6.3 Visible Light Detectors & Instruments

1. Astronomical Detection and Wavelength Sorting

  • Detectors for Astronomical Observations

    • The human eye was the first detector, but it has limitations:

      • Imperfect recording and retrieving (human brain).

      • Very short integration time (fraction of a second to add light energy).

    • Modern detectors offer significant advantages:

      • Provide a permanent record of cosmological information.

      • Allow for long exposure times (several hours for faint objects) by collecting light over extended periods.

  • Wavelength Sorting Instruments

    • Astronomers use instruments to sort light by wavelength before it reaches the detector.

    • Filters: Simple devices that transmit light within a specified range of wavelengths (e.g., a red filter blocks other colors).

      • Used to form images for measuring apparent brightness and color of objects.

    • Spectrometers: Devices that spread light into its full rainbow of colors.

      • Allow astronomers to measure individual lines in the spectrum of a source of radiation.

      • Both filters and spectrometers require detectors to record and measure light properties.

2. Photographic and Electronic Detectors

  • Photographic Plates

    • Served as primary astronomical detectors through most of the 20th century.

    • Mechanism: A light-sensitive chemical coating on glass, which when developed, provides a lasting image record.

    • Limitations:

      • Inefficient: Only about 1% of incident light contributes to image formation; the rest is wasted.

      • Less accurate measurements of brightness compared to modern alternatives.

  • Charge-Coupled Devices (CCDs)

    • Modern electronic detectors, similar to those in digital cameras.

    • Mechanism: Photons hitting the detector generate a stream of charged particles (electrons) that are stored and counted.

    • Pixels: Each spot where radiation is counted is called a pixel (picture element); modern detectors have millions of pixels (megapixels).

    • Advantages over Photographic Plates (Answering Learning Objective 1):

      • Efficiency: Record 60-70% of all photons, with best silicon and infrared CCDs exceeding 90% sensitivity, allowing detection of much fainter objects (e.g., small moons, icy dwarf planets, dwarf galaxies).

      • Accuracy: Provide more accurate measurements of astronomical object brightness.

      • Digital Output: Output is digital (numbers) for direct computer analysis.

3. Infrared Observations

  • Unique Difficulties for Infrared Observations (Answering Learning Objective 2)

    • Infrared (IR) radiation extends from ~1 \mum to 100 \mum or longer and is “heat radiation.”

    • Main Challenge: Distinguishing faint heat radiation from cosmic sources from the much greater heat radiated by the telescope itself, the observatory, and Earth’s atmosphere.

      • Typical Earth surface temperatures are ~300 K, and according to Wien's Law, objects at this temperature radiate with a peak wavelength around 10 \mum.

      • To infrared eyes, everything on Earth, including the telescope and camera, is brightly aglow.

  • Solutions for Infrared Observations (Answering Learning Objective 2)

    • Detector Cooling: Protect the infrared detector from nearby radiation by isolating it in very cold surroundings, often near absolute zero (1 to 3 K) by immersing it in liquid helium.

    • Telescope Heat Reduction: Reduce radiation emitted by the telescope structure and optics, and block this heat from reaching the IR detector.

4. Spectroscopy

  • Purpose of Spectroscopy

    • One of the astronomer’s most powerful tools.

    • Provides information about celestial objects' composition, temperature, motion, and other characteristics.

    • More than half of the time on large telescopes is dedicated to spectroscopy.

  • How a Spectrometer Works (Answering Learning Objective 3)

    • Light Entry: Light from the astronomical source (an image produced by the telescope) enters the instrument through a small hole or narrow slit.

    • Collimation: A lens collimates (makes parallel) the incoming light rays.

    • Dispersion: The collimated light passes through a prism (or grating).

      • Prism: Different wavelengths are bent by different amounts when entering and leaving the prism, causing them to leave in different directions, thus producing a spectrum.

      • Grating: A piece of material with thousands of grooves on its surface that also disperses light into a spectrum, functioning differently from a prism.

    • Focusing: A second lens behind the prism focuses the various images of the slit/entrance hole onto a detector (e.g., a CCD).

    • Spectrum Analysis: This collection of images, spread out by color, forms the spectrum that astronomers can analyze.

    • Considerations: As spectroscopy spreads light into more bins, fewer photons go into each bin, requiring either a larger telescope or significantly increased integration time, or both.