6.3 Visible-Light Detectors and Instruments

Detectors

The radiation needs to be detected after being collected by telescopes 

Integration time:

• the total duration a light-collecting sensor is exposed to a signal

Long exposure

• The technique where a detector is used over a long period of time 

Filters

• A type of sorter that is used before light can go to a detector, need to sort wavelengths 

  • Common one: Red transparent plastic - used to determine apparent brightness and colour of objects

• Photographic and Electronic Detectors: 

• Old method: Photographic plates - glass coated with light-sensitive chemicals to record spectra (low efficiency, ~1% light use, but permanent records)

• Modern method: CCDs (very efficient, 60–90% light use, digital output, more accurate)

  • When photons hit a CCD they generate electrons that are accounted and stored as pixels to form digital images

  • Measure brightness 

Infrared Observations

• HEat radiation, everything goes off heat

• Difficult to distinguish between infrared radiation that reached earth or if its the telescope or the earth

• Detectors are isolated in very cold surroundings → Near absolute zero, inside liquid helium 

Spectroscopy

 the study of light broken into its component wavelengths (spectrum)

• reveals a star or galaxy’s composition, temperature, motion, and other properties.

  • More than half of telescope time is spent doing spectroscopy

Function of spectroscopy

• Light from celestial objects pass through a small slit and enter the spectrometer

• Lens collimates the light and forms a parallel beam 

• Light passes through a diffraction grating which bends the different wavelengths 

• Second lens helps focus disperse light onto a CCD detector