The coronagraph

 [This post participates in the Carnival of Space #185 at The Spacewriter's Ramblings blog]

The coronagraph is the technique for which a disc, which covers the Sun disc, interposes between the observer and the detector, as happens when the moon eclipses the Sun. When this happens appears the Solar corona, thanks to the disc covers the solar disc and the photosphere is not visible. The corona is the main object of study with a coronagraph. The corona is very faint in the visible, compared with the bright photosphere: the corona is most easy to detect in UV and x-rays due to the high temperatures and a higher level of ionization.

STEREO COR1 coronagraph. Source: NASA

To the telescope is attached the disc, normally a small circle plate called “occulting disk”. The opaque disk can vary its size for hiding a specific percentage of the corona. The instrument must be built carefully to avoid reflexes in optics which also can be affected by the diffraction, reducing the quality of the image. To solve this problem the telescope has internal obstructions called “Lyot tops”, which helps to reduce in more than 109 times the internally scattered light. Examples of telescopes:
    Ground based telescope: MK3 coronagraph in Mauna Loa Solar Observatory
    Space  based telescope: LASCO instrument in the SOHO satellite and COR1 in STEREO

Solar observatory in Mauna Loa
This technique also allows to observe the solar winds and comets near to the Sun, which otherwise would be impossible due to the Sun bright. With LASCO now is more easy to study the CMEs (Coronal Mass Ejections). CMEs observations are easier from space based observatories due to that in the space there is no bright sky and there is more contrast. In some times to observe the CMES is not only covered the Sun disc, also is covered the nearest corona to the Sun. Understanding the CMEs can help to prevent problems in satellites and telecommunications: observing an “halo CME” indicates that a CME comes toward us.

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