Thanks, Eloy. The feature at 100 MHz in the Y polarization is definitely the most prominent. Maybe it's related to FM? The X polarization also has smaller features around the gap at ~130 MHz, possibly also related to flagging, and a discontinuity at ~180 MHz. I was postulating the that latter might be due to different 30.72 MHz chunks being observed on different nights. The explanation for the overall larger values at higher frequencies seems totally plausible.
On Wed, Oct 7, 2015 at 7:50 AM, Eloy de Lera Acedo <[email protected]> wrote: > Hi John, > > Do you mean the feature at 100 MHz? That one is not predicted by the > modeling and indeed it is very strange because it does not show up in the > other polarisation. We will check with our Australian colleagues, see what > they think. That should not be there. > > The reason why the A/T measurements give larger values than the model can > be related to this: > > "For Hydra A, our model predicts the flux density at 300 MHz to be ∼1/3 > that at 74 MHz. However, detailed measurements specifically targeting Hydra > A at 74 and 330 MHz [35] show that the flux density of the compact central > region reduces less steeply with frequency, and would be only ∼1/2 that > at 74 MHz. Unlike the surrounding diffuse emission, the compact central > region is not spatially filtered at the higher frequencies, thus our > modelled "known" flux density causes an underestimate of the actual flux > density, resulting in a higher than actual A/T . In future processing of > measurement, we expect significant convergence as the models of calibrator > sources are improved for the MWA and other wide-band low-frequency > telescopes." > Regarding A/T, in this paper (Fig. 15) you have our best estimates for A/T > calculation for SKA before re-baselining (before halving the number of > elements). > > http://rd.springer.com/article/10.1007/s10686-015-9439-0 > > Cheers, > Eloy. > > On 7 Oct 2015, at 15:31, Jonathan Pober <[email protected]> wrote: > > I'm not used to thinking about A/T this early in the morning, but are > Figures 14 and 15 what we've always wanted to see from the SKA? Measured > (and simulated) frequency responses? > > http://arxiv.org/abs/1510.01515 > > (There seems to be a prevalance of 15's here, which numerology.com tells > me corresponds to "Loving, forgiving, tolerant" -- for those of you perhaps > looking for guidance in your relation to the SKA.) > > In Figure 14 there are a few sharp features beyond what the model > predicts. I need to read the paper in more detail to figure out exactly > what the measurement is. Those could be due to sidelobes from other > sources affecting their spatial filter, although they are using the full > 128 tiles of the MWA for cross correlation, so the PSF should be good. > They're also stitching together the band from several 30.72 MHz chunks, so > those could be artifacts at band edges. > > Maybe Eloy and/or Nima have some thoughts on these results? Very nice to > see this work taking place and moving ahead. > > Cheers, > > Jonnie > > > -- > *Dr. Eloy de Lera Acedo* > > *Senior Research Associate* > Astrophysics Group > Cavendish Laboratory > University of Cambridge > JJ Thomson Avenue > Cambridge CB3 0HE > > *Teaching Associate and Bye-Fellow* > Downing College > Regent Street > Cambridge CB2 1DQ > > Telephone: (+44) (0)1223 (3)37365 > Fax: (+44) (0)1223 337563 > Email: [email protected] > Webpage: http://eloydeleraacedo.weebly.com > >
