[fitsbits] Start of the CONTINUE keyword Public Comment Period

[William Pence]

Rob Seaman wrote:
> Perhaps the discussion from 1993 and 1994 could be included verbatim,  
> or linked by thread off the web page?  I'd hate to hold a debate on a  
> topic that was already batted around a dozen years ago.  

There is a link on the registry web page to the (very large!) set of 
comments that were made about this convention when it was first 
introduced, but the archived FITSBITS discussions for this time period 
are just broken up into monthly text files.  You have to scan through 
the whole text file to pick out the relevant postings.  It would take a 
lot of editing to make a file just containing the CONTINUE thread.

> Is the  
> registry page the entire text of the proposed convention?  Like Tom  
> said, you need to cover the obvious corner cases, at least by  
> verbiage like "it is undefined what string is conveyed if a card  
> ending is '&' is not followed by CONTINUE".

What you see is the entire existing text of the convention, but I plan 
to augment it with some additional text, based on the comments that are 
made here during the public comment period.

> 1) Couldn't the reordering question be dealt with via appending &NNN  
> instead of &?
...
> 2) What is the reason for not strictly requiring that such a  
> continuation string end with ampersand-quote? 
...

These are valid questions, but they are a bit 'off-topic' since the 
purpose of the registry is to document existing conventions, not defend 
the way they were designed.  If we were designing a new continuation 
convention now we might do things differently.

> 3) One suspects a number of conventions will depend on valueless  
> keywords.  

Agreed.  The ESO HIERARCH keyword (which will be submitted for inclusion 
in the Registry in the near future) is another well known example that 
is somewhat complementary to the CONTINUE convention.  The HIERARCH 
keyword convention effectively allows keyword names much longer than the 
8-character limit of real keywords

> The CONTINUE convention, however, is different in providing a more  
> general capability that could be used, for instance, within any other  
> convention.  Deleting a CONTINUE card could in principle change the  
> meaning of any header in arbitrary ways. 

As you further discussed in a subsequent posting, this CONTINUE 
convention could be used in principle with any string-valued keyword, 
including the mandatory or reserved keywords like TFORMn, TTYPEn, 
EXTNAME.  In practice, however, the continue convention has mainly only 
been used for very instrument-specific keywords like

TDDES12 = 'D[0~3] & E[0~63] C[(S[msLimit1]~S[msLimit2]),((S[msLi&'
CONTINUE  'mit2]+1)~S[msLimit3]),((S[msLimit4]+1&'
CONTINUE  ')~S[msLimit5])] '

> I wonder if perhaps we  
> shouldn't rather be discussing how best to add this capability to the  
> core FITS standard.  

This was one of the reasons for setting up the registry in the first 
place.  The first step is to simply document the existing convention in 
the registry;  as a second step, this may lead to a wider discussion of 
how this sort of capability could be added to the FITS standard.

Back in 1993 when this convention was first discussed, we failed to 
reach any consensus, largely, I think, because many people at that time 
felt that there was no compelling need for any keyword strings longer 
than 68-characters.  In the rare cases where anyone need longer strings, 
ad-hoc work-arounds were devised, rather than coming up with a general 
solution to the problem.

Maybe this thinking has changed in the past 14 years(?).  If enough 
people agree that this is an issue worth addressing, it should not be 
hard to come up with a technical solution for handling long string 
keyword values (not necessarily identical to the current CONTINUE 
convention) that could be considered through the official approval 
process for inclusion in the FITS Standard.

Bill Pence
-- 
____________________________________________________________________
Dr. William Pence                       pence at milkyway.gsfc.nasa.gov
NASA/GSFC Code 662       HEASARC        +1-301-286-4599 (voice)
Greenbelt MD 20771                      +1-301-286-1684 (fax)