Harvard University Astronomy Lab and Clay Telescope

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The Spectrum of the Blue (or cloudy) Sky

Spectral lines are universal signatures of elements; for example, sodium lines have exactly the same wavelengths from our lamp as in the Sun, if we measured in the center of the Sun where there is no radial velocity componet from its rotation. Precise measures of the wavelengths of lines seen from stars can be used to uniquely identify the elements. We are not going to point the telescope at the Sun because the Sun is much too bright for our detectors. We will instead point away from the Sun to capture the sunlight scattered by the earth's atmosphere towards our detector.

Procedure:

• We will begin by visually inspecting the emission lines of a few lamp sources (e.g. Sodium, Helium, Hydrogen) with the spectrograph in the lab. Your TF will guide you through the process of aligning the lamps and using the eyepiece to observe the emission lines. Each element emits light at distinct wavelength each of which corresponds to a specific color. As you observe the emission lines, take note of the colors at which the elements emit light.
• Next, we are ready to head up to the telescope! With the spectrograph ready to go, point the telescope AWAY from the Sun. Take a short (~few seconds) exopsure and check that the spectrum is not overexposed. If you want to check the counts, click view --> display crosshair from the toolbar, center the crosshair on the center of the spectrum and the counts will be displayed. Click here if you need instructions on how to take spectra. When you have determined a good exposure time (counts should be no less than 15,000 and no more than 40,000), take 3 exposures. It is always good to have a few observations; in case one is flawed for some reason, you will still have others as a check.
• The final step will be to take calibration spectra. Since elements emit light at distinct wavelengths, we take a spectrum of a lamp source (in our case, we have a Mercury-Argon calibration lamp) to compare which wavelength corresponds to which pixel in our spectrum. We can then compare the wavelength to pixel determination of our lamp source to our solar spectrum. This is called a wavelength solution and will be done in the analysis section below (with some guidance from your TF). Calibration spectra must be taken immediately before or after the solar spectrum being very careful not to bump the telescope or spectrograph.

Analysis:

• As described above, we first need to determine a wavelength solution (following these steps) so we can determine which elements are present in the solar spectrum.
• Now that you have your spectrum of the blue (or cloudy) sky, you need to analyze it to find some different kinds of atoms present on the sun.
• You should identify 3 lines in the solar spectrum.
  • This Wikipedia link is a nice visual reference to the solar spectrum and line identification list. In the solar spectrum, there are often many lines for a simple type of atom; for example, there are two seen from hydrogen and sodium.
  • Using the Spectra software that you used to find your wavelength solution, you can determine the wavelength of the absorption features in the spectrum by clicking "Calculate Line Wavelength" when centered on a specific line. Now search the line identification list to determine which line corresponds to the wavelength you found.