[fitsbits] Start of the FITS-IDI Convention Comment Period

[Eric Greisen]

Harro Verkouter wrote:
> Hi all,
> 
> After reading through the document I'd like to make a few comments.  
> Some of those apply to the implementation of the/a reader, nonetheless  
> they seem to indicate incompatibility between documentation and  
> practical use-cases. This is written from the perspective of the EVN  
> FITS-IDI data-export software, which has been implemented exactly  
> following the Flatters(1999) document [and thereby finding  
> discrepancies between documentation and implementation].
> 
> * I have noticed that in most cases the order in which the keywords  
> are written (also the non-mandatory FITS keywords) may have to be  
> identical as in the documentation or else your FITS-IDI file may be  
> rejected. I do not know if this situation has changed over the last  
> few years, I did not experiment with writing keywords "out-of-order"  
> since finding out that it most likely 'breaks' your FITS-IDI file.
> 
> * The "SORT" optional keyword to the UV_TABLE header (p.15) does not  
> seem to get honoured. Our FITS-writer writes the hard-coded value  
> "TB" (Time-Baseline) since that is the order in which the data is  
> forcibly written, nonetheless it is never recognized by the reader,  
> making the user have to run uvsort (or wossname of the task) to make  
> the data recognized as being sorted in TB order. It may well be that  
> this behaviour is triggered by our FITS-writer doing something else  
> wrong however it is not documented - making it quite unfixable yet.
> 
> * I'm glad to see the POLARX/Y situation being somewhat resolved. I  
> beg to differ, however, from the statement in the document "The units  
> were changed from the meters specified by the earlier documents, but  
> seldom used in actual implementations.". Especially since it is  
> exactly this early misdocumentation (documentation says "meters", s/w  
> interprets them as "arcseconds") that has caused many of our users  
> headaches (EVN data).

I think all of these comments apply to the implementation of the AIPS 
task FITLD rather than to the IDI convention itself.  I hope that I have 
fixed the earlier versions of FITLD so that the columns may now occur in 
any order.  I just tested it and the SORT values are honored - but note 
that 'T*' is changed by FITLD to 'TB' or to '**' if unsorted data are 
found.  Leonia Kogan has studied the POLARX/Y question and determined 
that the software can, for data within the last 30 years, tell which 
units were used.  AIPS has been corrected to make that test and enforce 
the new (and usual) convention.

A question was raised outside this forum about adding PULSAR BIN as an 
optional UV data axis.  I have not added that to the document since its 
support would force much labor on my part for aips and other software 
people as well.  Until there are users who promise to use it, I am 
inclined to stay away from that addition.

Cheers,

Eric Greisen