[evlatests] Oddities at L-band (pointing, Tcal, Tsys, Psum, etc.)

[Rick Perley]

    Answers to Barry's questions:

Barry Clark wrote:
> On 08/13/2013 10:05 AM, Rick Perley wrote:
>
> [snip]
>
>   
>>      3) I used a central subband (#8 -- 1480 MHz, nice and stable) to
>> look at the PSum values.  They should all be reasonably close to each
>> other if 'set and remember' is working right.   I'm prepared to accept
>> that 'reasonably close' means with 50% of some median value.  But this
>> is not the case.  The median value for the subband appears to be about
>> 10 counts.  In LCP, all antennas are within 50% of this.  But in RCP,
>> there is a very different story:  ea02R, ea07R and ea13R are all at 1
>> count, or less!  That's a factor of 10 too low!!!
>>     
>
> What do the Psums in the other subbands do?  If there is a lot
> of interference in some subband, you could have big numbers there
> that make the average Psum sensible.  If not, it could be that
> the wire between the T304 detector and the sampler has a lot of
> attenuation.  Do both polarizations do this?
>
> [snip]
>   
    They all behave in the manner described above.  A quantitative 
example for ea02 (almost exactly the same values are seen in ea07.  ea13 
is discrepant only in the A/C side -- B/D looks o.k.).
   
         Subband#       AC PSum           BD PSum
             1                      .41                        
.25          (band edge attenuation)
             2                      .65                        12
             3                      1.1                         8.2
             4                      .95                         8.2
             5                      .82                         8.5
             6                      .61                         7.6
             7                      .58                         5.9
             8                      .73                          7.1
             9                      2.2                          20    
(RFI Zone of Death!)
            10                      3                            23
            11                      2.5                         15.7
            12                      1.5                         10.3
            13                      1.3                          9.5
            14                      1.0                         8.0
            15                      .83                         5.9
            16                      .47                         3.7
----------------------------------------------------------------------

    So, there is no evidence from these values, nor from the spectra, 
that there is any signal in the RCP side which would have caused the 
levels within the subbands to be as low as they are. 

    This problem seems only to be in the RCP side.  All LCP values are 
within 'norms' (+/- 50% of the median)


>
>   
>>      5) In perusing the Tcal tables, some curiosities are found:
>>
>>      a) ea07 has a Tcal about 8K -- 5 times higher than any other
>> antenna!  Is this right?
>>     
>
> Right or not, that's what the Soida documents say.
>   
    Righto.  I do note that the visibilities from this antenna (both 
polarizations) are about as expected, and the Tsys values, although 
higher than normal , are in line with the observed amplitudes.  (In 
other words, the listed Soida documents are in keeping with the data.)  
This raises the question of why is this receiver so different  than the 
others ...?
>   
>>      b) ea11, ea14 and ea17 all have incredibly high Tcal values listed
>> for the first two subbands (values exceeding 55K for ea11!!!), but
>> normal (1.5K) values for all other subbands.  Can this be real?  I
>> didn't think the noise diodes could have output powers 30 times higher
>> than the mean in the bottom 200 MHz of the band...
>>     
>
> This is due to the (ahem) feature that the Tcal stowed in the SDM
> is the interpolation to the subband reference frequency (lower
> edge) combined with the fact that the bandpasses of these receivers
> appear to go really whacky below about 1150 MHz.
>   
    The Tcal values shouldn't know or care what the bandpass looks 
like.  While it is true that these antennas do have a significant 
low-frequency rolloff in their spectra, there are others which decline 
similarly, but without the spectacular (and unphsysical) rise in Tcal in 
the lowest subbands.
    I'll be keen to see how the improved interpolation scheme works. 

> _______________________________________________
> evlatests mailing list
> evlatests at listmgr.cv.nrao.edu
> http://listmgr.cv.nrao.edu/mailman/listinfo/evlatests
>