Mini-Cal: What is a 'mini-cal'?
"Mini-Cal" is short for mini-calibration. A mini-cal allows us to
determine how the hardware and electronics in our receiver and detectors
are performing. (It does not tell us about how the antenna is
performing. We discuss that under "antenna efficiency" below.) Doing a
mini-cal involves taking several measurements of things whose brightness
we know. In that sense it is similar to looking at a flux calibrator -
the difference is that instead of pointing the whole antenna at a
calibrator, we either insert a known target right in front of our
receiver (we actually have a small target - called a hot-load - that is
mounted on a swing arm and can move in front of our receiver), or we
inject power into our electronics using something called a noise diode.
The advantage of doing it this way is we can do a mini-cal anytime we
want, wherever the antenna happens to be pointed. The disadvantage is
that we are not testing the entire system. Because the test signals
enter the system at our receiver, they never pass through the big dish
antenna or the subreflector. That means they do not tell us how those
parts of our instrument are working.
For those of you wanting some more specifics, a mini-cal consists of 5 measurements:
- The zero-level of the power meter. This is what the power meter registers when no power is input to it.
- Sky (we use the sky as a low-power signal).
- Sky +noise-diode (adding a little extra signal with the diode helps us determine how linear our system really is).
- Hot-Load (this is a target whose temperature is known).
- Hot-Load + noise-diode (again, adding the noise diode helps us determine the system linearity).
Every time we do a mini-cal, we determine a quantity we call the
"Receiver Gain". It was the first plot shown on the main page. It tells
us how to convert the power measurements made in our power-meters to a
brightness temperature entering the receiver. (Note that it does not
tell us the brightness entering the antenna!)
In summary, to track changes in our electronics that happen in
between our looks at a flux calibrator, we do mini-cals. Typically they
are done about every 20 minutes.
The EndToEndGain is only valid at the moment we observed our flux
calibrator. To adjust this to the time we were observing our target, we
use the Mini-Cals. Remember, the mini-cals tell us how our electronics
are performing at any instant, and they give us a quantity we call the
Receiver Gain. We do a mini-cal every 20 minutes or so, and it gives us a
Receiver Gain in units of Kelvin per micro-Watt. To determine the
receiver gain at the time we observed our flux calibrator, we do a
linear interpolation between the Receiver Gain found just before and
after observing the flux calibrator. Let's assume we measured a Receiver
Gain of G1 at time T1, a few minutes before looking at our flux
calibrator. Let's also assume we have another gain measurement of G2
taken at T2, a few minutes after looking at our calibrator. If the
calibrator was observed at time Tcal, we estimate the Receiver Gain at
time Tcal with this linear interpolation:
ReceiverGain_Cal = (G2 - G1) / (T2 - T1) * (Tcal - T1) + G1
Similarly, the Receiver Gain when looking at our target is given by:
ReceiverGain_Source = (G4 - G3) / (T4 - T3) * (Tsource - T3) + G3
where G4 and T4 are for a mini-cal just before observing the source, G5
and T5 are from the mini-cal after observing the source, and Tsource is
the time at which the source was observed.
We therefore can correct the EndToEndGain for changes in our receiver over time with this equation:
TimeCorrectedGain = EndToEndGain * ReceiverGain_Source / ReceiverGain_Cal
Another way of saying this is that the time correction to the EndToEndGain is a multiplicative factor given by:
TimeCorrection = ReceiverGain_Source / ReceiverGain_Cal.