Home About Contact us Related links No AI
Top Species list A-Z Family groups 1-26 Plumage Nests Habitats Key plants Passerines v non-passerines Glossary Sponsors Credits Photos Aboriginal information General information
Top Species list A-Z Family groups 1-26 Plumage Glossary Credits General information
Top Species list A-Z Family groups 1-26 Plumage Glossary Credits General information
Top Australia Chile/Argentina Germany/Netherlands USA/Canada
Top Australia Chile/Argentina Germany/Netherlands
Top Favourites Birds of Australia Australian weather shots Dew drops Dew drop animations How dew drop lenses work Birds of Oman Astronomical telescopes Art in nature
Top Scientific research Popular-scientific pages Credits

Astronomical Telescopes Spectrometers

Spectrometers normally contain cameras similar to, if not identical with, imaging instruments. However, instead of filters, as mentioned on that page, one uses a so-called dispersive element in front of the camera, i.e. a device that splits up light into its rainbow colours (the Solar spectrum is shown here).

To illustrate this, imagine that that Solar spectrum is measured with the camera displayed on the page about imaging instruments. Have you noticed the dashed white box that has been marked there? This is the area onto which spectra of sources within the field-of-view of the camera are projected, if a dispersive element is indeed used. The result might then look about as shown below in the figure.

Spectrum projected onto an imaging camera.

Now the pixels of the camera are used to store the light, as it is split up into the colours of the rainbow. After proper instrument calibration the position at which photons are measured on the CCD chips will tell what colour the incoming light had that was registered there (because the colours that I preserved for convenience will normally be lost and only a greyscale wedge obtained).

Dispersive elements

Dispersive elements used frequently in astronomical telescopes are prisms, gratings, or combinations thereof.

Non-dispersive spectroscopy

There is now also a way to obtain spectral information from the light received from an astronomical source WITHOUT using a dispersive element! In fact, in the example used above, where it is mentioned that normally only a greyscale wedge is obtained, this particular imaging camera (as shown in the figure) can in fact also measure which energy (or wavelength or colour) the incoming photons have. So the colour derived from the position that they hit after being dispersed by a grating can be cross-checked by the colour derived directly from the individual pixels. The spectral resolution (i.e. the ability to measure minute differences in colour shades) of the pixels is lower than the precision derived from measuring the positions of the dispersed photons. That is the reason why one does in fact use a dispersive element, rather than rely on the colour decoding capability of the CCD pixels themselves.