. The primary difference is the pieces of the spectrum examined by each filter set, the overlaps between filters within a set and the width of those filters. The SDSS filters have almost no overlap between them and let wide pieces of the spectrum pass. Hence, they are known as a wide-band system.

5. Why do we need southern hemisphere standard stars if the SDSS is a northern hemisphere survey?

... The filter set for the SDSS has many astrophysical benefits when compared to other wide-band filter systems. Though the SDSS proper is a northern hemisphere survey, there are several other groups and individual investigators who will observe objects in the southern hemisphere using these same filters and they need standard stars to compare their observations to and use these observations to compare to the northern hemisphere data.

... Our group has under taken this effort, using essentially the same people, reduction software and selection and observing techinques that were used in the setup of the initial u'g'r'i'z' standard stars in use in the northern hemisphere. In this way, we hope to maintain a consistent approach to entire standard network.

6. Where can I get more information on standard stars?

7. Do you have any good pictures that illustrate how a star changes brightness in different parts of the spectrum?

... Yes. If you go to: http://home.fnal.gov:/~dtucker/Souterhn_ugriz/gifanimations.html you will find a gif movie of a really red star near the equator. The five images were taken in our What you'll notice in the movie is a star that appears to "explode" into view in the r', i', and z' frames.

8Where can I get more information on the Sloan Digital Sky Survey

The SDSS survey information can be obtained from http://www.sdss.org.
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