[fitsbits] INSTRUME and TELESCOP

[Bob Garwood]

I think it's import not to confuse the internal usage and data formats 
of a specific package or tool with a corresponding FITS file intended to 
hold the same data.

If the FITS file format was developed at the same time as the software, 
there's probably a close relationship.  If the FITS format was developed 
primarily to aid in export of data from one piece of software to 
another, I wouldn't necessarily expect that to be true - certainly not 
in both pieces of software. 

If there was a general SDFITS convention, demonstrated by exchange of 
real data between multiple different pieces of software (which there 
isn't), then you could reasonably expect that what CLASS wrote as 
TELESCOP would match the definition of what ASAP wrote as TELESCOP.  
CLASS could still internally combine a telescope name + other 
information, but on output to SDFTIS that shouldn't be true.  Similarly 
for BACKEND or INSTRUME, etc.  There's going to be differences in usage 
within a tool and if there was a standard for exchange, those 
differences would be handled by their respective writer and readers so 
that there would be as much uniformity as necessary in the FITS format 
used for exchanging the data.  But there isn't such a standard so right 
now, things look frustratingly similar but that information is unlikely 
to be understood in the same way if you were to try and use SDFITS to 
pass data from one tool to the other.  The few successful attempts I'm 
aware of typically involve steps like "I know this SDFITS file came from 
aips++, so I need to do this to that field to make sense of it."  It 
would be ideal if those types of steps wouldn't be necessary.

-Bob

Mike Nolan wrote:
>>    I'm struggling with this because I'm stuck in the middle between our
>>    Spanish station where the astronomers use CLASS and our Australian station
>>    where they use ASAP.  We are now setting up a new telescope at Goldstone
>>    with a very powerful CASPER-based backend. How do we archive data in a
>>    common format?
>>
>>     
>
> Aha, now we'll all understand: We have exactly the same problem. Today's
> users want class (or whatever), but it's not an archival format. I opine that
> you'll need a separate archival format that should follow the standards
> papers as closely as possible. If you want, you can try to add keywords
> to make class (or whatever) happy, but you're likely to need a converter.
> No fun when you're talking a GB/s of data, but there you are. Most of the
> reduction programs want something like the TELESCOP keyword, but they're
> probably less picky about what's in it. As an example, in our class writer,
> I encode the sub-band in the telescope name so that you can sort on it,
> since class doesn't really have another way to do that. A few years ago,
> we had a class guru visit, and he wrote scripts to fix everything up.
>
> Class and ASAP were developed on opposite sides of the globe in
> pre-internet times, and they just don't want quite the same thing, and
> don't quite deal with modern data rates. Aips++ was supposed to solve
> all this, but it just added one more.
>
>
> Cheers,
>
> -Mike
>
>