[fitswcs] Distortions

[Mark Calabretta]

Hi Bill,

That's the most straightforward and probably the best way to do it if
you're using the generalised polynomial distortion.  (In this case,
WCSLIB should translate it automatically to TPD for efficiency.  If you
don't want it to, e.g. for testing, then specify the distortion function
to WCSLIB as "Polynomial*".)

Auxiliary variables could be used to apply the offsets i0 and j0, but
that's an unwarranted complication in this case.  Also, depending on
what else is going on in the header, you might be able to use a sequent
distortion, rather than prior, and have the offsets applied simply via
CRPIXja.

Just to be clear, Paper IV uses a shorthand notation for the distortion
parameter keywords.  So when you write

DP3.NAXES = 2
DP3.AXIS.1 = 1
etc.

they would appear in the header as

DP3 = 'NAXES: 2'
DP3 = 'AXIS.1: 1'
etc.

Regards,
Mark Calabretta



On Wed, 5 Jun 2019 21:01:19 +0000
"Thompson, William T. (GSFC-671.0)[ADNET SYSTEMS INC]" <william.t.thompson at nasa.gov> wrote:

Mark:

Actually, I think I figured it out.  I was getting confused by the discussion of the auxiliary variables, which I now realize isn't relevant.  I believe the correct formation would be

DPDIS3 = 'Polynomial'
DP3.NAXES = 2
DP3.AXIS.1 = 1
DP3.OFFSET.1 = i0
DP3.AXIS.2 = 3
DP3.OFFSET.2 = j0
DP3.NTERMS = 1
DP3.TERM.1.COEFF = d1
DP3.TERM.1.VAR.1 = 1
DP3.TERM.1.VAR.2 = 1

Is this correct?

Thanks,

Bill Thompson