The Electronic Camera in Astronomy

1. The Electronic Camera in Astronomy

2. Detectors: The Photographic Plate • 1839 Louis Daguerre creates first photgraphic plate: thin layer of silver-iodine on copper base • Soon after, John Draper photographs moon • Numerous improvements are made over the years to create more sensitive and more easily used plates • 1873 Hermann Vogel discovered a method to photograph light in colors other than blue (originally the only color that was absorbed) • Eastman-Kodak continues to improve color techniques and resolution of grains

3. Detectors: CCDs • 1960's memory storage device with light sensitivity – possibility of imaging? • Starting in the early 1980's the first electronic cameras were developed • Over the years, these devices have become standard imaging tools in the world of astronomy • Recently CCDs have become much more mainstream. Your digital camera uses a CCD to record your memories!

4. So What is a CCD? CCD stands for Charge-Coupled Device All this means is that every photon (packet of light) that hits the detector becomes coupled with an electron. The detector consists of a series of tiny “buckets,” called picture elements or “pixels” to collect the electrons. These pixels are made of doped silicon, which means that electrons are easily removed when bumped by photons. As light hits the detector, each pixel slowly builds up a charge of electrons for each photon it collects. This charge can then be read off pixel by pixel and recorded digitally for later reconstruction

6. So which is better? Photographic Plates Size: ~9in x 11in can be larger or smaller as desired, and cost permits Resolution: 3 microns Sensitivity: ~1% Development time: ~hours Format: Glass plate CCDs Size: from 5mm x 10mm up to ~2cm. Also used in mosaic Resolution: 5 microns Sensitivity: 85%-99% Readout time: ~seconds or better for newer cameras Format: Digital

7. CCDs CONS PROS • “Read noise” is typically 5 electrons; can be reduced by cooling the chip • Large range of maximum/minimum electrons that can be counted in each pixel (typically 64,000) • 1 photon = 1 electron this remains constant even as you approach saturation • High quantum efficiency (input/output reading): 90% • Background noise in each pixel is not constant and requires statistical correction • The quantum efficiency of each pixel is not the same - use flat-fielding • Pixel defects are common: dead/hot pixels/lines • Long read time for larger arrays • CCDs die at 1micron (best for visible wavelengths)

8. Photometry Measuring the brightness (Magnitudes) of astronomical objects Learn about stellar evolution and ages of stars Learn details of the physics and chemistry driving the Universe

9. Astrometry Accurately measuring distances to and distances between astronomical objects Formerly this was the only technique available to astronomers for learning about the physics and mechanics of the Universe

10. Today's Experiment • In groups of 3 if possible, choose a computer • Open CCDOps and open the image located inC:\FKCLUST filename: FKCLUST.ST6 • Follow lab procedure. Label stars as noted in lab manual. Choose central star as reference. Give reference Magnitude = 1 • DO NOT save changes when you finish. • Do not turn off the computer. Simply exit the program when you are finished. • Each group will have a chance to take an image with the CCD camera when I call you.