Astronomy Lab and Clay Telescope
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CCD Photometry in B and V of Vega and Albireo
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. Because the telescope and camera are so sensitive, and Albireo and Vega 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 Vega. Slew the telescope to Vega.
- Move the filter to B and take 3 images making sure that Vega 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 Albireo and take B and V images (3 of each). Try a 3 second exposure for B filter and 1 second for V filte, and adjust each for (again) ~30,000 counts, peak, in each filter.
- Note how the B vs. V images of Albireo compare: which star appears brighter in each filter?
- What is the orientation of the CCD images - i.e. make a sketch which way is N and which way is E by comparing what you see on the computer screen with what you saw through the eyepiece. Record all the above in table 1 (pdf or docx).
Reductions (in the lab):
- Open all your V filter Vega 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 star in the Albireo 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" - Vega in our case.
- Set the calibration constants using Vega 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 0.0 for our calibration star Vega, in each filter (Vega is "definted" to have mag 0.0). Center the aperture over Vega and click on it. This will set our calibration constants.
- Open all Albireo 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 Vega.
- Give a mean error for each star
- Repeat all steps for the B filter
Analysis (in the lab):
- Use your B-V values and the Basic HR Diagram Handout to determine the spectral type of the two stars in Albireo.
- Now, estimate the surface temperature for each of the stars in Abireo.
- 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 Albireo, d = 118pc. 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.
- You can then decide if the stars are truly a binary (formed at the same time): could the red star be a red giant on the HR diagram shown on figure 17-15 in your text and be co-eval with the blue star, if it is still on the main sequence?