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Published bySharyl Jacobs Modified over 9 years ago
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Announcements No class next week due to Dark Sky Observing Night on the same night. If the Dark Sky Night is cancelled, the class will meet. Homework: Chapter 8 # 1, 2 & 4 Lab this week: SDSS Advanced Projects: Hertzsprung-Russell Diagram
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Light Detectors
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Early astronomers were great sketch artists Donati drawings of the head of a comet Lord Rossi drawing of a spiral galaxy Carrington’s drawings of a solar flare
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The human eye is a remarkable instrument The Cilary muscles can deform the lens, changing its focal length to adapt for nearby or distant objects
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The eye’s photo detectors are called rods and cones
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The cones are concentrated in the center of the retina while the rods are spread out
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The rods are more sensitive than the cones
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It takes some time for the eye to adapt to the dark
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Photography came into use in the late 1800’s Henry Draper was one of the first to use photographic plates for astrophotography
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Techniques were developed to measure brightness on photographic plates Plate densitometers would measure the density of exposed photographic grains to determine brightness
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Photoelectric photometers provided an easier means of determining brightness
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The most common detector was a photomultiplier tube
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The SSP-3 Photoelectric Photometer is still available The readout must be calibrated for absolute photometry or compared to a comparison star’s readout of differential photometry.
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Modern solid state detectors eventually replaced photomultiplier tube Photodiode detectors are much smaller, more sensitive and much more energy efficient
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Image Intensifiers give real- time light amplification The intensified image is usually a monochrome image
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There are more military uses for image intensifiers than astronomical ones
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Today, almost everyone uses CCD’s for photometry
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The CCD is a simple MIS device This shows a front-illuminated CCD. The more sensitive CCD’s are back illuminated
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In a CCD pixel, photons are converted to electrons and stored in the potential well
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The readout works like conveyor belts of light buckets
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The result is an array of numbers representing brightness
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The array of data is translated into a grey scale image
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The digital camera works on the same principle as a CCD The color camera puts micro-filters in front of each pixel and then electronically adds the different colored pixels together to make a color image
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Video Astronomy
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There are two reasons to do video astronomy Reason 1: precise timing of astronomical events like occultation’s or meteor showers.
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Reason 2: a poor mans adaptive optics Ganymede imaged by Brad Hill taken on Thanksgiving morning in Nashville using a C14 and Flea 3 video camera
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