1. EVLA Spectral-Line Science Below 1200 MHz
E. Momjian & R. Perley (NRAO)
Abstract
The new L-band system of the EVLA and the array’s enhanced capabilities will make it possible to carry out observations below 1200 MHz and down to 940 MHz; a frequency range where the VLA has severe limitations and/or simply cannot observe. To test whether astronomical science can be delivered at such frequencies, we carried out observations on two HI absorbers using the B-configuration at 1139 and 1082 MHz. Although bellow 1150 MHz the band is dominated by RFI due to frequencies assigned to aeronautical navigation bands, we were able to obtain results consistent with the expected performance of the array with its current capabilities. Once completed, the EVLA will provide much improved sensitivity at the lowest L-band frequencies, making the array a major instrument in studies of redshifted HI and other spectral lines down to 940 MHz.
RFI at L-band
The figures below show the RFI environment between 940 and 2100 MHz, and 940 to 1200 MHz, obtained by monitoring the total power of an EVLA antenna for 24 hours (courtesy of B. Hesman and W. Brisken).
The RFI between 1025 and 1150 MHz is primarily due to aeronautical navigation systems. The strongest of these are the radar transponders at 1090 MHz (air to ground) and 1030 MHz (ground to air). The weaker RFI are due to two systems, civilian DMEs (Distance Measuring Equipment) and Military TACAN (TACtical Air Navigation) that occur every 500 kHz in the range 1025−1150 MHz. Night-time hours show a significant reduction in the RFI.
MHz
Optical Redshift: z =
HI frequency: 1139 MHz; a frequency that the whole array (EVLA+VLA antennas) can tune to. This frequency falls within one of the good frequency ‘windows’ of the VLA (Figure 1).
Array configuration: B (baseline range 0.21−11.4 km).
Total time: 1 hour (~75% on source) starting at ~7 am MST.
Bandwidth: 3.1 MHz with 127 spectral channels.
RFI: The Figures below show time vs. frequency for three baselines in the array, a short, an intermediate, and a long one.
Short baselines are clearly affected by RFI from navigation systems.
These RFI pulses are 3.5 msec or ~1.1 km long, which is shorter than most of the baselines of the B configuration, hence will not correlate on most baselines in this experiment.
Results: The Figures below show a continuum image of the source at 1139 MHz obtained by using the line free channels, and two HI spectra, one with all the antennas (total of 26) and another with the EVLA antennas only (total of 12). The rms noise values in both spectra are consistent with those anticipated at this frequency.
Short baseline
Intermediate baseline
Long baseline
VLA+EVLA
EVLA only
VLA vs. EVLA at L-band
, but with limited capabilities below 1220 MHz (see Figure 1)
Actual BW (MHz) *
Design BW (MHz)
EVLA
VLA
*Currently most EVLA antennas use the OMTs of the VLA. Only one EVLA antenna is equipped with the new prototype OMT. Full outfitting with the new OMTs is planned for 2009.
PKS 1082 MHz
Optical Redshift: z =
HI frequency: 1082 MHz; a frequency that only EVLA antennas can tune to.
Array configuration: B (baseline range 0.21−11.4 km).
Total Time: 1 hour (~75% on source), starting at ~2 am MST.
Bandwidth: 3.1 MHz with 127 spectral channels.
RFI: None!
Results: The Figures below show a continuum image of the source at 1082 MHz obtained by using the line-free channels, and the spectrum of the redshifted HI line. The rms noise in the spectrum is consistent with that anticipated at this frequency. The number of antennas in these observations was 13 (all EVLA).
EVLA only
Figure 1: The VLA antenna sensitivies between 1100 and 1250 MHz. The ‘windows’ of poor sensitivity are caused by resonances within the VLA’s dielectric phase shifter and/or microwave lens (Perley & Hayward 2008). This behavior is not seen in the EVLA antennas, because they don’t have the microwave lens and the dielectric phase shifter of the VLA. The arrow in the figure points to the frequency ‘window’ where the redshifted HI line of PKS is located at.
PKS
The Future
The results presented here clearly show that the EVLA will become a major instrument for spectral line observations below 1200 MHz.
Tests will be carried out to better understand the effect of the radar transponders used for aeronautical navigation during day time and in array configurations dominated by short baselines (i.e., C and D).
Moreover, tests below ~1 GHz will be carried out upon the availability of more EVLA antennas that are equipped with the new, fully upgraded, OMTs, which will deliver the full sensitivity at the lowest L-band frequencies.
Don’t forget to attend Juergen Ott’s presentation “The EVLA: prospects for HI” on Sunday to learn more about the EVLA.