Announcements No class next week. I will be returning from Kitt Peak late Thursday so I won’t be back in time Homework: Chapter 8 # 1, 2 & 4 Lab to do over the next two weeks: SDSS Advanced Projects: Hertzsprung-Russell Diagram
Light Detectors
Early astronomers were great sketch artists Lord Rossi drawing of a spiral galaxy Donati drawings of the head of a comet Carrington’s drawings of a solar flare
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
The eye’s photo detectors are called rods and cones
The cones are concentrated in the center of the retina while the rods are spread out
The rods are more sensitive than the cones
It takes some time for the eye to adapt to the dark
Photography came into use in the late 1800’s Henry Draper was one of the first to use photographic plates for astrophotography
Techniques were developed to measure brightness on photographic plates Plate densitometers would measure the density of exposed photographic grains to determine brightness
Photoelectric photometers provided an easier means of determining brightness
The most common detector was a photomultiplier tube
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.
Modern solid state detectors eventually replaced photomultiplier tube Photodiode detectors are much smaller, more sensitive and much more energy efficient
Image Intensifiers give real-time light amplification The intensified image is usually a monochrome image
There are more military uses for image intensifiers than astronomical ones
Today, almost everyone uses CCD’s for photometry
The CCD is a simple MIS device This shows a front-illuminated CCD. The more sensitive CCD’s are back illuminated
In a CCD pixel, photons are converted to electrons and stored in the potential well
The readout works like conveyor belts of light buckets
The result is an array of numbers representing brightness
The array of data is translated into a grey scale image
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
Video Astronomy
There are two reasons to do video astronomy Reason 1: precise timing of astronomical events like occultation’s or meteor showers.
Reason 2: a poor man’s adaptive optics Ganymede imaged by Brad Hill taken on Thanksgiving morning in Nashville using a C14 and Flea 3 video camera