From: Lucio Chiappetti <lucio@ifctr.mi.cnr.it>
Subject:      Re: Some FITS tables suggestions
Organization: Istituto di Fisica Cosmica e Tecnologie Relative
Date: Wed, 7 Oct 1992 21:19:26 GMT

In reply to a posting of mine (which in turn replied to some questions
by Bill Pence) :

On Mon, 5 Oct 1992 18:01:00 GMT thompson@stars.gsfc.nasa.gov said:
>>
>>>A common example of this problem is the Modified Julian Date
>>>which has a value something like 48043.8797453703700742
>>
>>  I am inclined to think this is somehow related to the need of expressing
>>  time quantities like this.
>
>        (He goes on to describe suggested definitions of the quantities ANGLE,
>        HANGLE, DATE, DATETIME, and TIME.  Definitions omitted to save space.)
>
>I strongly suggest looking into the document "Time Code Formats" from the
>Consultative Committee for Space Data Systems, CCSDS 301.0-B-2, April 1990.

  I knew about this standard from a previous posting (may be by the
  same author, I have not looked it up) in fitsbits or WGAS, which I
  have noted down
  (perhaps for the disastrous state of our postal system unfortunately I tend
   to consider any document NOT available via network as not existing)
>
>discussed there in some detail.  The suggested format for ASCII encoding is
>briefly outlined below for the following example (5 Oct 1992 at 1:23:45 UT)
>
>                           1992-10-05T01:23:45.000Z
>

   However perhaps I gave the WRONG impression that the topic of my mail
   was to establish conventions for ASCII encoding |
   That was not my primary aim | I intended to solicit a discussion about
   NAMES and CONTENTS of keywords from a semantic and logical point of view
   (the fact that those keywords need an ASCII encoding is specific to a
   FITS implementation, while my concern goes beyond FITS into the native
   representations used by analysis systems ... which MAY be non FITS but
   SHOULD be FITS-compatible)
   I am ready to comply with whatever ASCII encoding for date-times will
   be agreed for FITS in addition to the one currently used in DATE and
   DATE-OBS.
   My interim solution was not so different from the CCSDS scheme and
   had that in mind since it is very easy to map it ;

                        1992-10-05-01:23:45.00

   actually it is the notation used by VMS. I find more LEGIBLE to have
   an hyphen instead of a T, and no Z at the end (which makes me think
   of "Zulu" timezone notation), but it is really trivial to switch to
   the other notation.

   See more below on standards ....

On Wed, 7 Oct 92 16:03:42 +0100  <forveill@gag.observ-gr.fr> said:
>
>with the existing standard. Specifically, A VALID FITS FILE MUST USE SI
>UNITS. This tends to be repeatedly forgotten, even though it was already
>explicitly specified in the original FITS paper.
>X and Y should therefore be stored in metres, not microns, ENERGY in Joules
>not kev (or cm**-1, or Kelvin, or GHz),  and so on...

   Again I gave the WRONG impression that my concern was on units of
   measurement, while I was more concerned about the fact the file be
   self-documenting about the units used, and about establishing conventions
   for names, contents and "binary" representation.

   Concerning SI units and alike, when I was a student I was an enthusiast
   of the SI, and I swore never to use cgs and similar things.
   When I later started my research life I realized one needs to be more
   flexible. I am not scandalized to use Angstroms or keVs (actually I
   cannot THINK of an X-ray spectrum if not in photon/cm2/s/keV vs keV,
   and of an UV spectrum if not in erg/cm2/s/A vs A  ... and I believe
   this is what everybody does ... I have never seen a paper on Ap.J. with
   photons/m2/s/J or J/m2/s/m
   Similarly the examples I made in my posting were just examples (although
   I will not find very useful to give photon energies in Joules or to
   use metres instead of microns to measure positions on a CCD plate: the
   latter example is particularly specific to Bill Pence's posting, since
   it avoids an unnecessary factor 10**6 and therefore 6 digits of -
   fictitious - precision)

   Concerning standards, therefore they also are subject to a silent
   judgement from the community, based on comfort and legibility.
   There is a nice paper by Arnold Toynbee, the historian, explaining why
   the French Revolution Calendar was not able to supplant the traditional
   calendar.
   Everybody uses Internet domains for e-mail addressing, which are legible,
   instead of the awful X.400 notations.
   Some years ago there was some proposed standard to express ALL decimal
   numbers in exponential notation in power of 1000, so a date in year
   1992 had to expressed as 1.992d3 or something like that : it fell in
   the place where everything is forgotten.

On the contrary other standards, of which FITS looks a working example,
meet the general agreement and ARE used.

I would however just reiterate my concept, that I regard the two points
treated above (ASCII-encoding and units) as completely minor in my line
of thought, while I would once again solicit a discussion on names of
keywords (and their content).

I feel that this would be beneficial AT LEAST to the X-ray community which
is now working separately at many separate missions, in fields like :

   list of photons :

   should we call photon detector-pixel positions X and Y, or XPOSITN and
   YPOSITN (8-char FITS names) or whatever ?
   should we call photon energy expressed as a PHA channel (not keV) with
   the name of ENERGY (I would say no and reserve it for keV, Joule or
   whatever physical unit) or PHA or what else ?

   time profiles (this perhaps also beyond X-ray astronomy) :

   it is important to agree a convention on the format of such files as
   binary tables, on the names of the columns and on the additional
   keywords needed to specify time of each bin

I hope to listen to some discussion on the latter topics in the near
future.

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