Subject: Proposed BINTABLE binary table extension to FITS

From: thompson at stars.gsfc.nasa.gov (Bill Thompson)
Date: 27 Sep 91 23:07:39 GMT

I have been looking at the document "Binary Table Extension to FITS: A 
Proposal" (DRAFT 20 September 1991), by W. D. Cotton and D. Tody.  Principally, 
I have been looking into the question of entering multidimensional arrays into 
the binary tables.  So far I have come across two different possible 
conventions on how to do this.  The proposal discusses one such convention, the 
TDIMnnn convention, in Appendix A, and discuss the possibilities of other 
conventions in their section 4.  Don Wells advocates something called the 
"Green Bank Convention".  Cotton and Tody also discuss the possibility of more 
complicated structural information being entered into elements of binary 
tables.

Since everyone I've talked to seems to think that binary tables are the wave of 
the future, then it may be advantageous to think about how the binary tables 
concept will be extended in the future.  The TDIMnnn convention requires that 
the keywords TDIM1, TDIM2, etc. be reserved for the use of this convention.  
Certainly other conventions will also impose similar requirements.

What I propose is that another keyword be reserved as part of the basic binary 
tables (BINTABLE) extension.  This keyword (let's call it CONVENTN for now) 
would signal the presence of additional constraints on the creation of user
defined keywords.  For instance, if the value of CONVENTION were 'GBC', then 
this would mean that the additional keywords MAXIS, MAXIS1, MAXIS2, etc. were 
reserved.

The idea is that rather than try to anticipate all the possible reserved words 
now, we could reserve one additional keyword which allows us to anticipate 
future improvements.  Otherwise we would have to register each improvement as a 
different version of XTENSION, which I feel is more awkward.

The following rules would apply to the use of the CONVENTN keyword. 

	1.  The CONVENTN keyword would not be a required keyword, but would be 
optional.  If the CONVENTN keyword is absent, then only the standard 
restrictions apply on making up names for keywords.

	2.  A file using the CONVENTN keyword would be written in such a way 
that a FITS reader that did not support the use of the CONVENTN keyword, or
that did not recognize its value, would still be able to read the file
according to the standard of the Binary Tables Extension. 

	3.  Only registered values would be allowed for the CONVENTN keyword.  
These values, and the associated description of the convention, would become 
part of the definition of the Binary Table Extension through some controlled 
process.

I throw this out for general discussion.

Bill Thompson