|Scans 11-16||Focus and Peak|
|Scans 17-56||Total power nods, 30 sec each scan.|
Result of peak scan on 3C286
The DCR (digital continuum receiver) recorded two IF bands of 800 MHz bandwidth from each of the two beams and using two polarizations, hence 8 channels total. Due to an error in configuration, beam 3 was observing a band centered at 23 Ghz, and beam 4 at 24 GHz. The intention had been for each beam to observe two frequencies. Nevertheless, the setup is summarized in the following table.
|1||9||L3||23 GHz||5.15 K|
|2||10||L4||24 GHz||3.18 K|
|3||11||L3||23 GHz||5.15 K|
|4||12||L4||24 GHz||3.18 K|
|5||13||R3||23 GHz||5.27 K|
|6||14||R4||24 GHz||5.01 K|
|7||15||R3||23 GHz||5.27 K|
|8||16||R4||24 GHz||5.01 K|
dTr = Tsys/sqrt( Npol * Tint * BW)
Atmospheric noise To estimate the atmospheric term (dTa)
we take the data for each
30-second scan, convert to units of temperature, and determine the
rms residuals to a linear fit. The linear fit simulates the effect
of baseline removal or beam switching at a few second rate.
For off-source scans, this rms is an estimate of atmospheric fluctuations;
for on-source scans, it includes the additional effects due to
a strong source.
Some typical blank sky scans are illustrated in the next graph, (Linear fit not removed).
To estimate the gain fluctuations, we look at the difference between
the noise-cal-on and noise-cal-off phases. The calibration noise
source is injected before the first amplifier stage and is switched
on and off at a 1 Hz rate. The DCR integrates the power from these
two phases separately. Thus for each integration we can look at the
"delta_cal" or the difference between the cal-on and cal-off phases,
and knowing the value of the calibration (Tcal) in temperature units,
one can convert this difference to temperature units. The delta-cal
is proportional to the gain (i.e., the conversion between counts and
temperature). The rms of the delta_cal over a scan gives an estimate
of the gain fluctuations.
Specifically: dTg = Tsys * rms(delta_cal) / median(delta_cal)
The following graph shows Tsys * delta_cal / median(delta_cal) for a few typical scans. The plot is for beam 3, offsource.
It is of interest to note that beam 4 seems to perform
considerably better than beam 3. The next graph shows
the similar plot for beam #4.
Since beam 3 appears to be excessively noisy, we will consider only beam 4 and will look at the off-source data. This will give results relevant to detecting faint sources. The results, with rms temperatures in milli-Kelvin (mK) are summarized as follows.
|statistics for beam 4|
|Tintegration||dTa (mK)||dTg (mK)|