[mmaimcal] Matching of frequency profiles for ALMA and ACA correlators

[Mark Holdaway]

Robert,

It seems like it might be beneficial to have some imaging simulations
of this issue.  However, given that other errors (like pointing and
voltage pattern) are at a significantly higher level than 0.4%, it seems
that at first glance this should be OK.   However, the effects of the
pointing errors may be more randomized than the effect of the
spectral sensitivity functions being systrematically different.
It seems, if we were to simulate this, we would need to look at several
different sorts of objects with different spectral signatures -- 
rotating things,
expanding things, etc.   A new dimension to the library of simulation 
objects.

    -Mark

rlaing at eso.org wrote:

>The purpose of this e-mail is to start a discussion on a small issue which came 
>up during the PDR of the ACA correlator. It concerns the differences in
>frequency profile for the 64-station (hereafter ALMA) and ACA correlators. The
>former is an XF design, and the response for a narrow spectral line is a sinc
>function in the frequency domain. The latter is an FX design, and the response
>is a sinc^2 function.  
>
>See
>http://edm.alma.cl/forums/alma/dispatch.cgi/revsactive/docProfile/100643/d20050510035512/No/2005-05-09%20CORL-62.00.00.00-006-A-REP.pdf 
>
>(pp 45-46) and
>
>http://edm.alma.cl/forums/alma/dispatch.cgi/revsactive/docProfile/101003/d20050611223211/No/3-2Compatibility-freq-ch_profile.pdf
>
>for the material presented at the review.
>
>If we combine ALMA and ACA data without doing something to equalize the
>frequency profiles, there will be imaging errors. The ACA team have looked at
>reproducing the ALMA frequency profile from a linear combination of ACA channels
>(second document above). This gives fairly small differences except in the case
>of a uniform weighting function at the highest spectral resolution (see second
>document, above).  [It is, in any case, arguably the wrong way round, as the
>sinc^2 response of the ACA (FX) correlator has lower sidelobes.]
>
>In practice, it is likely that some weighting function other than uniform will
>be used to reduce sidelobes at the cost of spectral resolution. For example: 
> - A triangular weighting function in the time (lag) domain for the ALMA
> correlator will produce exactly a sinc^2 response. This matches the form of the
> ACA correlator response, but for a lower resolution. 
> - Hanning weighting (which is actually quite close to this) will also suppress
> the sidelobes very effectively (and the ACA correlator can reproduce this
> profile to within 0.4% - see second document) by weighted frequency binning.
>
>If the ALMA correlator is used at its highest resolution with uniform weighting,
>this approach does not work, since the ACA correlator cannot then bin over
>narrower channels.
>
>The design issues are therefore as follows:
>
>For anything other than the maximum frequency resolution, we should use 
>appropriate weighting functions (Hanning or triangular) to match the profiles.
>This should probably be the default option.
> 
>The match between the frequency responses of the two correlators at high
>spectral resolution can be improved by increasing the length of the FFT in the
>ACA correlator (to give narrower spectral channels) and then re-binning with
>appropriate weights.  Is this justified by the need to match the frequency
>profiles of the two correlators to better than the levels shown in the review
>documentation? [There are cost and schedule implications.]
>
>Do we need imaging simulations in order to answer this question, or are the
>existing calculations adequate?
>
>Regards
>
>Robert
>
>
>
>
>
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