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Knowledgebase: GO - General Observer
Where was Spitzer EXACTLY during my observations so that I can calculate light-time corrections?
Posted by Elena Scire on 21 November 2008 11:01 AM

This information is in the fits headers for data processed with pipelines > S16. Here is a sample snippet:

SPTZR_X =       6084048.364647 / Heliocentric J2000 x position (km)
SPTZR_Y =    -138993559.088106 / Heliocentric J2000 y position (km)
SPTZR_Z =     -62551818.282086 / Heliocentric J2000 z position (km)
SPTZR_VX=            29.396946 / Heliocentric J2000 x velocity (km/s)
SPTZR_VY=             0.639668 / Heliocentric J2000 y velocity (km/s)
SPTZR_VZ=             0.729644 / Heliocentric J2000 z velocity (km/s)
SPTZR_LT=           508.824176 / One-way light time to Sun's center (s)
SCLK_OBS=        837186592.637 / SCLK (~sec. since 1/1/1980) at ramp start.
ET_OBS  =        205991150.144 / Ephemeris time (seconds past J2000 epoch)
DATE_OBS= '2006-07-12T15:44:44.960' / UTC date & time at ramp start.
MJD_OBS =        53928.6560759 / Mod. Julian Date (UTC) (days) at ramp start.
HMJD_OBS=        53928.6558680 / Corresponding Heliocen. Mod. Julian Date
UTCS_OBS=        205991084.960 / UTC seconds past year 2000 at ramp start.

The inertial reference frame for the calculations is ICRF, with its origin at the Sun's center. The target used for computing helocentric Julian data is specified by RA_RQST and DEC_RQST in the FITS header. Note that since the range of the target is unavailable, it is impossible to include a correction for the target's Spitzer/Sun parallax. If you'd like to calculate this information yourself, please see this file.