Flux Calibrator: Why do we observe flux calibrator sources?
Flux Calibrator: Why do we observe flux calibrator sources?
The most straightforward way to calibrate is to look at something
whose brightness you know. We will refer to these reference objects as
"flux calibrators". For example, we happen to be pretty sure that 3C48
emits 3.313 Jy at X-band. If we point our antenna at it and measure 10
micro-Watts in our power meter, then we would guess that anything else
we look at that shows a power reading of 10 micro-Watts is also emitting
a flux of about 3.313 Jy. We might also assume that if we measure 20
micro-Watts, the thing we're looking at is probably emitting 6.626 Jy.
(This assumption is also referred to as the "linearity assumption." This
means that if the flux from a source changes by a certain percentage,
the power we measure changes by the exact same percentage. It's a pretty
good approximation for DSS-12.) If looking at a known object tells us
everything we need to know, why are there other steps to calibration?
It's because the performance of the antenna changes with time and
position on the sky (see the "Why do we have to re-calibrate our telescope every time we observe?"
discussion.) So when we look at a flux calibrator, we determine how our
system performs at that instant in that part of the sky. We have to
make adjustments to estimate how our system is performing a few minutes
later looking at a different part of the sky.
Flux Calibrator Details
Flux calibrators are astronomical sources whose flux we believe we
know very well. (See the "Parameters" listing below.) Since we know the
flux of these targets, and we can measure the power received in our
detector, we can determine the end-to-end system gain with this
equation:
EndToEndGain = (Flux_Cal / SizeCorrection) / (Power_Cal)
EndToEndGain has units of Jansky per micro-Watt, and tells us how to
convert a power reading in our detector to the amount of flux coming
from an object on the sky. It is only valid for one place in the sky,
and one instant in time.
Flux_Cal is the known flux (in Jy) coming from the source.
SizeCorrection is a correction factor we apply to Flux_Cal. If a source
is larger than our antenna beam, we do not pick up all the flux it
emits. The fraction that we do receive depends on the size of the
source and the size of our beam. We have determined this correction
factor for each of our Flux Calibrator sources, and it is listed on the
Parameter Values page.
Power_Cal is the power, in micro-Watts, measured by our detectors when doing a cross-scan of the calibrator