WCS questions

[William Thompson]

swalton at galileo.csun.edu (Stephen Walton) writes:

>[Sorry if you see this twice;  the first post didn't propagate
>very far, apparently.]

>At the recent RISE 96 workshop at HAO in Boulder we discussed the 
>possibility of adopting the standard in the solar community that the
>linear part of the WCS of a solar image result in (x, y) coordinates
>such that x**2 + y**2 = 1 at the solar limb, with (x=1, y=0) at the
>solar West limb and (x=0, y=1) at the solar North limb.  This makes a
>good deal of sense from the perspective of solar astronomers, who often
>want to work with the radial distance from the center of the disk.  At
>the workshop, I was asked to write a small sample program, probably in
>Fortran, to show how to add WCS information to a solar image whose
>geometry is given in the header in some other format.

>How much violence would it do to the WCS draft to have a new CTYPE
>pair, perhaps "PLAT----" and "PLON----" (Planetary latitude and
>longitude), for which the units of (x, y) would be normalized distance
>from object center rather than degrees?  I'd prefer SLAT and SLON for
>"solar," of course, but these seem to have been reserved for other
>purposes :-) .

Good luck.  I'm personally not convinced that the WCS will ever be compatible
with datasets that are not in some way related to the celestial sphere.  It's
just a completely different mindset.  For example, the response from Mark
Calabretta

>You should be able to organize it so that the intermediate result obtained
>by applying the PC matrix (but not the CDELTn) give the required (x',y').
>The CDELTn would then scale these to (x,y) in "degrees" as required by WCS.
>This could be made a defining characteristic of PLON/PLAT CTYPEs without
>needing to alter the current proposal.  While WCSLIB does not provide the
>intermediate (x',y') directly it should be a simple matter to apply the
>linear transformation routines (linfwd(), and linrev()) to that end.

still seems to me to be saying that the WCS folks expect to be able to project
the data onto the sky.  Otherwise, what is meant here by degrees?

The idea of defining the coordinates to be in units of the solar radius seems
like a good one, at least at first blush.  There are some questions that do
come to mind:

1.  How do you define the solar limb?  The solar radius differs depending on
    what wavelength one is observing in.  The size of the sun in the EUV is
    significantly different from that in the visible.  The most reasonable
    thing to do would appear to be to use the photospheric value, which is well
    known.  On the other hand, some observers may prefer to determine the size
    of the sun using a limb fit.

2.  If the official value for the (photospheric) solar radius changed, would
    that invalidate older FITS files?

In spite of the above questions, there are some advantages to referencing all
data to a normalized solar limb.  It would allow for easier comparison of data
taken at different points in the Earth's orbit, with different apparent solar
sizes.  Also, and probably more critical, it would simplify intercomparison of
data taken from spacecraft not in Earth orbit with data taken from ground-based
observatories.  For example, the current Solar and Heliospheric Observatory
(SOHO) spacecraft sits at the inner Langrange point, which is about 1% of the
way towards the sun.  This means that SOHO sees a 1% bigger sun than is seen
from ground-based observatories.  This difference can cause some confusion when
pointing is discussed, and needs to be kept in mind in all data analysis.

An alternative to expressing the data in solar radii is to use kilometers, or
possibly megameters.

I don't think I like the idea of using the terms PLAT and PLON to refer to a
coordinate system which is not really a latitude/longitude system.  I think
those terms should be reserved for true latitude and longitude, e.g. S40WN9.
In that case, the data should be expressed in degrees.  There is, of course,
the possibility of confusion between heliographic and Carrington coordinates.

William Thompson