[fitswcs] Intro

[Don Wells]

Dear friends of FITS WCS,

Erik Deul writes:
 > .. from Leiden Observatory, The Netherlands, where I work on (helping) 
 > designing and implementing data processing pipelines for several large scale 
 > optical and near infrared imaging surveys.. So I am basically a
 > user of WCS..

I am glad that you have joined our discussions, Eric. We need to know
of any peculiar geometric properties of your imaging systems which
should be supported by the canonical distortion notation for FITS WCS.

 > ..  I already have
 > running pipelines implemented way before this debate by definition a 
 > non-standard representation was adopted..

Do you have any documentation describing your geometric
representation?  If so, please tell us the URL or else post a copy to
[fitswcs].  We should know what distortion terms you use, in order to
compare with the set used by the WF-PC and by the DSS, which are the
terms we are considering for adoption as the universal FITS notation.

 >.., implementation of new
 > functionality (switch from single CCD to multiple CCD astrometric
 > calibration, see mail by Miguel Albrecht) is currently in an advanced state
 > I was seeking for a standard way to implement the astrometric distortion 
 > information for each CCD in the FITS extended structure that stores the 
 > multi CCD imagery..

Numerous optical observatories have built or are building multi-CCD
cameras. All of them need to solve the same problem that you want to
solve. I expect that you will agree that the FITS community should
adopt a single WCS notation which will be sufficient to represent the
geometries of all of these cameras. I hope that you will also agree
that our strategic goal should that all astronomical imaging software
should support this notation. For example, NRAO's AIPS package, which
supports NRAO's VLA [Very Large Array] and VLBA [Very Long Baseline
Array] should implement the optical distortion notation which will
represent your cameras, so that users of ESO instruments or of NRAO
instruments can compare positions of sources seen in ESO imagery with
sources seen in NRAO imagery without the usual uncertainties,
confusions and inconveniences of incompatible WCS implementations. The
same notation should be implemented by AXAF [Advanced X-ray
Astrophysics Facility] programmer,s by the Starlink programmers, by
the IRAF group, etc, etc...  The worldwide astronomy community needs a
universal WCS solution.

 > Now it seems to me there is a big silence on this mail-exploder as
 > of August 21th. ..

Sigh... like a number of the other people on [fitswcs], I am deeply
involved in a big construction project, the GBT [Green Bank Telescope]
in my case (see the URLs in my personal web page if you are
curious). During the past five or six weeks I needed to concentrate
100% of my effort on drafting a planning document and on coding
simulations to support arguments in the document and on talking with
other GBT project personnel about it. The pressure has eased a bit
now, and I can devote some time to WCS again.

 > And there is still no final conclusion. Is it brewing; can
 > we expect something on the next ADASS.

As Chair of the IAU FITS Working Group, I want this [fitswcs] group to
establish a working consensus on WCS issues *before* the ADASS meeting
in Urbana, Illinois, five weeks from now. During the discussions in
July and August we resolved several (all?) of the difficult issues
which stopped progress on FITS WCS 1-2 years ago. I believe that our
last difficult issue is to agree on a notation for a universal optical
distortion representation.

			       -=-=-=-

The geometry of most optical cameras can be represented by the
distortion notation used by the DSS [Digital Sky Survey]. It appears
to me that there is, effectively, a consensus existing among the
[fitswcs] readership that a solution of that general form would be
acceptable and sufficient. I see several remaining questions:

1) What keyword notation for the distortion coefficients? In
   particular, should we adapt the DSS numbering convention or should
   we 'clean up' the notation. Somebody needs to make a choice.
   Probably that somebody is me.

2) Which axis-association convention for celestial projection
   distortion keywords? Should all of the distortion keywords be
   associated with the latitude-like axis, as we have done in the
   existing WCS conventions (e.g. CROTA2 for the usual case of RA on
   axis 1 and DEC on axis 2)? My personal opinion is 'yes', but some
   other [fitswcs] participants have argued against this. Somebody
   needs to make a choice. Again, probably that somebody is me.

3) Do we need more than one axis point? Prior FITS WCS conventions
   have defined the terms relative to the tangent point specified by
   the CRPIXi. I.e., the FITS reference point (reference pixel) is
   assumed to be at the tangent point. This is the correct description
   of the geometry of optical cameras which have only one optical
   axis. However! Some optical cameras, e.g. the HST's WF-PC cameras,
   have reimaging optical elements with their own axes which are not
   collinear with the axis of their main telescope optics. This means
   that, in principle at least, some of the geometric terms should be
   expanded about the telescope axis and other terms should be
   expanded about one or more other axes. WF/PC-1 and -2 have four
   separate reimaging cameras working in quadrants of the HST's field.
   ESO is building an analogous camera for the VLT (are you working on
   it, Eric?). For the WFPC2 this situation is discussed in section 7
   (p.174) of Holtzman etal, PASP 107, 156 (1995): '..the dominant
   distortion is expected to be cubic arising from the field
   flattener..'. The polynomical terms used in STSDAS software for
   each of the four cameras of WFPC2 are 2-D cubics expanded about the
   center pixels of the cameras. The wording of the Holtzman
   discussion leaves me slightly uncertain whether CRPIXi is set to
   the center pixels of each CCD or is set to the pixel position of
   the pyramid apex in the pixel system of each CCD. I.e., does the
   WFPC2 implementation have only one axis point, or does it have two?
   If WFPC2 uses two axis points (which I suspect), then our
   distortion keywords will need a convention to express the second
   point relative to the tangent point defined by CRPIXi. The
   [fitswcs] subscribers who are associated with re-imaging cameras
   should declare their requirements.

 > Anxiously waiting for an emerging standard. > Erik

Just like the rest of us!

-Don
-- 
  Donald C. Wells         Associate Scientist         dwells at nrao.edu
                    http://www.cv.nrao.edu/~dwells
  National Radio Astronomy Observatory                +1-804-296-0277
  520 Edgemont Road,   Charlottesville, Virginia       22903-2475 USA