Lecture Notes: Modern Telescopes and Charge-Coupled Devices (CCDs)
Introduction to CCDs
- Charge-Coupled Devices (CCDs): Modern telescopes use CCDs instead of photographic plates.
- Function: Directly connect incoming photons from an object to its image on a computer.
Working Principle of CCDs
- Photoelectric Effect:
- Fundamental principle CCDs are based on.
- Describes how a photon with sufficient energy can free an electron from an atom.
- Analyzed by Albert Einstein in 1905.
- Silicon Wafer:
- Used in CCDs because it easily releases electrons when exposed to visible light.
- Tiny positively charged capacitor collects freed electrons.
Quantum Efficiency
- Definition: Measurement of the effectiveness of a CCD in converting photons into electrons.
- Efficiency Rate:
- Highest quality CCDs achieve up to 90% quantum efficiency.
- Human eyes (rods and cones) have only about 1% quantum efficiency.
Image Capture Process
- Exposure:
- Photons start producing electrons as soon as the shutter opens.
- Freed electrons are collected by the capacitor until the shutter closes.
- Data Transfer:
- Voltage across the capacitor indicates the number of collected electrons.
- Information is relayed to a computer.
- Pixel Structure:
- Each integrated circuit represents one pixel.
- CCDs consist of thousands or millions of these pixels arranged in an array.
Example: Hubble's Wide Field Camera 2
- Configuration:
- Contains a 2K by 4K array, resulting in an 8 megapixel CCD.
Capturing Color Images
- Filter Use:
- Repeated observations with different filters are needed for color images.
- Example - Planetary Nebula MYCN18:
- Located 8,000 light years away.
- Image composed from three separate images using:
- Blue filter for oxygen.
- Green filter for hydrogen.
- Red filter for nitrogen.
These notes summarize the lecture content about the technology and functioning of charge-coupled devices as used in modern telescopes, the concept of quantum efficiency, and the process of capturing color images with CCDs.