Subject: Re: OGIP/93-013 - A list of standard strings for HE missions,instr..etc

From: "Ian M George, Code 668, NASA/GSFC, USA (Usque ad mortem bibendum)" <[email-masked]>
Date: Tue, 1 Mar 94 16:37 EST

Barry Schlesinger listed a few questions/comments in his recent HEAFITS post 
(1994 Feb 23) regarding the OGIP Memo OGIP/93-013 "Standard Strings for 
Mission, Instrument, Filter & Detector Names for OGIP FITS files". 
Below we give our response.

A number of relatively minor changes have been made to the memo resulting 
from all the comments received, and a new version (1993 Feb 28) is available 
via anonymous ftp on legacy.gsfc.nasa.gov from
	caldb/docs/memos/ogip_93_013.ps		(Postscript)
	caldb/docs/memos/ogip_93_013.tex 	(LaTeX source)

Barry writes:
> The formsats given for the keyword=value statements appear to be
> missing blanks.  A keyword of fewer than eight characters must be
> filled with trailing blanks. There must be a space between the "=" and
> the quote beginning the characters value.  Also, the recommended form
> for character string values is that they be at least eight characters,
> with trailing blanks, if necessary.  While this format is not required
> for optional character values, it should be followed unless there is a
> compelling reasong not to.  A sample keyword=value statement would be
> as follows: 
> DETNAM  = 'SDB     ' 
	... Yes we were aware of the rules/conventions concerning character
	string keywords. Yet, it had not occured to us that the computer-style 
	font used for these strings may have lead to confusion in the minds 
	of some readers. However in order to be 100% clear & true, the latest 
	version of the memo has the correct FITS syntax (inc trailing blanks) 
	everywhere appropriate within the text.
			
> Because of portability issues, underscore is preferable to hyphen.  
> Hyphens are standard for some FITS usages for historical reasons, but 
> the recommendation now is to use underscore rather than hyphen if 
> possible.
	... We understood the recommendation to use underscores rather 
	than hyphens was primarily aimed at keyword names, rather than 
	character string values. Since there are no hyphens in the 
	keywords we quote, and since it would be almost impossible 
	to 'enforce' a re-naming of most of the instruments which 
	commonly use hyphens as part of their instrument/detector names,
	we consider the suggested strings are acceptable.
	It's worth reiterating that our primary goal in the memo was 
	not to invent a beautiful multi-mission naming scheme, rather 
	to simply formalize/list the common strings used by scientists.

> On page 6, there is a note that all strings in subsequent sections are 
> to be considerered case sensitive.  What is meant by this statement?
	... We mean that the values of the strings should be considered
	case sensitive, again in an attempt to stick to the strings 
	commonly used by scientists, or already in use within OGIP 
	FITS files. In fact you will see that upper case is used 
	throughout, with the exception of a handful of filter strings.

> Having two meanings for the values of IPC or SSS for INSTRUME for 
> EINSTEIN can be confusing. The two non-recommended definitions should 
> not be allowed, unless they are there to grandfather in existing files. 
> If that is the reason, then the usages should be allowed but 
> deprecated -- designated as permissible for existing data sets but not 
> to be used in the future.
	... We agree, but in many cases (not just for the SSS & IPC) this is 
	the result of there being several similar instruments onboard,
	but with one instrument being used for the vast bulk of the 
	scientific observations. We think our disapproval is expressed, but 
	there are simply too many files out there already to do much more.

> Since data sets have been produced for a number of the experiments
> included, are the designated keyword values (names) for the different
> missions, instruments, detectors, and filters consistent with current 
> usage by the experimental groups?  In the same context, I note that 
> indexing is inconsistent from usage to usage, sometimes starting at 0, 
> sometimes at 1, sometimes at other values.  Does this indexing reflect 
> that used in by the teams in referring to their instrument?  Normally, 
> indexing should be consistent, beginning with 1, but the actual 
> designations given by the experimenters would override such a 
> principle.
	... Yes, we hope that the usage is that of the experimental groups,
	and yes, this is exactly the reason for the inconsistent indexing.
	... But have you ever tried to tell an experimental group how they 
	 should number their detectors !!! :-)

Regards
Ian M George
Lorella Angelini
HEASARC