[fitswcs] Question about sign convention for VELOSYS

[Steve Allen]

On Wed 2015-06-17T12:02:51 -0400, William Thompson hath writ:
> A question has arisen about the use of the WCS keywords SPECSYS and
> VELOSYS for solar observations.  The plan is to set
> SPECSYS='HELIOCEN', and to set VELOSYS to the radial velocity of the
> observatory relative to the physical center of the Sun, with a
> positive value if the movement is toward the Sun.  Is this the
> correct sign convention?

Are the velocities actually reduced to be with respect to the
heliocenter or are they simply laboratory rest frame velocities of the
spectrograph located on the surface of the earth, or in earth orbit,
or in solar orbit?

If they are laboratory rest frame, i.e. not reduced to the heliocenter,
then it makes sense that the convention is positive velocity into the
sun.  However in that case SPECSYS really wants to be something other
than 'HELIOCEN' because that is not the reference point for the
observation.

If they are actually reduced to the heliocenter then positive velocity
should be outward from the sun for SPECSYS='HELIOCEN'.

If you really want to preserve the laboratory rest frame convention
for velocities of material around the photosphere then I think this
wants to define a new SPECSYS='SOLPHOT' or something like that.

--
Steve Allen                 <sla at ucolick.org>               WGS-84 (GPS)
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