Salsa J V. 2.1 Image Processing software, available from European HOU site.
| target star in B
|target star in V
|standard star in B
|standard star in V
In this activity you
will determine the color of stars that have been observed with the
Leuschner telescope. There are four different stars with two images of
each, one using a blue filter (B) and one using a yellow or visible
filter (V). These stars will be referred to as your target stars. For
each target star there is also a standard star that was observed at
nearly the same time so you can assume the observing conditions are the
same. Given the known apparent brightness of the standard stars, you can
calibrate the target stars to determine their apparent brightness. This
procedure was explained in detail in chapter 5.
Perform the following procedure to get the apparent brightness
of each target star through each filter. Record all of your data in the
6.12. Use the Photometry function to find the intensity of each of the stars. To access this function click on the "Photometry" tool, represented in the toolbar by a dot in a circle with "123" over it (you can also access the tool as the "Plugin Photometer" from the Plugins menu). Then, click on the star and record the information that appears in the Photometry window. (In most cases the target or standard star is the
only star in the image; if not, it is clearly the brightest star in the
image). Then, making sure that the "Photometry" window is
active, select "Save As..." from the File menu. Give the data text file
a name (like "bstan2-mary") then click "Save."
6.13. Use the Brightness Conversion Table to get the
apparent brightness of each standard star.
6.14. Calibrate each
target star using this fact:
ratio of Counts = ratio of apparent
brightness of the target and standard stars
These ratios are
equal because the two stars were observed at nearly the same time by the
same equipment, so presumably under the same observing conditions.
There is one exception you must account for. The exposure time for
is three times longer than the exposure time for btarg2
means the telescope collected three times more light from the standard
star than it would if the exposure time had been the same as the target
star’s. You must correct for this in your ratio. You can find the
exposure time for any image yourself by selecting "Show Info..." under the Image menu. Scroll down about a page or more and find “exp. time.” The time
is given in seconds.
6.15. Use the Brightness Conversion Table
at the end of this Investigation to find the apparent magnitude of each
target star. Find the closest magnitude that corresponds to your value
for apparent brightness.
6.16. Calculate the B–V index for each
6.17. Use the B-V Index Table on the next page to approximate the
color and temperature of each target star. If your B–V index falls
between two values in the table, estimate the answer.
Sample Data Recording Table
Known Apparent Brightness of Standard Stars
| standard star
| apparent magnitude
B-V Index Table
Brightness Conversion Table