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lab4 - AST 3722C Spring 2008 Lab#4 Even More Observing Due...

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AST 3722C - Spring 2008 Lab #4: Even More Observing. Due before class on April 8. This is a somewhat experimental lab designed to give you more experience with the night sky. Choose one of the two parts to complete. Part A. Using a CCD with the LX90. Each LX90 telescope comes with a tiny CCD camera called a “Lunar-Planetary Imager” (LPI). We haven’t covered CCDs specifically in class yet but they are essentially the same as what you have in your digital camera. They became very popular with professional astronomers in the early- to mid-1980s and have remained popular since then. They are the detector of choice for the vast majority of observations in visible wavelengths. The ones that are used at professional observatories are somewhat different in that they are cooled to liquid-nitrogen temperatures (77 K), which makes them quite sensitive to photons. Obviously one can’t keep a dewar of LN 2 around all the time, or in a digital camera, so the CCDs that we’ll use are less sensitive. But they can still be useful for taking images of the sky. The main advantage of CCDs is that they are “linear” detectors. What does this mean? If you have N photons striking a pixel, you get M counts in that pixel. If 2 N photons hit, you get 2 M counts in that pixel. And if you get nN photons, you’ll get nM counts. This linearity property makes it easy to quantify the brightnesses of objects. It doesn’t work as n → ∞ (in other words there is a limit to how many photons the CCD can absorb and still respond linearly), but as long as you know the sensible maximum allowable number of counts and don’t expose your CCD too long, then the CCD will respond linearly.
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