Astronomical Techniques Summary

1. Radiation Processes

• Blackbody Radiation:
  • Emission of radiation from an object in thermal equilibrium.
  • Wien’s Law: Peak wavelength λpeak∝1/T\lambda_{peak} \propto 1/T (inverse relation with temperature).
  • Planck’s Law: Describes the intensity of radiation at a specific wavelength and temperature.
  • Applications: Temperature determination of stars and other astronomical objects.
• Synchrotron Radiation:
  • Emitted by relativistic charged particles spiraling in magnetic fields.
  • Found in pulsars, supernova remnants, and active galactic nuclei (AGN).
  • Polarized, revealing magnetic field structure.
• Bremsstrahlung:
  • Produced by decelerating charged particles near nuclei, often in X-ray sources.
  • Observed in hot gases, such as galaxy clusters.
• Compton and Inverse Compton Scattering:
  • Compton: High-energy photons lose energy scattering off electrons.
  • Inverse Compton: Low-energy photons gain energy interacting with relativistic electrons (important in gamma-ray astronomy).

2. Telescopes

Refracting Telescopes

• Concept: Use lenses to focus light.
• Advantages:
  • Simple design, no obstruction of light.
  • Sealed tube protects optics from the environment.
• Disadvantages:
  • Chromatic aberration: Different wavelengths focus at different points.
  • Limited by lens size (lenses sag under their own weight).
• Examples: Yerkes Observatory (40-inch refractor).

Reflecting Telescopes

• Concept: Use mirrors (parabolic or spherical) to focus light.
• Advantages:
  • No chromatic aberration.
  • Mirrors can be much larger than lenses, enabling high light-gathering power.
• Disadvantages:
  • Mirrors require regular maintenance (recoating).
  • Can suffer from spherical aberration (corrected in parabolic designs).
• Examples: Hubble Space Telescope, Keck Observatory.

Optical Telescopes

• Purpose: Observe visible light (400–700 nm).
• Components:
  • Primary mirror: Collects and focuses light.
  • Secondary mirror: Redirects light to detectors.
  • Adaptive Optics: Corrects for atmospheric turbulence in real time.
• Applications:
  • Imaging stars, galaxies, nebulae.
  • Spectroscopy of stars and exoplanets.

X-ray Telescopes

• Concept: Use grazing incidence mirrors (Wolter telescopes) to reflect and focus X-rays.
• Advantages:
  • Reveal high-energy processes like black holes, neutron stars, and galaxy clusters.
  • Can image hot gas and detect X-ray binaries.
• Disadvantages:
  • Must be placed in space to avoid atmospheric absorption.
• Examples: Chandra X-ray Observatory, XMM-Newton.

Gamma-ray Telescopes

• Concept: Detect high-energy photons using pair production or scintillators.
• Advantages:
  • Study the most energetic processes: gamma-ray bursts, AGN, and supernovae.
• Disadvantages:
  • Poor angular resolution compared to optical or X-ray telescopes.
  • Require space-based observatories (e.g., Fermi-LAT) or Cherenkov techniques (e.g., H.E.S.S.).
• Examples: Fermi Gamma-ray Telescope, H.E.S.S.

Infrared Telescopes

• Concept: Detect heat radiation (1–100 μm) using cooled detectors.
• Advantages:
  • Observe cooler objects like brown dwarfs, exoplanets, and molecular clouds.
  • Penetrate dust clouds that obscure optical observations.
• Disadvantages:
  • Requires cooling to reduce thermal noise.
  • Atmospheric absorption necessitates high-altitude or space-based placement.
• Examples: Herschel Space Observatory, James Webb Space Telescope (JWST).

Radio Telescopes

• Concept: Use large parabolic dishes to collect radio waves.
• Advantages:
  • Study cold interstellar gas, pulsars, and cosmic microwave background (CMB).
  • Can operate under all weather conditions.
• Disadvantages:
  • Poor angular resolution due to long wavelengths (improved with interferometry).
• Examples: Arecibo Observatory, Very Large Array (VLA).

Microwave and Submillimeter Telescopes

• Purpose: Study the CMB and dust in galaxies.
• Examples: Planck Satellite, ALMA (Atacama Large Millimeter/submillimeter Array).

3. CCDs (Charge-Coupled Devices)

• Concept: Convert light into electronic signals using the photoelectric effect.
• Advantages:
  • High quantum efficiency (~80%).
  • Linear response, suitable for photometry and spectroscopy.
  • Real-time readout.
• Disadvantages:
  • Sensitive to cosmic rays.
  • Require cooling for long exposures to reduce thermal noise.
• Alternatives:
  • CMOS Detectors: Faster and cheaper but less sensitive.
  • Bolometers: Measure temperature changes for infrared detection.

4. Celestial Coordinate Systems

• Equatorial Coordinates:
  • Right Ascension (RA): Analogous to longitude.
  • Declination (Dec): Analogous to latitude.
  • Fixed and independent of observer’s location.
• Galactic Coordinates:
  • Aligned with the Milky Way plane.
  • Used for studying Galactic structures.
• Alt-Azimuthal Coordinates:
  • Based on observer’s horizon.
  • Suitable for real-time tracking but changes with time and location.

5. Spectroscopy

• Spectrographs: Disperse light into its spectral components for analysis.
• Reflection Grating Spectrometers (RGS): Use reflective gratings for precise spectral analysis, especially in X-ray and UV.
• Multi-object Spectroscopy: Captures spectra of multiple objects simultaneously using fiber optics.

6. Imaging and Resolution

• Point Spread Function (PSF): Quantifies the spread of light from a point source; narrower PSF = higher resolution.
• Signal-to-Noise Ratio (SNR):
  • A measure of detection quality.
  • Improves with longer exposure times (t1/2t^{1/2}).