Display and Study Processed Images

Start ds9 again, select the dark-subtracted image file of the solar spectrum, and examine it in the image window. The two dark lines at the center are the sodium lines, D1 (589.594 nm, the weaker one, is on the left) and D2 (588.9973 nm, the stronger one, is on the right). Many of the other lines in the spectrum of skylight are actually not from the sun, but from water vapor in the earth's lower atmosphere. The strength of those lines may vary with the cloud cover and humidity. A few lines due to iron in the sun are also in this region, but they are weaker than the sodium lines. Somewhere near the center of the image (row 250 or so) use the mouse to find the column at the center of each of D lines. It helps to look at the magnified window at the upper right. Read the coordinates out from the panel on the left.

1. What are the two x readings for these D1 and D2 lines. The difference is the number of pixels separating the two wavelengths. Call that $dx=x_2 - x_1$. Calculate the difference in the wavelengths $d\lambda=\lambda_2-\lambda_1$, and the scale $dx/d\lambda$ in nm/pixel. Now you can estimate the wavelength of any other line at $x$ by calculating

$$\lambda=\lambda_2 - (d\lambda/dx)\cdot(x-x_2)$$

Here the signs are chosen with the understanding that you will use a positive number for $d\lambda/dx$, and that longer wavelengths are on the left at lower $x$.

Now that you know the scale, look at the image of the spectrum of the match.

2. How close is the wavelength of the D1 emission line in the match to D1 in the sun? Comment on this.

3. Choose a row across the spectrum where the stronger D2 line is not saturated. It may be that for the match spectrum the signal is so large that the CCD is not responding linearly, or may even show a negative value, in rows near the center. Avoid using those rows. Using the same row for both D1 and D2, what is the signal at the center of each line? What is the ratio of signal $D2/D1$? The theory the interaction of the electron spin with its orbital motion predicts that this ratio is exactly 2.0.

Look at the two lines in the spectrum of the fluorescent lights.

4. What is the separation of these two lines in pixels? In nm? Most of the visible light produced by the lamps comes from fluorescence of a coating inside the tube in response to excitation by an ultraviolet line from mercury. Mercury has spectral lines with wavelengths of 579, 577, and 546 nm. Which of these lines would you expect to appear in this spectrum? What is the difference in the wavelength of the two lines you observed (multiply the difference in pixels by the scale in nm/pixel)?