Hi Danny, It is an internal document, I am going to ask if it is ok for me to forward it to you, probably it is fine but I will double check.
I am trying to think about this one a bit more… I am not sure I fully understand some of the reasoning behind it. Cheers, Eloy. > On 13 Oct 2015, at 19:50, danny jacobs <[email protected]> wrote: > > Eloy could you send the link to this full paper? I'm having trouble finding > it (Stephan has a lot of papers about "calibratibility", the most relevant of > which SKA-TEL.LFAA.SE.CAL-AADC-TN-001- seems to be not available online in > the form listed on his website.) > > Let me see if I can summarize the arguments. Spectral leakage from > foregrounds can be completely eliminated at the sensitivity of a 10 minute > integration by a combination of direction dependent calibration in the > primary fov and by the sidelobes being very low outside of the fov. Thus the > only physical requirement is on the beam inside of a 10 minute coherent > average on an EoR imaging slice of 1MHz. > > Where I'm a little confused is how we can get foreground subtraction to EoR > precision at a 10 minute cadence. Suppose there is a spectral ripple in the > feed at ~-25dB, below the noise level of a 10 minute integration. We can > integrate longer to try to calibrate it out, but technically that violates > the spec. > > Theres also a more general critique to be made, that the spec starts by > assuming perfection in calibration and foreground removal, something that has > yet (as far as I know) to be demonstrated in practice. Presumably there are > practical things we can do on the instrument side to make this process > easier. > > Cheers, > ~Danny > > > > On Thu, Oct 8, 2015 at 1:16 AM, Eloy de Lera Acedo <[email protected] > <mailto:[email protected]>> wrote: > Hi Danny, > > I think the 60 MHz spacing is not related to delays at all, although I may be > wrong… I never heard that argument within SKA. > > That requirement was derived from the following argument (from a “calibration > requirement” document by Stephan Wijnholds): > > ——————————————————— > " > > This requirement is currently formulated as “The stability of the average > station band shape post RFI-mitigation shall be within +/- TBD %” [1]. > Combined with the requirement on station beam stability, this indicates, that > in the envisaged mode of operation of the LFAA element, the station > calibration should ensure that the station beam behaves spectrally and > spatially according to a specified full-polarization beam model thus allowing > sufficiently accurate prediction of the station response during processing of > the station signals. To set a meaningful quantitative requirement on the > spectral and spatial stability of the station beam, we thus need to see what > accuracy is required for array level calibration. > > We expect to be able to exploit our knowledge of station configuration and > antenna response to predict the station beam after station calibration. In > practice, station calibration will be done with finite accuracy and we will > have imperfect knowledge of the antenna response, for example due to > manufacturing tolerances. This may result in systematic deviations from the > predicted station response. Fortunately, we can correct such errors by > direction dependent calibration [2][3] at a cadence of typically 10 minutes. > Below, we derive a requirement on the required station beam accuracy for > sources inside the FoV. For the impact of sources outside the FoV, we rely on > suppression of these sources by the station sidelobes. Combined with the > assumed 10-minute update rate, the station beam accuracy requirement > naturally leads to requirements on the spectral and spatial beam stability. > > In [4], two distinct approaches are presented that relate the impact of > station beam errors to the thermal noise. Although the mathematical bounds > used in these two approaches can lead to slightly different proportionality > constants (the factor 1/√2 in the equation below), both lead to a similar > result stating that the relative errors on the station beam (relative to the > main beam peak) should be smaller than > > <Screen Shot 2015-10-08 at 09.12.54.png> > > where ∆S0/√(B·t) is the noise level of a single station after integration > over bandwidth B and time t and Srms is the total RMS flux in the image. > > The instantaneous thermal noise level ∆S0 = 2 kB·Tsys / Aeff follows directly > from the sensitivity requirement and Srms can be calculated based on the > source statistics from [5] and the size of the FoV of a 35-m station. We will > assume that the sensitivity of the full LFAA system will be 100, 760, 1025 > and 975 m2/K at 50, 100, 160 and 220 MHz respectively and that the data are > split in slices of about 1 MHz for EoR tomography [6]. Since direction > dependent array calibration can be exploited to correct systematic errors in > the beam model at time scales of order 10 minute, we will use t = 600 s. This > leads to the following reformulation of requirement SKA1-SYS_REQ-2621: > > Spectral stability. The spectral stability of the station beam bandpass post > station calibration and RFI mitigation shall be within 1.3%, 0.4%, 0.6% and > 1.1% at 50, 100, 160 and 220 MHz respectively compared to then full > polarization parameterized beam model. > > " > > ——————————————————— > > > Cheers, > Eloy. > >> On 8 Oct 2015, at 00:48, danny jacobs <[email protected] >> <mailto:[email protected]>> wrote: >> >> Maybe there is? Looking at the requirements in the "SKA phase 1 (level 1) >> requirements" I see the following >> >> "The spectral stability, on a time scale of 600 sec.,of the station beam >> bandpass, post station calibration and RFI- mitigation, shall be within 1.3 >> %, 0.4 %, 0.6 % and 1.1 % at 50 MHz, 100 MHz, 160MHz, and 220 MHz >> respectively compared to the full polarization, parameterized beam model. >> >> Translating into the HERA framework: >> I read this as a suppression of all variation to the -22dB level at 100MHz. >> Since these numbers are given in ~60MHz bins we could read this as >> requiring -22dB at 16ns or longer. >> >> How does this relate to the HERA spec? The HERA specification is -60dB >> suppression at reflection time scales of 60ns and longer. (This spec is >> rough and will change slightly as we refine it but this is in the ballpark.) >> >> We can translate the (current) HERA spec to SKA spec language: Spectral >> stability shall be better than 1e-4 % on spectral scales of 17MHz or shorter. >> >> So looked at this way, the HERA spec and the SKA spec are about 4 orders of >> magnitude apart. Recently we've discussed the possibility that the HERA >> spec is a little more stringent than we absolutely need, so we'll see where >> we land. >> >> >> Digging further into the SKA spec: >> >> Looking back even further into the level 0 (science) requirements that are >> cited as the origin of the 0.6% spectral stability requirement I find this >> "SKA1 shall possess relative calibration accuracy of adjacent frequency >> intervals of better than 40 dB for Δν/ν=10^-2 to Δν/ν=10^-5" >> >> At 100 MHz this corresponds to a ripple constraint of -40 at 1000ns or >> longer. This one is hard to understand as flowing from a 21cm science case. >> Its also hard to see how that flows into the ~1% error allowable in the >> level 1 spec. >> >> What am I missing here? >> >> >> >> >> >> >> >> On Wed, Oct 7, 2015 at 4:15 PM, Eloy de Lera Acedo <[email protected] >> <mailto:[email protected]>> wrote: >> Hi Miguel, >> >> Yes, understood. I have to say also that SKA does not have any requirement >> for spectral ripple in the sense HERA does. I never heard about anyone >> thinking about doing avoidance with SKA for example, or at least that never >> got translated into requirements. The only concern I have appreciated is >> about calibratabilty and absolute A/T across the field of view. >> >> Eloy. >> >>> On 7 Oct 2015, at 23:28, Miguel Morales <[email protected] >>> <mailto:[email protected]>> wrote: >>> >>> Hi Eloy, >>> >>> I’d like to mention that the spectral features we are interested in are at >>> the per element not the per station. While per element spectral features do >>> average out in frequency, they don’t go away, they’re just moved to the >>> station beam (spatial). As we need exquisite control of both, this doesn’t >>> really help us. So the golden metrics for me is per antenna response and >>> how repeatable it is antenna-to-antenna. >>> >>> Best, >>> >>> Miguel >>> >>> >>> >>> >>> >>>> On Oct 7, 2015, at 8:40 AM, Eloy de Lera Acedo <[email protected] >>>> <mailto:[email protected]>> wrote: >>>> >>>> Hi John, >>>> >>>> I am checking with our australian colleagues, it should not be there. I >>>> can’t remember now if we ever discussed this. This is also a very small >>>> array of course (16 antennas) so it has some features that won’t be >>>> present for larger stations. >>>> >>>> Eloy. >>>> >>>>> On 7 Oct 2015, at 16:32, Jonathan Pober <[email protected] >>>>> <mailto:[email protected]>> wrote: >>>>> >>>>> 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] >>>>> <mailto:[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 >>>>> <http://rd.springer.com/article/10.1007/s10686-015-9439-0> >>>>> >>>>> Cheers, >>>>> Eloy. >>>>> >>>>>> On 7 Oct 2015, at 15:31, Jonathan Pober <[email protected] >>>>>> <mailto:[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 <http://arxiv.org/abs/1510.01515> >>>>>> >>>>>> (There seems to be a prevalance of 15's here, which numerology.com >>>>>> <http://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 <tel:%28%2B44%29%20%280%291223%20337563> >>>>> Email: [email protected] <mailto:[email protected]> >>>>> Webpage: http://eloydeleraacedo.weebly.com >>>>> <http://eloydeleraacedo.weebly.com/> >>>>> >>>> >>>> -- >>>> 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 <tel:%28%2B44%29%20%280%291223%20337563> >>>> Email: [email protected] <mailto:[email protected]> >>>> Webpage: http://eloydeleraacedo.weebly.com >>>> <http://eloydeleraacedo.weebly.com/> >>> >> >> -- >> 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 <tel:%28%2B44%29%20%280%291223%20337563> >> Email: [email protected] <mailto:[email protected]> >> Webpage: http://eloydeleraacedo.weebly.com >> <http://eloydeleraacedo.weebly.com/> >> >> >> >> -- >> >> National Science Foundation Fellow >> Arizona State University >> School of Earth and Space Exploration >> Low Frequency Cosmology >> Phone: (505) 500 4521 <tel:%28505%29%20500%204521> >> Homepage: http://loco.lab.asu.edu/danny_jacobs/ >> <http://loco.lab.asu.edu/danny_jacobs/> > -- > 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 <tel:%28%2B44%29%20%280%291223%20337563> > Email: [email protected] <mailto:[email protected]> > Webpage: http://eloydeleraacedo.weebly.com > <http://eloydeleraacedo.weebly.com/> > > > > -- > > National Science Foundation Fellow > Arizona State University > School of Earth and Space Exploration > Low Frequency Cosmology > Phone: (505) 500 4521 > Homepage: http://loco.lab.asu.edu/danny_jacobs/ > <http://loco.lab.asu.edu/danny_jacobs/> -- 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 <http://eloydeleraacedo.weebly.com/>
