[fitsbits] Start of the FITS Region File Public Comment Period

[Craig Markwardt]

William Pence <William.Pence at nasa.gov> writes:

> This is to announce the start of the 30-day Public Comment Period on
> the FITS Region File convention which has been submitted for inclusion
> in the Registry of FITS Conventions.  This is the 12th in a growing
> series of conventions submitted to the Registry which is maintained by
> the IAU FITS Working Group.

Glad to see this is being documented!

Section 2
  * "The names of the columns (TTYPEi) indicate the coordinate system in
  use, in the usual way, and these coordinate names are to be identified
  in the MFORM1 keyword (MFORM1='X,Y'); the mandatory keyword MTYPE1
  provides a name for the coordinate pair."

Comment: I'm not sure what the "usual way" refers to.  I think it
means, "using the CTYPEi coordinate labels of the source image."

I'm also unsure of what MFORM1 is meant to mean (this is a
Chandra-specific convention).  Is 'X,Y' a placeholder or literal?
What are the allowed values of MTYPE1, or can it be anything?


  * "The following columns are optional..."

Comment: Is there any convention for null values?  It seems there
could be many 'unused' fields in a region file.  For example, if there
is a region composed of the union of a circular and rectangle, the "R"
column will be used for the circle but not the rectangle.  Also, the
rectangle will require X and Y to be 4-vectors, but the circle will
only use one X and Y value each.  Are the extraneous values simply
ignored?


General comment: Region files are image and projection dependent.  For
example, if someone lays down a circular region on a tangent plane
projection image in DS9, the same region will not be circular in
another projection.  The situation is even more complicated for
non-circular regions.  This means that region files are not globally
interchangeable, but rather specific to a particular analysis with a
known image and known projection.  I think this should at least be
noted.

Craig Markwardt

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Craig B. Markwardt, Ph.D.      EMAIL: cbmarkwardt+usenet at gmail.com
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