[evlatests] 3-bit polarization/stability test

[Bill Cotton]

Rick,

   You know, no quasar is really unpolarized at high enough frequency.
Also, 3C84 is "strongly" circularly polarized at cm wavelengths - ~0.5%.

-Bill

"RP" == Rick Perley <rperley at nrao.edu> writes:

RP>     ... was going to send this to specific individuals ... but it's 
RP> easier to do it this way ...

RP>     We talked yesterday about a longish observation of OQ208 for the 
RP> main purpose of determining variability of the antenna/electronics 
RP> polarization.  There are worries about temperature sensitivities, so an 
RP> observation of a source through dawn was felt to be optimum.  The 
RP> previous test was of 3C286, which has the distinct disadvantage of being 
RP> very highly polarized, making it difficult to isolate small 
RP> antenna-based effects.   Polarization tests are much better done with a 
RP> strong unpolarized object. 

RP> Unfortunately, at this time of year, the highly preferred source -- 3C84 
RP> = 0319+4130 (30 Jy) is an evening source, so OQ208 = J1407+2827 seems 
RP> preferred. 

RP>     But I think 3C84 should still be utilized, and the test be done in 
RP> late afternoon-evening, when temperature differentials are nearly as 
RP> strong as they are in the morning.  3C84 rises just before sundown.  The 
RP> maximum rate of change of temperature in the vertex room is likely to be 
RP> during these evening hours.  I suggest we do this *tonight*, and that we 
RP> use Ku-band.  Amongst other things, we know the polarizer purity at 
RP> Ku-band is better than at X-band, -- making electronics polarization 
RP> changes even easier to detect. 
RP>     One of the distinct advantages of 3C84 is its very high flux density 
RP> -- 15 times that of OQ208.  This is very useful in finding other issues, 
RP> and measuring -- for example -- the bandpass stability. 
RP>     It is now clear at the site, and the winds are lightening up.  A 
RP> nice calm and frosty night is forecast.  It's a good time for this test. 

RP>     Rick
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