[evlatests] Strange differential bandpasses

[Ken Sowinski]

On Fri, 27 Aug 2021, rperley via evlatests wrote:

> OK, now that a probable cause is located, we have to think about how to 
> manage this.
> Other than installing 'window wipers' on the windows (ha! -- think about that 
> for L and S bands ...),
> the only good mitigation is more frequent bandpass calibration.  Probably 
> every couple of hours, if the weather is likely to lead to dew condensing on 
> the windows.  (All feeds from C upwards have heat lamps, so I would hope the 
> problem is much rarer for this bands).

I had forgeotten that L and S do not have feed heaters.  Is that
because they are too large to be heated practically?  Gimbaled umbrellas?

Ken


> For *most* continuum observations, the effect of this changing reflection is 
> pretty minor -- (so long as the imaging is done over spectral widths much 
> larger than the observed period in frequency space).  For continuum 
> observations requiring channelwidths less than this, self-calibration (if 
> viable) will fix things up.  All other cases will need better bandpass 
> calibration, if the amplitude of the effect (a few percent) is important.
>
> Rick
>
>
> On 2021-08-27 12:49, Wes Grammer via evlatests wrote:
>>  This all makes sense. The radome on its own wouldn't produce much of
>>  a mismatch/reflection, but with dew it would be much greater. The RF
>>  mismatch at the vacuum window is higher because of the thickness and
>>  density of the foam window, so a standing wave could be set up between
>>  the two interfaces.
>>
>>  Since 2013, we've had to replace the original foam windows in the S-
>>  and C-band receivers with denser ones because their marginal strength
>>  caused them to deform over time. The denser foam had a slightly higher
>>  dielectric constant ,which led to a small increase in mismatch.
>>
>>  -Wes
>>
>>  On 8/27/2021 10:08 AM, Rick Perley via evlatests wrote:
>>
>>>  So, on a probably drier day, the effect is nearly invisible.
>>>  Support for the idea that water/dew on the window is a contributing
>>>  factor ...
>>>
>>>  Rick
>>>  On 8/27/21 9:42 AM, Drew Medlin wrote:
>>>  Here's an observation from a drier time of year. 2021-03-27,
>>>  afternoon observation, operator recorded Sky cover 40%. Cumuliform
>>>  clouds.
>>>
>>>
>>  20A-092.sb39518402.eb39548999.59300.832225300925.ms-J0303+4716-bb12-PHASE-amp_vs_freq-LL_RR.png
>>>
>>>
>>>  Drew Medlin
>>>  dmedlin at aoc.nrao.edu
>>>
>>>  Scientific Data Analyst - VLA Scientific User Support Group
>>>  National Radio Astronomy Observatory
>>>
>>>  On Aug 27, 2021, at 9:18 AM, rperley <rperley at nrao.edu> wrote:
>>>
>>>  Certainly is reasonable.  In the case I analyzed, the start of the
>>>  run was around sunrise, the end around noon.  The period of the
>>>  oscillations (and the phase) did not appear to change, however.  The
>>>  change in amplitude was quite obvious.
>>>
>>>  I don't know if there was rain on the day I looked at -- but perhaps
>>>  dew on the window would do the trick ...
>>>
>>>  Rick
>>>
>>>  On 2021-08-27 11:11, Ken Sowinski wrote:
>>>  On Fri, 27 Aug 2021, Rick Perley via evlatests wrote:
>>>  Drew, et al.:
>>>  Perfect!  Period shown in that C-band plot is about 100 MHz, which
>>>  corresponds pretty closely to the expected value corresponding to
>>>  the length of the horn.  Good evidence for a mismatch between the
>>>  window and the OMT….
>>>  The change in the sinusoiid should correlate with temperature.
>>>  Might the reflections have been exacerbated by moisture from
>>>  all the recent rain?
>>
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