[fitswcs] Question about sign convention for VELOSYS

[William Thompson]

On 06/17/15 13:19, Steve Allen wrote:
> 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.

I think the options are either SPECSYS='HELIOCEN' if the wavelengths have been 
Doppler corrected, with VELOSYS having the opposite sign of what I said, or 
SPECSYS='TOPOCENT', in which case VELOSYS must be zero.  In the latter case, 
there is no standard way of expressing what the Doppler correction would be. 
I'll pass this information along.  I thank everybody for all your help.

Bill Thompson

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

-- 
William Thompson
NASA Goddard Space Flight Center
Code 671
Greenbelt, MD  20771
USA

301-286-2040
William.T.Thompson at nasa.gov