3 Telescopes and techniques

Summary

Observations of neutron stars and pulsars extend over more than 19 decades of the electromagnetic spectrum, from low radio frequencies (around 30 MHz) to high gamma-ray energies (above 200 GeV). The techniques used in telescopes between these extremes range from the coherent detection of radio waves to photon detection techniques more usually associated with nuclear physics. There are nevertheless elements in common over the whole range, which we will refer to in this brief survey.

1. (1) The signal is weak, requiring large collecting areas and long integration times.

2. (2) Identification of objects requires accurate positions and discrimination from adjacent sources.

3. (3) Pulsed sources require high timing accuracies, often around 1 microsecond.

4. (4) Measurements must discriminate against unwanted backgrounds, either of astronomical origin, such as radio emission or cosmic rays from the Milky Way Galaxy, or from terrestrial sources, especially man-made radio signals.

The terrestrial atmosphere is transparent to radio waves (except at short millimetric wavelengths where molecular absorption occurs, and at long metric wavelengths where ionospheric refraction and reflection occur). Radio telescopes can therefore be built at ground level, and can extend in size almost indefinitely, giving both high sensitivity and high angular resolution. X-rays and gamma-rays are absorbed in the atmosphere, and direct detection of such high-energy photons can only be achieved using space-based telescopes, where telescope apertures are limited by the capabilities of launch vehicles to a few metres in diameter.