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

Light Detectors

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

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 availableThe 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 mans adaptive optics

Ganymede imaged by Brad Hill taken on Thanksgiving morning in Nashville using a C14 and Flea 3 video camera