[evla-sw-discuss] Atmosphere

[Bryan Butler]

On 4/3/06 08:57, Barry Clark wrote:
> 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?

if it is not worse, then there is no harm in using it (other than "it's 
different" or "it's more complicated" [maybe]), and potential benefit 
(commonality of model, familiarity of terms, etc., between the two projects). 
so unless you can demonstrate that it is in fact worse, i would strongly 
recommend adopting it.

> 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.
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