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Harvard University
Astronomy Lab and Clay Telescope
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CCD Photometry in B and V of Betelgeuse and Rigel
Photometry is the quantitative measurement of both the flux and color of a star. This lab will guide you to understand how to calculate the magnitudes of the stars from CCD images. We will observe two bright stars in the constellation Orion, Betelgeuse and Rigel. Because the telescope and camera are so sensitive, and Betelgeuse and Rigel are so bright, we will use an Aperture Mask when taking images to prevent our CCD camera from saturating. Using this aperture mask effectively makes our telescope aperture smaller and allows us to collect moreaccurately measure the star flux without the complications of saturated images (which would introduce large systematic errors).
Procedure (at the telescope):
- First, we'll take our calibration images of Alcyone, which is the brightest star in the Pleiades Cluster. This star will be the standard that we base our observations on. Slew the telescope to Alcyone.
- Move the filter to B and take 3 images making sure that Alcyone has no more than 40,000 counts in its peak pixel for each exposure. Start with a short exposure time of 0.5 seconds and adjust accordingly to get ~30,000 counts, peak.
- Move the filter to V and take 3 more images, again with exposures to yield ~30,000 counts.
- Now slew the telescope to Betelgeuse and take B and V images (3 of each). Try a 3 second exposure for B filter and 1 second for V filter, and adjust each for (again) ~30,000 counts, peak, in each filter.
- Finally, slew the telescope to Rigel and take B and V images (3 of each). Try a 3 second exposure for B filter and 1 second for V filter, and adjust each for (again) ~30,000 counts, peak, in each filter.
- Note how the B vs. V images of Betelgeuse and Rigel compare: which star appears brighter in each filter?
- Record all the above in table 1 (pdf or docx).
Reductions (in the lab):
- Open all your V filter Alcyone images in MaximDL.
- Right click on an image and this will allow you to choose a stellar aperture size (in pixel units). The software will count up all the counts recorded in the images from within that radius, so the radius should be large enough to include all of the star image. Try 5pix. By right clicking you can also adjust the gap ring and sky annulus radius, to say, 2pix and 3pix. The gap and sky annulus should not be so big as to get close to the other stars in the image.
- MaximDL will take the star counts and subtract the sky annulus background counts it measures adjusted (multiplied) for the difference in areas between the star (aperture ring) and the sky (outside surrounding ring). This is shown in the equation below (which we discussed in class):
- V = -2.5log(Nv/Tv) + Cv and B = -2.5log(Nb/Tb) + Cb
- Nv(for V filter) is the number of counts (minus sky and normalized for same area apertures), Tv is the exp. time in V, and Cv is the "constant" for the V filter. The same is true for the B filter. MaximDL will store the constant values from the star you entered as the "calibration star" - Alcyone in our case.
- Set the calibration constants using Alcyone as our Standard Star
- Open the Information Window in MaximDL (view -> information window), if it is not already open.
- Make sure the window is fully expanded when you click calibrate
- Set the mode to Aperture
- In the magnitude calibration box, click extract from image and set from fits and then set the magnitude equal to 2.87, which is the V-magnitude for our calibration star Alcyone. Center the aperture over Alcyone and click on it. This will set our calibration constants.
- Open all V-band Betelgeuse and Rigel images and record the V magnitudes in table 2 (pdf or docx).
- Since you have already set the calibration constants, all you need to do is click on each of the stars in Albireo and the information window will give you the magnitude for each star using the calibration constants from Alcyone.
- Give a mean error for each star
- Repeat all steps for the B filter for both Betelgeuse and Rigel using Alcyone as your standard (Note: Alcyone's B magnitude = 2.81).
Analysis (in the lab):
- Use your B-V values and the Basic HR Diagram Handout to determine the spectral type of the two stars.
- Now, estimate the surface temperature for each of the stars.
- First, determine the color ratio: (Nb/Nv)calib = 10-(B-V)/2.5
- Use the color ratio and Figure 17-9 in your textbook to find the corresponding temperature of each star.
- Determine if the star is a main sequence star (luminosity class V) or a giant (luminosity class III, II, or I)?
- To do this you need to know the distance to Betelgeuse and Rigel, d = 197pc and d=260pc. By substituting this and your apparent V magnitude into the distance modulus equation, m-M = 5logd-5, where m and M are the apparent and absolute V magnitude, and d is the distance in pc units. You can then derive the approximate absolute magnitude and luminosity class for each star by comparison with the values plotted on the Basic H-R Diagram Handout.