[mmaimcal]TP detectors as atmospheric monitors, continuum band mismatch

[Mark Holdaway]

> Al wrote:
> 
> Several items came up in discussion today which I thought I would report 
> back for discussion/comment.
> 
> Webber noted that bandpass shape/extent may differ between correlator 
> with tunable filter and TP devices--hence the TP device may find itself 
> contaminated with a line not seen in the correlator.  This would pose a 
> problem in image reconstruction using TP and interferometer data.  The 
> tunable filter sections overlap to avoid aliasing; this cuts their 
> nominal bandpass below what they would have in the baseline correlator. 
>   The do have a bypass more.  Use bypass? We may want to define 
> 'standard continuum band' without strong spectral lines.  This has been 
> discussed before.

Just as at the VLA and VLBA, there will be default comntinuum bands set up
for ALMA.  But note that at the VLA, they continually redefined the
continuum bands as the interference enviromnet shifts.  In our case, the
bands may get redefined for different atmospheric conditions.  There will
be interference that develops, even at Chajnantor, in bands 1,2 & 3 in the
next few decades (from high frequency airborn radar applications, some
satelites).

Additionally, some users will use other nonstandard continuum
bands for various scientific reasons.

Having the same hardware treating the total power and interferometric
data, ala the HOMOGENEOUS ARRAY, has always been attractive to me
as it avoids problems like this.

> Baudry asked if we had considered using the output of the total power 
> detectors as a continuum channel which we could compare or complement 
> the WVR data on line variations with.  In a discussion, we noted that 
> the WVRs (effect seen at OVRO, VLA, IRAM) seem to have a variable 
> scaling factor over time which might be traced to 'dry component' 
> variations.  

It is my opinion that the dry fluctuations would be smaller than the
scaling factor fluctuations,  If these are due to dry fluctuations,
that will seriously impact WVR.

> Lucas noted that this had been noted at IRAM, though the 
> continuum channel did not show this.  Baudry noted that we might wish to 
> use the 8GHz TP detector channels for this sort of monitoring.  I think 
> this is possible but I'm not sure about the sensitivity of those 
> devices.  Earlier in the week, we had decided that owing to 1/f noise it 
> was probable that only the 2 GHz channels would be useful for TP 
> continuum owing to their lessened susceptibility to 1/f noise.

Keep as many options open as we can easily afford for future staff
scientists to find useful correlations and do useful things. It is our job
to define at least one clear path through the instrument, but clever
people will see other ways of using what we build.

   -M

> Discussion or comments?
> 
> Al
> 
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