[fitswcs] Time WCS

[Steve Allen]

On Fri 2002-02-01T16:35:47 -0500, William Pence hath writ:
> Representations of Time Coordinates in FITS

> 2.  The second 4 characters of the CTYPEj keyword specify the reference
> frame of the time measurements.  The same notation that is used for the
> SPECSYS keyword in the spectral coordinate system will be used:
>
>  code      location of time measurements
>  -------    -----------------------------
>   -SAT      on a moving satellite
>   -TOP      at a fixed location on the earth
>   -GEO      geocentric
>   -BAR      barycentric
>   -HEL      heliocentric
>   -LSR      local standard of rest (kinematic)
>   -LSD      local standard of rest (dynamic)
>   -GAL      Galactocentric
>   -LGR      local group
>   -SOU      source reference frame

There will need to be provision for "satellites" that are actually
fixed to the surface of, say, Mars.  The most recent draft of WCS
paper III has split the notion of SPECSYS into two keywords.  This was
specifically to leave open the option of defining each of the original
frames as any location in the solar system.

>    If CTYPEj = 'TIME-TOP' (topographic) then the following 3 keywords are
>    required to provide the positional information that is needed to perform
>    photon arrival time corrections:
>
>    OBS-LAT   latitude of time assignment position
>    OBS-LONG  longitude of time assignment position
>    OBS-ELEV  altitude of time assignment position

During various recent ruminations on paper III it was decided to
prefer body-centric Cartesian coordinates in meters.  The notions of
longitude and latitude come with much baggage about ancient
terrestrial geodetic systems that would best be forgotten.  Any GPS
unit with Cartesian readout mode gives a better answer than most maps.

>  D.  Optional keywords that define the exposure time of the observation:
>
>    TELAPSE   equals TSTOP - TSTART
>    ONTIME    total on-source exposure time; sum of GTIs
>    LIVETIME  equals ONTIME corrected for instrumental 'dead time' effects
>    EXPOSURE  equals LIVETIME corrected for any other effects (vignetting)
>    TIMEDEL   gives the inherent time resolution in the data in seconds,
>            i.e., the smallest possible difference between successive
>            time measurements.

These may want to be accompanied by something like TAVERAGE
representing the photon-weighted mindpoint of the observation.

--
Steve Allen          UCO/Lick Observatory       Santa Cruz, CA 95064
sla at ucolick.org      Voice: +1 831 459 3046     http://www.ucolick.org/~sla
PGP: 1024/E46978C5   F6 78 D1 10 62 94 8F 2E    49 89 0E FE 26 B4 14 93