Cross-Scan; What are 'cross-scans'?
We have discussed elsewhere that
everything emits radio waves, not just objects in the sky. The Earth's
atmosphere emits some, as do the ground and mountains surrounding our
antenna, and even our own equipment. Some energy from all of these other
sources enters our detector along with the object we are trying to
measure. Doing cross-scans is one way we can eliminate this effect. In a
cross-scan, we basically are just comparing how much power our
instrument measures when it is pointed straight at our target - let's
use Jupiter as an example - and how much power it measures when pointed a
little bit off of the target at an empty patch of sky. We assume that
the amount of unwanted energy we measure when pointed at empty sky just
to the side of Jupiter is the same amount of unwanted energy we pick up
from the direction of Jupiter. So if we subtract the "side" measurement
from the measurement we got looking at Jupiter, we are left with the
power from Jupiter. (Those of you who have done the "Bucket of Noise"
activity may remember discussing this.) Many of you have probably done
cross-scans by hand during your observing run. Did you take "baseline"
measurements and then "peak" measurements, and subtract the two? The
baseline was the "side" measurement made when the antenna was pointing
slightly away from your target. The "peak" was the on-source
measurement!
In summary, to eliminate unwanted radio emissions from our
measurement, we usually scan across our target, and use the difference
between "on-source" and "off-source" measurements as our signal. We do
these cross-scans on our Flux Calibrators as well as the objects we are
studying.
Cross-Scan Details
When measuring an object on the sky, we scan our antenna across the
object. We start well away from the target, where we think there is just
empty sky, move across the position where we think the object is, and
then back onto empty sky.
Can you explain the main features of this plot? Why is there a peak
in the middle? Why does the signal drop off smoothly to either side of
the peak? Why is the signal relatively low and unchanging at the
beginning and end?
We do cross-scans in both the declination (abbreviated dec) direction
and in the cross-declination (abbreviated x-dec, or xdec) direction.
For each cross-scan we do a least-squares fit in order to determine the
baseline level, and the location, width, and height of the signal in the
center. (We fit a straight line to the baseline, and a Gaussian to the
part where we see our target.) With our current data access tool, you
cannot see the raw data, all you can access is the fit to the data. The
peak height of the Gaussian above the baseline is the amount of extra
power we receive when pointed at the source, over what we get from the
nearby sky. That is the power measurement that is plotted, which gets
converted to flux or brightness temperature.
The other parameters of the fit are used in different ways. For
example, if the location of the peak of the Gaussian is not at the
center of the scan, that indicates that the antenna isn't pointing
exactly where we think it is. We use this information to adjust our
pointing next time we scan across the source.
