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+.help setjd Jan92 astutil
+.ih
+NAME
+setjd -- set various Julian dates in image headers
+.ih
+USAGE
+setjd images
+.ih
+PARAMETERS
+.ls images
+The list of images for which to compute Julian dates.  If the \fIlistonly\fR
+parameter is not set the image headers will be modified to add or update
+the calculated Julian date values.
+.le
+.ls observatory = ")_.observatory"
+Observatory of observation, used to define the time zone relative to
+Greenwich, if not specified in the image header by the keyword OBSERVAT.
+The default is a redirection to look in the parameters for the parent
+package for a value.  The observatory may be one of the observatories in
+the observatory database, "observatory" to select the observatory defined
+by the environment variable "observatory" or the parameter
+\fBobservatory.observatory\fR, or "obspars" to select the current
+parameters set in the \fBobservatory\fR task.  See \fBobservatory\fR for
+additional information.
+.le
+.ls date = "date-obs"
+Date of observation keyword.  The value must be in FITS format.
+This is one of DD/MM/YY (old FITS format), YYYY-MM-DD (new FITS format),
+or YYYY-MM-DDTHH:MM:SS (new FITS format with time).  The date should be
+in universal time though the \fIutdate\fR parameter can be used if
+this is not the case.  If a time is included it is used in preference
+to the \fItime\fR value.
+.le
+.ls time = "ut"
+Time of observation keyword with value given in decimal hours or HH:MM:SS.S
+format.  The date may be a local time or universal time as selected by the
+\fIuttime\fR parameter.  The time is used as is
+unless an exposure time keyword is specified, in which case
+the time will be corrected to the midpoint of the exposure from the
+beginning or end of the exposure.  This time is not used if a time
+is given in the date keyword.
+.le
+.ls exposure = "exptime"
+Exposure time keyword with value in seconds.  If specified the time
+is corrected to the midpoint of the exposure.  The time is assumed
+to be the beginning of the exposure unless
+the exposure time keyword name begins with a minus sign, for example
+"-exptime", in which case the time is used as the end of the exposure.
+.le
+.ls ra = "ra", dec = "dec", epoch = "epoch"
+If the heliocentric Julian date is requested the right ascension (in hours)
+and declination (in degrees) of the observation is determined from these
+keywords.  The values may be in either decimal or sexagesimal notation.
+An epoch keyword is optional and if given is used to precess
+the coordinates from the specified epoch to the observation epoch.
+If an epoch keyword is given but is not found in the header or can't
+be interpreted then it is an error.  The epoch keyword value may begin
+with 'B' or 'J'.  If the value is before 1800 or after 2100 a warning
+will be printed though the task will still compute the values.
+.le
+
+.ls jd = "jd"
+If specified compute the geocentric Julian day (Greenwich) at the
+midpoint of the exposure and record the value in the specified
+header keyword.
+.le
+.ls hjd = "hjd"
+If specified compute the heliocentric Julian day (Greenwich) at the
+midpoint of the exposure and record the value in the specified
+header keyword.
+.le
+.ls ljd = "ljd"
+If specified compute the local Julian day number.  This is an integer
+number which is constant for all observations made during the same night.
+It may be used to group observations by night in such tasks as
+\fBrefspectra\fR.
+.le
+
+.ls utdate = yes, uttime = yes
+Define whether the date and time of observation are in local standard
+time or in universal time.
+.le
+.ls listonly = no
+List the computed values only and do not modify the image headers.
+When simply listing the images need not have write permission.
+.le
+.ih
+DESCRIPTION
+\fBSetjd\fR computes the geocentric, heliocentric, and integer
+local Julian dates from information given in the headers of
+the input list of images.  This information may simply be listed or
+the values may be added or modified in the image headers.  Only
+those values which have a keyword specified are computed, printed,
+and entered in the images.  Thus, one need not compute all values
+and the dependent image header parameters required for computing them
+need not be present.  For example, if the coordinates of the
+observation are not available one should set the \fIhjd\fR parameter
+to an empty string.
+
+Often the date and time of observation are recorded either at the
+beginning or the end of an exposure.  To compute the Julian dates
+at the midpoint of the exposure the exposure keyword is specified.
+A negative sign preceding the keyword name defines correcting from
+the end of the exposure otherwise the correction is from the
+beginning of the exposure.  The exposure time must be in seconds and
+there is no allowance made for exposures which are interrupted.
+See also the task \fBsetairmass\fR which may be used to compute a
+universal time midexposure value.
+
+The date and time of observations should be given either in universal
+time.  However, if they are given in local standard time (there is no
+provisions for daylight savings times) the \fIutdate\fR and \fIuttime\fR
+parameters may be used.  Conversion between local and universal times, as
+well as the computation of the local integer date, requires the time zone
+in (positive) hours behind Greenwich or (negative) hours ahead of
+Greenwich.  This information is determined from the observatory at which
+the observations were made.  If the observatory is specified in the image
+header under the keyword OBSERVAT with a value which has an entry in the
+NOAO, local, or user observatory database then the value from the database
+is used.  This is the safest way since the observatory is tied to the
+actual image.  Otherwise, the \fIobservatory\fR parameter defines the
+observatory.  The special value "observatory" allows defining a default
+observatory with an environment variable or the \fBobservatory\fR task.
+Explicitly use the parameter \fIobservatory.timezone\fR use the value
+"obspars".  For more information see help under \fBobservatory\fR.
+
+The heliocentric Julian date is computed by defining a keyword for
+this value and also defining the keywords for the right ascension (in hours)
+and declination (in degrees).  An optional epoch keyword may be
+used if the RA and DEC are not for the observation epoch.
+
+The local integer Julian day number is the Julian date which begins at
+local noon.  Thus, all observations made during a night will have the
+same day number.  This day number may be useful in grouping
+observations by nights.  Note that in some time zones the UT
+date of observation may also be constant over a night.
+
+Among the uses for this task is to define keywords to be used by the task
+\fBrefspectra\fR.  In particular, the exposure midpoint geocentric Julian
+date makes a good sort parameter and the local Julian day number makes a
+good group parameter.
+.ih
+EXAMPLES
+1.  Compute all the Julian date quantities for 4 arc exposures with
+header parameters given below.
+
+.nf
+    demoarc1:
+	OBSERVAT= 'KPNO              '  /  observatory
+	EXPTIME =                  60.  /  actual integration time
+	DATE-OBS= '26/11/91          '  /  date (dd/mm/yy) of obs.
+	UT      = '12:11:30.00       '  /  universal time
+	RA      = '06:37:02.00       '  /  right ascension
+	DEC     = '06:09:03.00       '  /  declination
+	EPOCH   =               1991.9  /  epoch of ra and dec
+
+    demoarc2:
+	OBSERVAT= 'KPNO              '  /  observatory
+	EXPTIME =                  60.  /  actual integration time
+	DATE-OBS= '26/11/91          '  /  date (dd/mm/yy) of obs.
+	UT      = '12:41:30.00       '  /  universal time
+	RA      = '06:37:02.00       '  /  right ascension
+	DEC     = '06:09:03.00       '  /  declination
+	EPOCH   =               1991.9  /  epoch of ra and dec
+
+    demoarc3:
+	OBSERVAT= 'CTIO              '  /  observatory
+	EXPTIME =                  60.  /  actual integration time
+	DATE-OBS= '27/11/91          '  /  date (dd/mm/yy) of obs.
+	UT      = '11:11:30.00       '  /  universal time
+	RA      = '06:37:02.00       '  /  right ascension
+	DEC     = '06:09:03.00       '  /  declination
+	EPOCH   =               1991.9  /  epoch of ra and dec
+
+    demoarc4:
+	OBSERVAT= 'CTIO              '  /  observatory
+	EXPTIME =                  60.  /  actual integration time
+	DATE-OBS= '27/11/91          '  /  date (dd/mm/yy) of obs.
+	UT      = '12:21:30.00       '  /  universal time
+	RA      = '06:37:02.00       '  /  right ascension
+	DEC     = '06:09:03.00       '  /  declination
+	EPOCH   =               1991.9  /  epoch of ra and dec
+
+    cl> setjd demoarc?.imh
+    # SETJD: Observatory parameters for Kitt Peak ...
+    #              Image            JD           HJD   LOCALJD
+	    demoarc1.imh  2448587.0083  2448587.0127   2448586
+	    demoarc2.imh  2448587.0292  2448587.0336   2448586
+    # SETJD: Observatory parameters for Cerro Tololo ...
+	    demoarc3.imh  2448587.9667  2448587.9711   2448587
+	    demoarc4.imh  2448588.0153  2448588.0197   2448587
+.fi
+
+Note the use of the observatory parameter to switch observatories and
+the local Julian day number which is constant over a night even though
+the Julian date may change during the observations.
+
+2.  To compute only the geocentric Julian date from the "DATE" and
+"TIME" keywords in an image,
+
+.nf
+    cl> setjd obs1 date=date time=time exp="" hjd="" ljd=""
+.fi
+.ih
+REVISIONS
+.ls SETJD V2.11.2
+Y2K update:  Updated to use the new FITS format for the date.  If the
+time is given in the date keyword it is used in preference to the
+time keyword.
+.le
+.ls SETJD V2.11
+The checking of the epoch keyword value was improved.  Previously if
+there was a problem with the keyword value (missing or malformed) the
+task would use the epoch of the observation.  Now it is an error
+if an epoch keyword is specified but the epoch value can't be determined.
+Also a leading 'B' or 'J' is allowed and a warning will be given if
+the epoch value is unlikely.
+.le
+.ih
+SEE ALSO
+setairmass, hedit, refspectra, observatory
+.endhelp