Optical gain notes

The following figures show the effect of pointing and refocusing on system optical gain, both under relatively benign conditions and when the temperature is increasing rapidly.  Hourly focusing and pointing keeps the gain error within about 15% under benign conditions (nighttime, wind < few m/s).  Errors up to a factor two are possible when the Sun warms the antenna.

The following plot shows normalized flux density for the pointing source during a 5:00-9:00 AM shift (2009 session 38).  The lowest curve in each color is a spectrum of the calibrator just before poiniting and focusing; the others are after pointing.  The grey curve shows repeatablilty of the standard spectrum, scan #19 at the beginning of the run.  All are 60s integrations with secondary nodding but no sky companion for subtraction. 

The plot below shows the receiver total power through the run -- dips mark pointing -- so the atmospheric transmission changed relatively little for most of the run.  The spikes at the far right end of the trace show noise diode power (the noise diode is almost always off except for receiver gain calibration).  Comparison of spectra above shows that the noise diode has no effect on spectral shape, as expected from its synchronization with data collection.

This next plot shows the normalized apparent flux density for a pointing source taken during a morning to noon run, 2009 session 37.  The drop in apparent flux density and changing tilt across the spectrum can be attributed to dish surface distortions as the antenna surface heated.  The weather was overcast; times, air temperatures, and elevations are on the plot.   All are 60s integrations with secondary nodding but no sky companion for subtraction.

Questions or comments?  Please contact Andrew Harris.