[mmaimcal] CRE 173 B7 cross-polarization

[Al Wootten]

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