[evla-sw-discuss] Atmosphere

[Barry Clark]

Bryan Butler wrote:

> i have recommended a number of times that we should adopt the model for 
> refraction described in jeff mangum's ALMA pointing memo.  i still hold that 
> this is true.  is there some reason not to do this (i've asked before)?

Is it better than what we are using?

It's main advantage, as far as I can see, is that it describes refraction 
better for wavelengths shorter than 5mm.  At VLA frequencies, refraction
isn't significantly a function of frequency.  If that's it's only advantage, 
I don't see why we should bother.

According to the comments in CALC, the pure geometric contribution of
the atmosphere (the difference in path length due to the curvature of
the rays) is small enough not to depend on model very much, so they 
calculate it (via the Niell model) and throw it in anyhow.  It also 
calculates (I think - they aren't very specific) the ratio between the 
zenith phase path and the path to the source, given the Niell atmosphere, 
which may be useful.

> seems to me we ingest the variables on the java side.  for pointing this is the 
> obvious thing to do, right, since the refractive pointing is handled within the 
> java classes?

Agreed, I think it's better to handle the monitor data on the Java side,
for maintenance reasons.  Better not to support two paths.  As I said, 
at the moment the atmopshere is applied to the pointing in the middle of 
things, in C.  I could leave it there or move it into Java.