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©2011 John Sohl, Department of Physics, Weber State University, Ogden, UT 84408-2508 - page 3 / 7





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center the spectrometer slits on the galaxy. Adjust the position of the slits to cover the brightest part of the galaxy. You may want to change the “Slew Rate” to 1 or 2.

Step 2. Collect Spectroscopic Data.

Click on the “Take Reading” button. You will get a screen with a blank graph on it. Click on the “Start/Resume Count” menu item at the top of that window. Your computer screen should now look something like that shown in Figure 4. (At first you will get what looks like random dots on the screen (“noise”) but it should soon settle down into something like Figure 4.) In the bottom right corner of this window you will see “Signal/Noise:” This is called the signal to noise ratio and is a measure of how “clean” the data is becoming. You will need a signal to noise ratio of at least 10.0 but the higher the number the better. As is true in real life, to get better results takes time! On nearby galaxies that are bright you will get excellent data in a minute or two. On very distant, dim, galaxies it may take you 10 minutes to collect enough photons to get a reasonably useful result.

Figure 3 Example of the pop up window obtained when taking a spectrometer reading. The dots represent counts of photons collected at various wavelengths. You need to collect data until you have a signal to noise ratio of at least 10.

Once you have obtained a sufficiently good signal, click on the “Stop Count” item on the menu bar. You will now get a screen with a line connecting all the dots, see Figure 5. If the signal is too noisy to cleanly identify any spectral lines then you will need to resume the counting process. To do this click on the “Start/Resume Count” item on the menu bar.

Assuming that the data are good enough to use, you must now measure the wavelengths of the two spectral lines. Bring the mouse cursor onto the graph and click and hold the left mouse button, a cross hair will appear. Center this cross

Figure 4 Once you have stopped the photon counting you can measure the wavelengths of the spectral lines. The calcium K line is on the left and the H line is on the right for the pair. Ignore the “G-Band” line on the far right. Use the mouse to measure the wavelengths by locating the cursor near the line then hold the left mouse button down to get a cross hair to appear, center that cross hair left to right on the spectral line. The results appear on the screen, 4012.0 angstroms in this case.


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