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©2011 John Sohl, Department of Physics, Weber State University, Ogden, UT 84408-2508 - page 4 / 7





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hair on the spectral line. The up and down position is not important, it is the right/left position that you are interested in. Notice that you now get a “Wavelength:” (in units of “angstroms” which is the same thing as 1.0 X 10-10 meters) indicator near the top of the window.

This is the number you will need to record in your data table. You need to record the value for both the spectral lines that are near each other. The spectral line on the left is the calcium K line and the one on the right is the calcium H line. Notice that as you move the cursor the smallest resolution you can get is 2 angstroms. If need be, you can estimate in between those two values if your cursor will not center on the peak.

You will also need to record the “Object:” and the “Apparent magnitude:” for your galaxy. In the example of Figure 5 these would be Coma1 and 12.30 respectively. Note that you can also use the “Record Measurement” menu item from the top of the window. You can then use the main instrument window (Figure 3) to print out all your data when you are done. Do this with the menu bar at File >> Data >> Print.

Step 3. Collect Data for a Different Galaxy.

Once you are sure that you have collected all the data you need from the spectrograph, click on the “Return” item on the menu bar. This will return you to the main telescope view. Click on the “Change View” button to return to the finder telescope view.

At this point the top menu bar item “Field...” should be dark. Click on it. (If this item is grayed out you need to verify that your view is the “Finder” and not the “Instrument.”) You will get a pop up window that looks like Figure 6. Choose a field, click on it with the

Figure 5 The “Field...” window. You will need to collect spectroscopic data from at least one galaxy

mouse then click “Ok”.

from each of the fields shown in this window. I.e.,

You will need to collect spectroscopic data on at least one galaxy from each of the six

you will need data from a total of at least 6 galaxies.

available fields. Once you have selected the next field repeat steps 1 through 3 for a new galaxy. Start at the top with Ursa Major II and work your way down.

Procedure summary: position the finder telescope field-of-view-box on the galaxy, change the view to “Instrument” and position the slits on the brightest part of the galaxy you have chosen. Take a data reading until you get a signal to noise ratio of 10.0 or higher. Stop the data counting and use the mouse to determine the wavelengths of the two spectral lines. Record all needed data on the worksheet (columns 1 - 4). Change fields to gather data on a new galaxy and repeat this process for a total of at least 6 galaxies, with at least one from each field starting at the top and working down.

Once you are completely finished taking all data, return to the main telescope window and then click on File >> Exit in the menu bar at the top of the screen.

Special Note: If you wish you can speed this process up by requesting additional telescope time. A bigger telescope will collect more light faster. There is also a large demand for these telescopes so you must apply for time. You will not always get that time. You can try if you wish, but you don’t have to. Go to the menu bar item “Telescope” and figure it out from there. Again, you don’t have to, but it will speed things up, especially on the dimmest and most distant galaxies.

Step 4. Data Analysis. The procedure for analyzing your data is described below followed by a completely worked out


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