
If the galaxy is properly centered in the slit, the Spectrometer Reading
window will appear when the spectrometer "Take Reading" button is
pressed.  Select "Start/Resume Count" from the menu to begin taking
data.  You will see the counts build up in each channel of the
spectrometer.  The identification and apparent magnitude of the object,
as well as the total photon count and integration time, also appear on
the screen.  You should record the ID and apparent magnitude.  (In your
calculations you will also need the absolute magnitude of the galaxies.
This is available under "Help - Absolute Magnitude", unless your
instructor has informed you otherwise.)

Note that the spectrum display has been normalized so that the highest
count in any channel becomes 1.0 and all other counts are scaled
accordingly for plotting.  This type of display emphasizes the reduction
of noise and the increased visibility of the spectral lines as the
integration time increases.

You will notice that for the brighter galaxies the spectrum builds up
quite quickly, and the H & K lines are apparent very soon after the
integration begins, while the spectra of the fainter galaxies build up
much more slowly and the spectral lines cannot be distinguised from the
noise for some time.  You may select "Stop Count" from the menu at any
time.  This causes the integration to pause and the current readings for
each channel are connected by lines to make the spectrum more readable.
Select "Start/Resume Count" again to continue the integration.  You
should integrate each spectrum until the signal to noise ratio is in the
neighborhood of 10.0.  This will yield well-defined and measurable H &
K lines for all galaxies in this exercise.

When the signal to noise ratio has reached ~10.0, select "Stop Count"
and measure the spectrum.  If the left mouse button is pressed when the
cursor is on the graph, the cursor will change to cross hairs which
facilitate alignment on the spectral lines.  When you have carefully
positioned the cross hairs at the center of a line, record the wavelength
shown on the screen.  Repeat the measurement for the other line.  (The
current resolution of the cross hair on the screen is 2 Angstroms.  This
can easily be interpolated to 1 Angstrom in cases where the cross hair
cannot be placed in the exact center of the line.)

When you have finished measuring a spectrum, select "Return" to access
the telescope control panel, and continue with the next galaxy.  Be sure
you have recorded all measurements before returning to the control
panel, as all data is lost when the Spectrometer Reading window closes.

