Half way round the sky

Planck has now observed half the sky, as anyone following the "Sky Coverage" plots will have noticed.  The images below show the coverage from two different perspectives, one in Galactic coordinates, and one in Ecliptic coordinates.

Sky observed in Galactic coordinates
Galactic Coordinates.
Image credit: IAS
Sky observed in Ecliptic coordinates
Ecliptic coordinates.
Image credit: ESA
Sky coverage of Planck as in November 2009 in two different coordinate systems.  Click on the images to see the source pages.


Planck's axis points almost directly away from the Sun, and as it spins on its axis at 1rpm, its detectors look at the sky in a ring around the sky - always around 85o away from the axis.  The axis moves around the sky at around 1o per day, so it takes one year to swing round the whole sky.  One side effect is that some parts of the sky, near the "top" and "bottom" of the rings, will be observed more than others.

In Galactic coordinates, which are aligned with the Galactic plane, the ring around the sky which Planck is now observing is slightly distorted - just as the north and south poles are in an atlas.  This shows that Planck has now observed most of the Galactic Plane, including the Galactic Centre.  Planck's scientists are now scouring the data to find out more about the microwave light emitted by our own Galaxy, at an unprecedented resolution and wavelength range.  The areas of the sky at high Galacitc latitude - i.e. away from the Galactic Plane, will take longer to observe.  A full analysis of the sky will take several years, to account for all the effects that the instrument itself has on the data.

Ecliptic coordinates are aligned to the plane of the Earth's orbit, and therefore roughly aligned Planck's orbit too.  The point at which Planck's axis is pointing moves across the centre of the picture from left to right, with the ring of sky being observed forming a circle around that point - albeit a distorted circle, again because of the projection used.  In mid-August, the central point was just right of centre, and now it is just left of centre.  This plot shows better the way the sky at the north and south scliptic poles (the top and bottom of the map) are observed.  This is the reason why Planck has already observed 50% of the sky, despite being less than 50% through the first all-sky survey in terms of time.  The first all-sky survey will last for another 4-5 months.