ATCA Atmospheric Seeing Monitor (ASM)
With the start of science operation of the ATCA's high-frequency receivers in the 12 mm and 3 mm bands, a tool became necessary to support observers in assessing whether the atmospheric conditions are good enough for their hiqh-frequency observing programs. Since May 2004 this information is provided by the ASM.
The ASM is a two-element interferometer operating at 30 GHz, with a baseline length of 230 metres. Each antenna is 1.75 m in diameter (see photo). The antennas are fixed in a certain position, pointing towards the Optus-B3 satellite (in the meantime the satellite has been decommissioned and another one is used), which has a 30 GHz beacon onboard.
Each antenna is equipped with a low-noise amplifier (LNA) in its prime focus that operates at the observed radio frequency (RF). From there the signal is conducted into the airconditioned cooler box seen in front of the antenna in the photo, which contains electronics to further amplify and down-convert the RF signal to 240 MHz. The 240 MHz output is transported to the screened room of the ATCA control building, where it is fed into the ASM backend (the so-called "holography rack").
The "holography rack", which was developed do perform holography measurements of the ATCA antennas, is also a key component of the ASM. Its components are described on the holography page. The down-converted signal from both antennas is fed into lock-in amplifiers to achieve stable phase lock between the two incoming signals. The phase of one antenna's incoming signal is used as reference with respect to which the phase of the second is plotted.
Photo of the holography rack developed by ATNF, which is used for both antenna holography and as the backend of the Narrabri Atmospheric Seeing Monitor. As displayed it was used during a field test of one of the two ASM stations during the final phase of construction
Normally the holography rack is kept inside the screened room of the ATCA observatory in order to prevent radio interference from its electronics components. It is shown in its usual place in the photo below.
Photo of the holography rack developed by ATNF, which is used for both antenna holography and as the backend of the Narrabri Atmospheric Seeing Monitor. Here it is displayed in its function as the ASM backend, inside the screened room at Narrabri
ASM output and user interface
The output from the ASM backend is stored in a database, from where it picked up, undergoes some basic processing and is subsequently displayed by one module of the new Compact Array monitoring software, MoniCA.
Processing of the ASM data is necessary, because the satellite is not kept exactly in one spot above the surface of the Earth, but moves slowly following a figure-8 pattern. Hence the incoming signal will display slow phase changes with time. These are subtracted from the observed data, based on the satellite ephemerides provided kindly by Optus, after which (if the model predictions for the satellite orbit match the actual motion) the residual phase noise will reflect the turbulent properties of the atmosphere along the line-of-sight to both antennas - see figure below.
Predicted phase for given satellite ephemerides (upper panel). The observed phases are displayed in the central panel. The bottom panel shows the atmospheric contribution to the phase, obtained as the difference between observed and predicted phase (plot produced by M. Kesteven)
MoniCA's GUI is user-configurable to display various quantities fed back by the ATCA. Amongst many others, there is also a default setup for ASM output display. The relative phase variation recorded by one of the ASM antennas compared to the other, after correction for the satellite's motion, is translated into a pathlength difference, in micrometers, which is plotted against time. A 1-day display window is used for a detailed view, while a 7-day window provides a more long-term overview. One fiducial day's ASM output is displayed below.
MoniCA ASM output for 29 August 2004. The long-term trend (upper panel) and short-term phase variability (lower panel) are displayed. All display parameters are in practice configurable. (Plot produced by D. Brodrick)
The example above shows data of a day during which an rms pathlength scatter of up to 1000 micrometres was measured (which is not really good for millimetric observations). A clear diurnal pattern is visible - the previous nights during that week, with rms noise levels of order 100 micrometres, offered perfect conditions for high-frequency observations, while on the night of 29 August the conditions worsened visibly. 21 and 23 August 2004 were exceptionally good days for mm observations at Narrabri.
The ASM measurements are available to ATCA observers on a display screen in the control room for on-the-spot decisions on the suitability of the observing conditions for their programs. The ASM database can be used offline to collect atmospheric stability statistics over long periods of time.
Relative phase differences between the two incoming signals depend on the length of the baseline of the interferometer and on the observing frequency. Using the appropriate values for the currently used configuration and the frequency of the planned observations, these values can be scaled to be used by an observer to directly assess the expected phase (in)stability under the given atmospheric conditions.
Note that the ASM measures only phase variability, not the atmospheric transmission, for which water vapour radiometers are in development.
A short summary of the ASM is also available in an ATNF Newsletter report.