Feb 28 MiniRF: preliminary calibrations and results.
Comparison with Cygnus A
Cygnus A is one of the brightest sources, with a well-determined and non-variable flux density of 1187 JY at 2384 MHz (Ott et al 1992).
In scan numbers 23 and 24 we observed Cygnus A and a nearby blank sky region. The attenuators were set to the "standard"
that was used for all miniRF observations, hence gains should have been the same for both miniRF and Cygnus A.
The spectra were integrated over the portion containing the miniRF spectrum, namely 2383 to 2385.1875 MHz as was done to
construct the total power beam patterns displayed on the Feb 28 observation page
To do this, the raw counts in the spectra were added up for channels 400 through 623. These totals are shown in the table:
| || integrated counts in || || power/CygA |
| scan || XX || YY || XX+YY |
| scan 23: On Cyg A || 4.827e6 || 3.915e6 || 8.742e6 |
| scan 24: off Cyg A || 4.67e4 || 1.187e5 || 1.654e5 |
| On minus off || 4.780e6 || 3.796e6 || 8.576e6 |
| scan 43: miniRF AZ scan || 6.613e7 || 7.248e7 || 1.386e8 || || 16.16 |
| scan 63: miniRF EL scan || 6.564e7 || 7.343e7 || 1.391e8 || || 16.22 |
So the power of the maximum of the miniRF beam pattern is about 16.2 times Cygnus A.
What is the power at the input of the GBT feed due to Cyg A?
It would be the flux density times the effective area times the bandwidth;
i.e. 1187e-26 * 5101.1 * 2.1875e6 = 1.3245e-13 watts ==> -98.78 dBm
- Thus the miniRF power is 16.2 times that, hence -86.68 dBm.
This value is an average over 0.5 second containing a burst pattern. The duty cycle within 0.5 second
- 100usec * 128 / 500ms = 0.0256 ==> -15.9 dB
So our estimate of the peak power of a pulse would be:
- -86.68 + 15.9 ==> -70.77 dBm
and this is not too far off from Roger's estimate of this peak received power of -75.1 dBm.
Summary of calculations
| Cyg A power at input to feed
| miniRF 0.5s ave = 16.2xCygA
| miniRF pulse peak
- 23 Mar 2009
Frank - is it possible that the averaging time to use is 1.0 sec instead of 0.5 sec?
If that were the case, then the averaging correction would be 10.3 dB, which would then make the measured power (-76.4 dB) be almost exactly what Roger calculated as expected (-75.1 dB).
- 13 May 2009