"Coronagraph" just refers to an instrument which studies the Sun's outer atmosphere, the "corona". From Earth this is most easily seen during a total eclipse. SOHO have two coronagraphs which study the Sun from space. A very common way to observe the corona is to cover the bright disk of the Sun. This creates a sort of mini-eclipse and allows us to see the Sun's fainter outer atmosphere.

One of the coronagraph instrument on SOHO, LASCO actually contains three coronagraphs, each with a different sized occulting disk so that we can see the fainter and fainter corona, further and further away from the Sun's surface.

Another of SOHO's coronagraphs, UVCS, doesn't use an occulting disk - it just shifts its pointing around the Sun, avoiding the Sun's disk

Below are images from the three LASCO coronagraphs and UVCS. The white circles show the size and location of the Sun's surface. As with much SOHO data, the color isn't "real". We color code the images so we can tell them apart quickly.

LASCO Coronagraph 1 LASCO Coronagraph 2
LASCO Coronagraph 3 UVCS
The dark line on the lower left is the
support for the occulting disk.

Ground Based Coronagraphs

It is possible to use coronagraphs from the top of a tall mountain on Earth as well, although you can't see the corona as far from the solar disk as you can in space. Below are examples of
coronagraph images taken from the Earth.
Mauna LoaPic du
White light coronometer image from Mauna Loa Solar Observatory in Hawaii H-alpha coronagraph image from Pic du Midi Observatory in France

What Can You See in a Coronagraph Image?

Coronal Streamer Streamers seen with the SOHO/LASCO C2 coronagraph. Streamers are structures formed by the Sun's magnetic field. They can last for months.
Sometimes the Sun's magnetic field becomes unstable and erupts in huge magnetic bubbles of plasma known as Coronal Mass Ejections (or CMEs) that blow out from the Sun's corona and travel through space at high speed. The image of a CME to the right is a composite made with data from both the SOHO/LASCO C2 and C3 coronagraphs. CME
May 2000 Conjunction Other objects you can see in LASCO coronagraph images include planets (to the left is an image of an impressive conjunction from 2000) stars, and comets.

In addition, the images sometimes show artifacts due to high energy particles hitting the detector, bit of dust, or material coming off of the spacecraft. You can read more about these here:
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Last modification: November 14, 2014
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