Southern Standard Stars for the u'g'r'i'z' System:
Frequently Asked Questions
What is a standard star?
What is a filter, and why are there so many of them?
What is photometry?
Do filter sets differ?
Why do we need southern hemisphere standard stars if the SDSS
is a northern hemisphere survey?
Where can I get more information on standard stars?
Do you have any good pictures that illustrate how a star
changes brightness in different parts of the spectrum?
Where can I get more information on the Sloan Digital Sky Survey
... 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.
... 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.
... Photometry is the science of determining how bright an object is.
... 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.
... 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.
... Yes. If you go to:
you will find a gif movie of a really red star near the equator.
The five images were taken in our
- The initial list of 164 Northern and Equatorial u'g'r'i'z' standards
can be found at these URLs:
These two URLs provide the calibrated data in two formats:
(1) the 5 magnitudes and (2) the g' magnitude and the colors for
the other bands.
- The Standard Star Newsletter can be found at this URL:
This newsletter contains information on standard star progress in
several of the filter systems in use by the astronomical community.
- Band passes for several of the astronomical filters can be
found at this URL:
What you'll notice in the movie is a star that appears to "explode"
into view in the r', i', and z' frames.
The SDSS survey information can be obtained from
- u' -ultraviolet- filter
- g' -green-yellow- filter
- r' -red- filter
- i' -near infrared- filter
- z' -infrared- filter
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