Monday, 17 December 2012

Routine display of all-sky images

For those of you who have been wondering about the strange plots on the right-hand side of our weblog page, here is the explanation.

These are all-sky radio images, taken using KAIRA. Our array is capable of imaging the entire sky instantaneously. In some ways, you can think of it as a fish-eye lens for radio astronomy. Because of the typical observation mode, we can taken these images regularly and put them on the web for everyone to see. In fact, we are the only LOFAR-based station that we know of that does this on a regular basis. Typically, the images are updated every 9 minutes or so. Here's a recent example:

Along the top of the image is the date (top left) and time (top right). The times are given in Coordinated Universal Time (UTC). Along the bottom are some details of the observation. The "mode" is the "RCU mode" (RCU = Receiver Unit) which specifies which filters are being used and which antenna array is selected. Typically, this is Mode=3 for our low-band antenna array observations. "Sb" is the subband (or receiver channel). Each receiver unit splits the signal up into 512 subbands which are sampled and processed. These all-sky observations are only one subband. The equivalent frequency of this subband is shown at the bottom right.

Around the edge of the plot are the cardinal points (North, East, South, West). Although it might seem that East and West are incorrect, this is actually what you expect when you look up. Imagine lying on your back, looking up. If North is above your head, then East is on your left and West to the right. This is also what you see on conventional star maps.

Because the images are regularly updated, you can watch the radio sources change position with time. The sequence below shows four images, separated by approximately one hour each. As you can see, the position of the radio objects move. This is because the Earth is rotating in the opposite direction. As a result, they appear to be moving around the north celestial pole.

The amount of time it takes for the sources to complete one full circuit is one sidereal day (approx. 23 hours 56 minutes). This is due to the mix between the rotation of the Earth and the orbit of the Earth around the Sun. This also means that for a given time of day, the radio sky will appear at a different position at different times of the year.

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