Subject: wcs.ps and scs.ps

From: Eric Greisen <egreisen at valen.cv.nrao.edu>
Date: Fri, 9 Apr 1999 13:24:25 -0400 (EDT)

Both wcs.ps and scs.ps have had modifications made.  The latter was
primarily the addition of a reference on relativistic velocities
suggested by Don Wells.  wcs.ps was changed to implement the
suggestion copied below.  Remarks about the actual (deprecated) status
of CDELT were added by me.  This status means that CDELT and its
offspring may be used in writing, but only if the new forms are not
used  (i.e. CDji, PVj_m and version codes other than blank).  The
political machinations have settled on the idea that CDELT is
deprecated but will have to be understood by readers for a long time
to come.  We are recommending that, either one uses solely older forms
of coordinates or solely newer forms.  Mark and I miss the simplicity
of the CDELT formulation, but understand some of the problems
perceived by the people who opposed the CDELT * PC formulation we had
proposed.  The desire for separate spelling for primary and secondary
forms (below) is an offshoot of the change from CDELT * PC to CDj_i.

Oh well, I think we are converging.  Is there anyone who does not
think so?  See

      ftp://ftp.cv.nrao.edu/NRAO-staff/egreisen/wcs.ps
      ftp://ftp.cv.nrao.edu/NRAO-staff/egreisen/scs.ps

Thanks, 

Eric W. Greisen

--------------- Mail received from the OGIP project -------------------

Subject: Re: wcs draft
Date: Thu, 08 Apr 1999 17:40:58 -0400

Eric,

Here is a revision to the Table 7 in your wcs paper that has been
recommended by representatives of several high energy astrophysics
groups.  The main change is that instead of having 'old' and 'new'
versions of these keywords in the table, we continue to use the same
keywords as have been used in the past when no version code letter is
used, and adopt the new root names for the keywords only when the version
code letter is used.  The advantage of this is that the large body of
software that reads and writes the old keywords will continue to work in
the future in most cases (when no version letter is needed).

Sincerely,

Lorella Angelini,  HEASARC/SAX
Keith Arnaud,      HEASARC
Mike Corcoran,     HEASARC/ROSAT
Tom McGlynn,       HEASARC
Koji Mukai,        HEASARC/ASCA
William Pence,     HEASARC
Steve Snowden,     HEASARC/XMM
Jonathan McDowell, AXAF Science Center
Arnold Rots,       AXAF Science Center

- ---------------------------------------------------------------
 Keyword                Image   BINTABLE vector  Pixel List
 Description            Array   Primary 2ndary   Primary 2ndary
 --------------------------------------------------------------
 Axis Type              CTYPEjs  jCTYPn jCTYns   TCTYPn TCTYns
 Axis Units             CUNITjs  jCUNIn jCUNns   TCUNIn TCUNns
 Reference value        CRVALjs  jCRVLn jCRVns   TCRVLn TCRVns
 Reference point        CRPIXis  iCRPXn iCRPns   TCRPXk TCRPks
 Transformation matrix  CDj_is   jiCDn  jiCDns   TCDn_k TCn_ks
 Coordinate parameter   PVj_ms   jPVn_m jPn_ms   TPVn_m TPn_ms
 Coordinate increment   CDELTj   jCDLTn jCDLns   TCDLTn TCDLns

------- end -------

---

From: nolan at deuteron.lpl.arizona.edu (Michael Nolan)
Date: 16 Apr 1999 16:11:07 GMT

Comments on the wcs paper (1999 April 9 version):

I can reasonably forsee the usefullness of airmass and unit optical depth
as axis dimensions.  

In table 4, adu means A/D converter.  Do you mean converter (as in
physical device), or do you mean DN?  I can see a value in both, and
have a specific need for the former, since we have four parallel IF
chains simultaneously sampled.

You removed the trig functions, which are mainly useful for projections,
and can be handled in paper II, but have other applications, such
as airmass again.  I suspect that if trig functions are needed, they
will simply be used withut difficulty.  The main problem would be how to
represent the inverse trig functions, as it would be better to choose now than
to have all of the possibilities be used later.

-- 
Mike Nolan +1 809 878 2612 ext 280 Fax: +1 809 878 1861 nolan at naic.edu
Arecibo Observatory/Cornell University POBox 995, Arecibo, Puerto Rico 00613

---

From: Eric Greisen <egreisen at valen.cv.nrao.edu>
Date: Mon, 19 Apr 1999 11:12:39 -0400 (EDT)

> I can reasonably forsee the usefullness of airmass and unit optical depth

     I am unfamiliar with airmass, does it have a clear definition?
Optical depth is unitless.  Can you describe how specifying that it is
optical depth in the units string (rather than the CTYPEn string) is
needed.

 > chains simultaneously sampled.

    I am also unfamiliar with these units.  Can Francois or one of the
units gurus at OGIP comment on this?

 > You removed the trig functions, which are mainly useful for projections,

    Actually they have no use whatsoever in celestial projections.
The only units strings are 'deg' in Paper II, which should be
announced in < 1 day I hope.

 > to have all of the possibilities be used later.

     I do not know what use inverse trig functions could have and how
to attach meaning to them as units.  Do you have any specific examples
in mind?  Please describe some in detail.  I agree that cos(elevation)
axes have occurred to me.  Are there other uses for even the forward
trig functions?  We need help from the community in such matters
because our experience has serious limitations.

BTW: I have put a new version of wcs.ps out with a few additions to
the pixel regularization section - a keyword to specify the magnitude
of the correction on each axis.  See
     ftp://ftp.cv.nrao.edu/NRAO-staff/egreisen/wcs.ps.gz

Thanks, 

Eric W. Greisen

---

From: nolan at naic.edu (Mike Nolan)
Date: Mon, 19 Apr 1999 12:02:24 -0400 (AST)

>      I am unfamiliar with airmass, does it have a clear definition?

Airmass is atmospheric optical depth normalized to zenith optical depth
assuming a spherically uniform atmosphere.  It is usually
approximated as sec(zenith angle), which is the only other case
(besides sin za) I can think of that needs explicit trig.  One
generally measures extinction as a function of airmass for calibration
of optical photometry.

I don't have any idea why one would need inverse trig functions, I
only wanted to note that they are one of the cases where you'd rather
choose the notation in advance.

-Mike

---------------------------
Mike Nolan <nolan at naic.edu>
+1 787 878 2612 Home: +1 787 895 6746 Fax: +1 787 878 1861
Arecibo Observatory / Cornell University
HC03 Box 53995, Arecibo, Puerto Rico 00612

---

From: Arnold Rots <arots at head-cfa.harvard.edu>
Date: Mon, 19 Apr 1999 13:22:49 -0400 (EDT)

> units gurus at OGIP comment on this?

adu is the (pseudo-)unit of the raw numbers as they come out of the
A/D converter device - to be transformed into energy.
In HEA, it typically (but nor exclusively) refers to the output of A/D
converters that process CCD read-outs,

  - Arnold

> Eric W. Greisen

--------------------------------------------------------------------------
Arnold H. Rots                                         AXAF Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 81                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head-cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
--------------------------------------------------------------------------

---

From: Peter Bunclark <psb at ast.cam.ac.uk>
Date: Tue, 20 Apr 1999 11:59:45 +0100

Mike Nolan wrote:

> atmosphere.  It is usually

I'd say ``it is usually calculated to high precision, often with a call to
slaAirmas()''.

Pete.