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|