[evlatests] Strange differential bandpasses

[rperley]

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).

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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