\n<\/p>\n
An unprocessed image of a star field taken by Proba-3\u2019s ASPIICS coronagraph and analysed by the Royal Observatory of Belgium. The vertical stripes in this image are due to an ongoing instrument calibration and will not appear in future images.<\/p>\n
To observe the solar corona, Proba-3 carries ASPIICS, short for Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun. This instrument, developed for ESA by Centre Spatial de Li\u00e8ge, Belgium, is made up of a large occulting disk mounted on the Occulter spacecraft and a solar coronagraph system carried by the Coronagraph spacecraft.<\/p>\n
While the two spacecraft were still attached to each other, the team tested the accuracy of the mission\u2019s pointing. Essentially, the operators rotated the spacecraft by telling it which stars it should be facing. ASPIICS images of the star field were then used to verify if the pointing was correct.<\/p>\n
Andrei Zhukov, Principal Investigator for the ASPIICS coronagraph at the Royal Observatory of Belgium, explains: \u201cWe looked at the star maps to see which stars would be visible for Proba-3 on that specific date. Choosing a single star would not be enough, because then you cannot be sure which star you are looking at. Ideally, you want at least three stars \u2013 a triangle like that can give you full orientation information.<\/p>\n
\u201cWe chose two bright stars from the constellation of Ophiuchus \u2013 meaning \u2018serpent-bearer\u2019 in Ancient Greek \u2013 which are marked with the Greek letters \u03b4 (delta) and \u03b5 (epsilon). They are situated close enough to fit into an ASPIICS image, together with a few weaker stars. The Ophiuchus constellation is actually visible to the naked eye from anywhere on Earth.\u201d<\/p>\n
Andrei then asked the control team to point the spacecraft towards those stars and to capture an image of the star field using the coronagraph\u2019s optical instrument.<\/p>\n
\u201cWith excellent precision, the spacecraft pointed exactly where we asked it to. When we got the images, we saw the two stars straight away. They are very sharp \u2013 this is great news, because it means that during the ten seconds it took to capture the image, the spacecraft were very stable.\u201d<\/p>\n
At least eight stars are visible in this ASPIICS image, which is enough to confirm the pointing of the telescope. Positioning both spacecraft precisely will be crucial for observing the corona. If their alignment is off even by a few millimetres, the Sun will not be fully covered by the occulter, leading to unwanted light interrupting the observations.<\/p>\n
In the precise alignment, the 1.4-m large disc on the Occulter spacecraft \u2013 the external occulter \u2013 will fully cover the Sun. Even then, however, so-called \u2018stray light\u2019 will spill over the occulter\u2019s edges, creating a haze that would interfere with the corona observations.<\/p>\n
To block the stray light, the coronagraph is equipped with another, internal occulter. In the star field image, this internal occulter is visible as a black ring, corresponding to a blackened section of one of the coronagraph\u2019s lenses.<\/p>\n
Andrei adds: \u201cYou can also see cosmic rays in the image, marked in purple. This is normal \u2013 in coronagraph images, cosmic ray hits often look like stars. There was a second image taken some time apart \u2013 in this one, the stars stay, and the cosmic rays appear in different locations.<\/p>\n
\u201cOverall, we were very pleased with the accuracy of the spacecraft pointing and the quality of the image. It made us even more excited to see the corona images, expected as early as March.\u201d<\/p>\n
The Royal Observatory of Belgium is hosting the ASPIICS Science Operations Centre (SOC) \u2013 a dedicated team responsible for creating operational commands for the coronagraph based on requests from the scientific community and sharing the resulting observations.<\/p>\n<\/p><\/div>\n<\/p><\/div>\n