<div dir="ltr"><div><br>All-<br><br></div><div>Recently, there have been a number of ~disparate ongoing discussions about<br>the labeling of X and Y polarization in the <1 GHz bands on the EVLA.<br>
</div><div>I'd like to clarify what I believe the desirable end-state should be, as <br>I've promised this description to a number of people, and especially because <br>I do _not_ think wire-swapping (as some discussions may be tending)<br>
is necessary, in fact.<br>
<br></div><div>The basic discrepancy in the current labeling scheme is that the dipoles<br></div><div>that are parallel to the elevation axis are labeled "X" (or "XLP"). This is<br></div><div>effectively the reverse of the IAU convention[1]. This is best understood by <br>
considering an instantaneous observation on the meridian looking south (i.e.,<br>telescope is tilted). At that instant, when sky and telescope coordinates are <br>aligned, it would be least confusing (and require the least s/w) if our telescope <br>
coordinate labels matched the IAU convention, which has "X" == "vertical" <br>(i.e., parallel to lines of R.A. in sky coords). Instead, in the current EVLA <br>labeling scheme, the "Y"-labeled dipole will be "vertical". Arguably, this is <br>
mainly semantics, but it has very practical and significantly confusing <br>consequences that are worth avoiding.<br></div><div><br>Since the "X" and "Y" labels are ultimately used in post-processing to trigger<br>
formal Stokes parameter conversions in a manner that _is_ consistent with IAU<br>conventions (namely, that the <XX> correlation nominally measures I+Q, <br>etc.), an additional 90 deg rotation is required for proper position angle <br>
calibration[2]. Application of the 90 deg pos angle calibration amounts to<br></div><div>converting to the sky polarization (IAU) coordinates, and therefore effectively <br>swaps visibilities w.r.t. to their correlation labels such that an "XX" correlation <br>
will thenceforth contain data that, in fact, originated in dipoles labeled "Y" on the <br>telescopes (and similar transpositions for the other 3 correlations). In other <br>words, discussions of actual dipoles between astronomers and engineers will <br>
be potentially confused by the particular state of the pos angle calibration of the <br>dataset in question. (Inasmuch as the nominal 90 deg rotation will be a <br>hard-wired correction [see [2], below], it may often be ambiguous whether or <br>
not it has, in fact, been applied to the data appearing in some plot. One must be <br>explicitly cognizant of the pos angle calibration state, in any case.)<br><br></div><div>So, <b>I'd propose that we merely begin referring to the "vertical" (when antenna is<br>
tilted) dipoles as "X" or "XLP" and "horizontal" (parallel to elev axis) dipoles<br>
</b></div><div><b>as "Y" or "YLP". This implies only _re-labeling_ (lots of stickers, cable tags,<br></b></div><div><b>as well as engineering documentation updates), and (I emphasize) it _does_not_ <br>
compel any rewiring or cable-swapping</b>[3]. Since the nominal pos ang calibration <br>will no longer be required, the confusions described above disappear, and far <br>less implicit management of the orientation in s/w is required. I'd note also that <br>
the mere relabeling will also result in conventionally-ordered correlations in the output <br>dataset, i.e., [XX, XY, YX, YY], a mainly cosmetic feature that nonetheless probably <br>simplifies data transport issues (or at least potential confusions therein).<br>
<br></div><div>To be clear, the new labeling scheme would be:<br><br></div><div><b>"vertical" (when antenna tilted) == "X" == goes where "R" goes<br></b></div><div><b>"horizontal" (parallel to elev axis) == "Y" == goes where "L" goes</b><br>
</div><div><br></div><div>Currently, the correlator back-end is not distinguishing the linear feed<br></div><div>bands when labeling the output data. I.e., I believe we still get data labeled <br>
</div><div>[RR, RL, LR, LL]. In the current dipole labeling scheme, we should<br>be getting [YY, YX, XY, XX] for raw data (telescope frame), which will look quirky <br>cf any other linear feed telescope (only cosmetic, in principle, but s/w <br>
must cope and this has not been exercised). In the IAU-conformant labeling <br>scheme, we would get [XX, XY, YX, YY]. These labels originate in the Feed.xml and<br>Polarization.xml SDM tables. The IAU-conformant case requires<br>
</div><div>changing R to X and L to Y (or the corresponding changes to the appropriate<br>
enums) in the SDM-writer when the low bands are observed. (The existing <br>labeling scheme requires the opposite substitutions, as well as also recording <br>the 90 deg pos angle offset in Feed.xml.)</div><div><br>From the science operations perspective, note that introducing the <br>
IAU-conformant labels in existing SDMs will (trivially!) yield datasets <br>requiring no implicit correlation sorting or _nominal_ position angle <br>calibration in post-processing code. I.e., they will work correctly in <br>
(at least as regards polarization coordinates for science purposes) <br>_immediately_. When an astronomer says "X" it will mean the vertical<br></div><div>dipole, so confusion remains until the h/w labels are changed.<br>
</div><div>(If the dipoles themselves are grossly and systematically mis-oriented <br>_mechanically_, a ~conventional pos angle calibration using a reference <br>source will still be required, of course.)<br>
</div><div><br>(Someone should think about whether or not the relabeling compels<br>a swap in the Tcal value catalogs for these bands. Since we wouldn't <br>be changing the order in which the physical signals are presented to the<br>
band-independent parts of the system, it is not clear to me whether<br>a swap of Tcal values is necessary. It is perhaps possible that<br>the current Tcal value ordering may, in effect, be inconsistent with the <br>current dipole labeling!)<br>
<br></div><div>Cheers,<br>George<br><br></div><div><br>[1] According to Hamaker & Bregman (1996, A&AS 117, 161; see <br>
their Fig 1), the formal IAU definitions for polarizations in _sky_ <br>coordinates has "X" parallel to lines of constant R.A. (i.e., vertical<br>in images) and "Y" parallel to lines of constant Decl (i.e., horizontal). <br>
This arises from defining position angle zero to be a _vertical_ <br>orientation in sky coordinates (coinciding with the 'X' axis), and <br>measured positive counter-clockwise. With this convention,<br></div><div>
an XX correlation measures Stokes (I+Q) (i.e., with a plus sign), etc.<br></div><div><br>[2] Performing the 90 deg rotation is trivial, and SDM and MS have provisions<br>for effectively hard-wiring the application of this rotation via information<br>
extracted from the Feed.xml table. This is used routinely for ALMA which <br>has dipole feeds at a variety of non-standard orientations ("X" is always <br>the feed oriented radially on the off-axis feed circles). However, the specific <br>
case of a 90 deg position angle calibration causes particular confusion when <br>discussing specific dipoles as described in the text above.<br></div><div><br></div><div>[3] Swapping the cables (e.g., on the output of the low-band receiver)---<br>
such that the "X" labeled signal follows the path "RCP" takes for ordinary<br>
</div><div>EVLA bands---will simply reverse the labeling of correlations in the resulting<br></div><div>SDM: [YY, YX, XY, XX] --> [XX, XY, YX, YY]. Cosmetically desirable, but this <br>has no practically useful effect on the position angle calibration issue <br>
described above, and so does _not_ relieve the confusions attending<br>discussions of actual dipoles. As such, swapping cables amounts<br>to a lot of work with _only_ cosmetic (mainly) benefits. Note that both<br></div>
<div>swapping cables _and_ relabeling dipoles (equivalently, physically rotating<br>
the dipoles themselves by 90 deg, if that's possible) will relieve the pos angle <br>confusions but yield the unconventional correlation ordering: [YY, YX, XY, XX].<br></div><div><br></div></div>