[fitsbits] SDFITS Convention Comment: Hardware Keywords

[Bob Garwood]

I wrote this  yesterday but haven't sent it yet.  I'm not sure whether 
this covers your points yesterday or just readers or what.  Personally, 
while I agree there's plenty of room for SDFITS to be improved, I've 
mumbled that to various people for years without finding the time to 
follow through on that and so after > 10 years writing SDFITS (closer to 
20 years) I think its most important to document something that 
summarizes how SDFITS has been used in the past without trying to 
improve SDFITS right now.

So, here's what I wrote yesterday:

This convention is primarily to document existing usage - there's plenty 
of room for improvement, but the history of this convention has shown 
that were we to wait for those discussions to happen this convention 
would never get documented.  If there's some momentum for extending 
sdfits to include additional things - e.g. improving the axes 
descriptions in light of the WCS papers that didn't exist when this 
convention was written, or expanding the list of SHARED columns, I'd 
hope that those discussions could be put off until we've documented 
what's already being written [it would be useful to include an Arecibo 
example case here, I think.  I'm not sure where else SDFITS might 
already be in use].

Since I wasn't at the original meeting in Green Bank where the ideas 
behind the SDFITS convention was first discussed, this is simply my 
understanding of the intention after talking to a few of the people that 
were there.  I think the idea was that SDFITS would primarily be useful 
in the exchange of calibrated and reduced data between telescopes and 
data analysis packages and it was less useful for exchanging raw data. 
We use it at the GBT for raw data by liberally adding lots of 
telescope-specific columns.   By the time the data is calibrated the 
CORE columns plus the data axes descriptions are what's important.

So, I think in that spirit you should feel free to use whatever keywords 
you feel are necessary to fully describe your data, especially for your 
internal use and for historical archives, but it's unlikely that a 
package not specifically familiar with your data will know what to do 
with most of that. 

I've always felt that TELESCOP describes the whole thing collecting the 
photos.  ALMA is a TELESCOP with multiple antennas.  NRAO-GBT is a 
TELESCOP with one antenna.   SDFITS doesn't go into useful keywords, 
etc, for collections of spectra from multi-antenna telescopes.  It would 
be useful to have a registry of TELESCOP names.

We (GBT sdfits writer) put polarization information into a stokes-like 
axis: e.g CTYPE4='STOKES', CRVAL4=-5 (which translates to "XX", ala AIPS 
- in that usage negative values aren't really stokes IQUV but they are 
recorded in the "STOKES" axis nevertheless).

As per the GB convention that underlies SDFITS - the distinction between 
a keyword and a column is very fuzzy.   A keyword is a column with a 
constant value.  So there's no reason that FRONTEND couldn't be a 
keyword (in fact, that's what we write for the GBT).

-Bob


Tom Kuiper wrote:
> OK, thanks.
>
> I haven't participated in one of these before and so I don't know the 
> best way to proceed.  I realize that no one might react to my previous 
> e-mail and you wouldn't know if that indicated agreement, disagreement 
> or disinterest.  Would it be more helpful if I put my ideas in a 
> declarative form?  Then, if there is no reaction, agreement can be 
> assumed.  Here's an example:
>
> The TELESCOP keyword is required in the extension header and its 
> associated value will be a unique string which identifies the antenna 
> or antenna array and can be used as a key to a database of antenna 
> parameters.  NRAO will maintain a registry for TELESCOP keyword values.
>
> Regards
>
> Tom
>
> William Pence wrote:
>> Tom Kuiper wrote:
>>   
>>> William Pence wrote:
>>>     
>>>> This is to announce the start of the Public Comment Period on the
>>>> SDFITS binary table convention that is used for interchange of single
>>>> disk data in radio astronomy...  
>>>>       
>>> Bill, when does this Public Comment Period end?  I'm now working through 
>>> the process of writing the results of a particular series of 
>>> observations into SDFITS.  It'll be the first step in trying to get all 
>>> the DSN spectroscopic runs to produce compatible SDFITS files.  There 
>>> will be issues along the way.  I expect it'll take a few months.
>>>     
>>
>> The comment period lasts at least 30 days, but there is no upper time 
>> limit.  If there is a reasonable prospect that there will be continuing 
>> developments regarding a particular convention, then it can remain open 
>> for whatever time it takes to reach a conclusion.
>>
>> Bill
>>   
>
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