In LBA_SPARSE mode half of the dipoles, distributed across the station, are used. This mode grants an intermediate performance between LBA_INNER (not offered in Cycle 16-19) and LBA_OUTER (offered in Cycles 16-19), with a suppression of the magnitude of the side-lobes compared to the latter. The effective size of the station is around 65 m, which provides a primary beam FWHM of 4.9 deg at 54 MHz.
Note that because of the ongoing commissioning activities, observing with this antenna mode is only enabled as a shared risk mode and only the mode 'SPARSE_EVEN' is made available. Although a suboptimal performance of stations CS031, CS302 is known because of old station calibration tables installed at these stations, the overall performance is good and high quality imaging can be performed with this mode.
Zoomed central region of a large mosaic obtained from calibrated and imaged LBA_SPARSE_EVEN dataset with an exposure of 3 hours. An rms noise of ~1.3 mJy/beam is achieved at the resolution of 15".
In the following table a summary of the performance of the system is reported, based on a number of commissioning observations to test the LBA_SPARSE_EVEN mode. Also, a comparison with LBA_OUTER is performed in order to provide proposers simple guidelines to make their antenna mode choices.
Pros
Cons
LBA_SPARSE_EVEN
LBA SPARSE is recommended for survey observing that wants to maximize the area due to the larger FoV.
The station sidelobes are suppressed, resulting in less interfering sources in the far sidelobes.
More computational expensive than LBA_OUTER.
A couple of stations (CS031, CS302) currently behaving better in LBA_OUTER mode.
LBA_OUTER
LBA_OUTER is better suited for pointed observations on a single target of interest; because of the smaller image sizes needed to cover the FoV, less computational resources and less directions needed for DDE (direction-dependent) calibration.
Station calibration tables are different for LBA_OUTER and LBA_SPARSE_EVEN, resulting in certain stations (e.g. CS031, CS302) currently behaving better in the LBA_OUTER mode.
The station sidelobes are less suppressed, resulting in a larger impact of interfering sources in the far sidelobes.
