NAME

delaytime -- Correct for light delay time.

USAGE

delaytime infile outfile earth_ephem obs_ephem mu_ra mu_dec distance dist_unit

DESCRIPTION

The High Speed Photometer (HSP) aboard the Space Telescope is capable of timing observations to a temporal resolution of 10 milliseconds. The effects on the observed times of both the orbital motions of the Earth and the telescope may need to be corrected using times to the solar-system barycenter. This routine creates a file of observation times corrected for light delay from three sources: (1) general relativistic effects (up to 2 milliseconds), (2) displacement of the telescope from the center of the Earth (up to 20 milliseconds), (3) and displacement of the Earth from the solar-system barycenter (up to 500 seconds).

This task can simultaneously process several input files. Because this task requires that the user specify a target's proper motion and parallax, all input files should be for either the same target or for distant targets (such as extragalactic objects). The maximum time delay due to proper motions is 2.4 milliseconds per arcsecond of position displacement and 1.2 milliseconds per arcsecond of parallax.

Time accuracy is preserved by using double precision data in the output file. The pixel values in the output file are time offsets (in seconds) from a reference time defined by the output file header keyword REFMJD (in modified Julian date).

PARAMETERS

infile = "" [string]

Name of the input science data file (a file name template can be used to process several files).

outfile = "" [string]

Name of the output time file.

earth_ephem = "" [string]

Name of the STSDAS table containing Earth's state vectors (centered at the solar system barycenter). This table must have the following columns:

'X'		X component of the state vector (double).
'Y'		Y component of the state vector (double).
'Z'		Z component of the state vector (double).

And the following parameters:

'FIRSTMJD'	Epoch of the first row in the table, in 
		modified Julian date (double).
'GRID'		Epoch interval between successive rows in 
		the table, in days (double).
'LASTMJD'	Epoch of the last row in the table, in 
		modified Julian date (double).
'GRIDUNIT'	Unit of the grid (char).

obs_ephem = "" [string]

Name of the STSDAS table containing the observer's state vectors (centered at Earth's center). This table must have the following columns:

'X'		X component of the state vector (double).
'Y'		Y component of the state vector (double).
'Z'		Z component of the state vector (double).

And the following parameters:

'FIRSTMJD'	Epoch of the first row in the table, in 
		modified Julian date (double).
'GRID'		Epoch interval between successive rows in 
		the table, in days (double).
'LASTMJD'	Epoch of the last row in the table, in 
		modified Julian date (double).
'GRIDUNIT'	Unit of the grid (char).

mu_ra = 0. [real]

Target's right ascension proper motion at the epoch of J2000 (seconds of time per year).

mu_dec = 0. [real]

Target's declination proper motion at the epoch of J2000 (arcseconds per year).

distance = 0. [real]

Distance (or parallax) of the target from the solar system barycenter. This is in the unit specified in the dist_unit parameter.

dist_unit = "arcsec" [string, allowed values = arcsec|pc|ly|au|km]

Unit of distance (or parallax). Parallax should be specified in arcseconds ("arcsec"), while distance can be specified in parsecs ("pc"), light years ("ly"), astronomical units ("au"), or kilometers ("km"). CAUTION: this task will not calculate correct results for objects in the solar system.

EXAMPLES

1. Construct a file containing the time tags corrected to the barycenter of the solar system, form the input file abc.d0h. Use the Earth ephemeris timeseries$de200 and the HST ephemeris hst.tab. Assume that the target has zero proper motion and parallax.

ti> delaytime abc.d0h output timeseries$de200 hst.tab 0 0 0 "arcsec"

BUGS

REFERENCES

SEE ALSO

orderphase