[mmaimcal] CRE 173 B7 cross-polarization

[Steven T. Myers]

After reading it I still have no idea of what the level is
in science terms.  At -16db (the new spec) does that
mean 2.5% "leakage"?

The xpol from the slant grid looks to peak at the -6db
level in the beam, and is better within the 3db contour.
They give no pattern for the IR birefringence.  But in
the center of the beam we should still have reasonable
performance (impact: if you want good polarization maps
over a reasonable FOV then mosaic).

Also, since this seems to be due to optics, there is some
hope it is stable (and thus calibratable).  Do they
have any idea on stability?

   -s

On Fri, 1 Jun 2007, Al Wootten wrote:

> Folks,
>
> I commend to your weekend attention:
> 173.  	Band 7 cartridges: Cross-Polarisation
> CRE Number :ALMA-40.02.07.00-095-A-CRE
> (Attachments on edm)
>
> Description :
> The pre-production cartridges generally do not meet specification
> FEND-40.02.07.00-00290-00/T. We present in the attached pages an analysis
> that fails to account for the amount of cross polarisation that is measured.
> While we remain committed to investigate this issue before starting production,
> we must at present ask that the cross polarisation specification be set at a
> level that can reasonably be met by most cartridges. We ask for a relaxation
> of the specification to \u201316 dB, defined as the ratio of couplings to a
> calibration load through a wire grid, with the grid wires perpendicular and
> parallel to the nominal direction of the E vector.
>
> 173.2.    	Review comments Hans Rudolf
> 	 Hans Rudolf  	 2007-05-04 10:09   	Reply
> 	According to the remarks "We (IRAM) remain committed to analysing
> the cause for this and possible improvements" there is no need for a CRE,
> RfWs for the delivered cartridges should be prepared instead.
>
> However, I do not see an easy way to improve the x-polarisation performance.
> Most likely a redisigned optics would result in a major re-design of the band
> 7 cartridges, thus introducing cost and delay to the project. At this instance
> of the project such appears not to be an option.
>
>
> 173.3.   Further investigation of Xpol in B7 optics
> 173.3. 	 Bernard Lazareff 	 2007-05-31 11:38  	Reply
> 	SInce the CRE was submitted, we made dedicated measurements to analyze
> the cross-polarization of the B7 optics.
> 1) we find cross-polarization at a level consistent with what is expected from
> the slant geometry of the grid diplexer;
> 2) a design change that we had considered (rotating the grid to an angle that
> produces less Xpol) is not feasible because of the non-trivial relationship
> between the grid azimuth and the polarization PA in the telescope focal plane
> and at the location of the mixer feedhorns;
> 3) we find that there is another contribution to cross-polarization from the
> IR filter, probably resulting from the material anisotropy induced during the
> injection molding process.
> See attached memo (go to edm for the memo).
>
> As Hans says, it seems we have no options on this one.  We should, however,
> specify the compromises this CRE will force on ALMA polarization measurements,
> for which B7 is the band of choice.  We can always be the lone vote for
> a dissenting position.
>
> Clear skies,
> Al
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>

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|:| Steven T. Myers                      |:|  Tenured Astronomer       |:|
|:| National Radio Astronomy Observatory |:|  Ph:  (505) 835-7294      |:|
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|:| From 1-Nov-2006 to 8-Jun-2007 on Sabbatical at:                    |:|
|:| Los Alamos National Laboratory, Los Alamos, NM 87545               |:|
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