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+.help asthedit Jan96 astutil
+.ih
+NAME
+asthedit -- astronomical header editor
+.ih
+USAGE
+asthedit images commands
+.ih
+PARAMETERS
+.ls images
+List of images to be used.  The image header keywords are used in this task
+as variables which are read, modified, created, or deleted.  If the images
+do not have write permission or the \fIupdate\fR parameter is "no" then the
+image headers will not be modified.  If no images are specified then this
+task can be used as a calculator (though see \fBastcalc\fR).
+.le
+.ls commands
+A file of commands using the simple syntax given in the DESCRIPTION.  If no
+file name is given then the commands are read interactively from the
+standard input with a prompt given by the \fIprompt\fR parameter.  The
+command input ends with either EOF or "quit".  If a list of images and/or a
+table is specified the commands are repeated for each image or until the
+end of the table is reached.  Comments beginning with '#', blank lines, and
+escaped newlines are allowed.
+.le
+.ls table = ""
+Optional text file containing columns of values.  The table consists of
+one or more lines of whitespace separated columns of values.  Note that a
+string with whitespace needs to be quoted.  One line of the table is
+scanned for each image.  There must be at lest as many fields as are
+defined by the column names.
+.le
+.ls colnames = ""
+List of whitespace separated column names.  These are the names referenced
+in the command file by $<name>.  The leading '$' is not included in the
+column name specification.  There may be fewer columns than the number of
+columns in the table.  Dummy names must be used if some columns occur
+before a column to be referenced.
+.le
+.ls prompt = "asthedit> "
+When no command file is specified the input commands are read from the
+standard input (the terminal) and the value of the \fIprompt\fR string is
+printed as a prompt.  Note that if the input command file is specified as
+"STDIN" there will be no prompt even though commands will also be read from
+the standard input.
+.le
+.ls update = yes
+Update the image headers?  If no then any new, modified, or deleted
+keywords will not be recorded in the image headers.  This allows using the
+task only for computing and printing quantities.  Also this allows
+accessing read-only images.
+.le
+.ls verbose = no
+Print each keyword added or modified?
+.le
+.ls oldstyle = no
+Use the old style syntax of this task from versions prior to V2.11.  This
+parameter allows backward compatibility for command files previously
+developed.  Some aspects of the new syntax are still available.
+.le
+.ih
+DESCRIPTION
+\fBAsthedit\fR evaluates expressions using image header keywords, column
+names from a text table, CL parameters, internal variables, constants, and
+functions to create or modify image header keywords.  This task is
+particularly useful for adding keywords from a table and deriving keywords
+used by IRAF tasks which are not present in the images.  It differs from
+\fBhedit\fR in that it includes astronomical functions, operates from a
+command file which may perform many edits, and references columns from a
+text table.  The command file may be omitted in which case commands may be
+entered interactively for the first image and then the same commands will
+be repeated for any subsequent images.
+
+This task may be used interactively or with input from a command file
+(\fIcommands\fR).  If no command file is specified a prompt (\fIprompt\fR)
+is printed and commands are entered interactively.  The input is terminated
+with either the end-of-file character (EOF) or the command "quit".  Input
+command files simply contain the same input in a file and end with the end
+of the file or "quit".  The input commands, either those entered
+interactively or from a file, are repeated for each image in the image list
+and until the end of the input text table is reached, whichever comes
+first.  Generally this task is used on one or more images but if no
+image is specified the commands are executed just once and task behaves
+like an calculator.
+
+The command input consists of statements with each statement on a
+line by itself.  However long statements may be broken up with
+escaped newlines using the back-slash as the escape character;
+i.e. \<newline>.  Comments beginning with '#', blank lines,
+and whitespace are ignored.
+
+There are three types of statements: assignment, expressions, and
+conditional.  Each statement is on a line by itself though long statements
+may be broken up with escaped newlines (\<newline>).  Assignment statements
+have an image header keyword name (or variable name beginning with $), an
+equal sign (but see the \fIoldstyle\fR parameter), and an expression.
+Expression statements consist of only the expression with the value of the
+expression being ignored.  Expression statements are generally used with
+certain functions.  Conditional statements are blocks of if-endif and
+if-else-endif with assignment and expression statements between the
+if-else-endif statements.  These may not be nested.
+
+In earlier versions of this task there were only assignment statements
+and these did not use an equal sign; i.e. all statements consisted
+of an image header keyword and an expression separated by whitespace
+except that a keyword name by itself indicates deletion of a keyword.
+In order to interpret old command files the \fIoldstyle\fR parameter
+may be set to yes.  This will insert an equal sign internally.  It
+also only allows a subset of statements to not begin with a keyword
+or variable.  These are if, else, endif, print, printf, and quit.
+Note that with the old style syntax one may still include an equal
+sign.  It is recommended that the old style syntax not be used because
+of the greater flexibility in the new syntax.
+
+An image header keyword name is an arbitrary identifier which must begin
+with an alphabetic character or '$' followed by an alphabetic character and
+may use alphabetic characters, digits, or the characters '_', '$', or '.'.
+Keyword names are case insensitive.  Because some additional characters are
+allowed in the FITS definition of keyword names, such names may be
+referenced with the special '@' operator described below.
+
+One may also use internal variables which have the same identifier rules
+but begin with '$'.  Note that these variables are case sensitive (as are
+function names).  There are a few special predefined variables: "$I"
+contains the current image name, "$D" contains the current local date (in
+old FITS DD/MM/YY format), "$T" contains the current local time, "$GMD"
+contains the current Greenwich meridian date (in FITS YYYY-MM-DD format),
+"$GMT" contains the current Greenwich meridian time, and "$GMDT" contains
+the current date and time in FITS YYYY-MM-DDTHH:MM:SS format.
+
+Before the commands are interpreted for each image a line of a text
+file may be read.  This occurs when a file is specified by the
+\fItable\fR parameter.  The line is scanned and the values of each
+column are stored in the variable names specified by the \fIcolnames\fR
+parameter.  The values may be referenced in expressions by the
+specified column name preceded with '$'.  Note that additional lines
+may be scanned with the "fscan" function.  The user is then responsible
+for the table containing the correct sequence of lines when there
+are multiple images.
+
+In \fBasthedit\fR identifiers are image header keywords and lines
+for the table file are read automatically.  A related task is \fBastcalc\fR.
+In this task all variables are maintained internally and input and output
+are performed explicitly by functions.  There are functions to read,
+write, and delete image header keywords from a list of images.
+
+STATEMENTS
+
+The following gives a more formal description of the statement syntax
+and the special words "if", "else", "endif", and "quit".
+
+.nf
+	<keyword>
+        <keyword> = <expression>
+	$<variable> = <expression> 
+        <expression>
+        if (<expression>)
+            <statements>
+        endif
+        if (<expression>)
+            <statements>
+        else
+            <statements>
+        endif
+        quit
+.fi
+
+The result of the expression in the "if" statement is normally a logical
+value.  However, a numeric value of 0 is false while any other value is
+true and any string beginning with either "y" or "Y" is true with
+any other value being false; i.e. string values of yes and no may be used.
+
+The old style syntax allows the following statements.
+
+.nf
+	<keyword>
+        <keyword>     <expression>
+	$<variable>   <expression> 
+        <keyword> = <expression>
+	$<variable> = <expression> 
+        print (...)
+        printf (...)
+        if (<expression>)
+            <statements>
+        endif
+        if (<expression>)
+            <statements>
+        else
+            <statements>
+        endif
+        quit
+.fi
+
+Old style command files would only use the first two statements.
+
+KEYWORD NAMES AND VARIABLES
+
+Keyword names and variables may formally be defined as:
+
+.nf
+        [$]{a-zA-Z}[{a-zA-Z0-9._$}]*
+.fi
+
+where [] indicate optional, {} indicates a class, - indicates an ASCII
+range of characters, and * indicates zero or more occurrences.  In words, a
+keyword must begin with an alphabetic character, a variable or text file
+column name begins with '$' and an alphabetic character, and both may be
+followed by any combinations of alphabetic, digit, or '.', '_', and '$'
+characters.
+
+There are a few predefined variables which may be referenced in
+expressions.
+
+.nf
+        $I      The name of the current image (if used)
+        $D      The current date in the DD/MM/YY format
+        $T      The current (local) time as a sexagesimal string
+.fi
+
+The date and time are set once at the beginning of execution.
+
+Though not recommended it is possible to use any set of characters
+for a variable provided the variable is referenced as @"<name>".
+For example one could use @"date-obs" to include the character '-'.
+This option is primarily used for FITS keywords that use '-' as
+a hyphen character and must be escaped from interpretation as the
+an arithmetic subtraction operator.
+
+EXPRESSIONS
+
+Expressions consist of operands and operators.  The operands may be any
+image header keyword, previously defined variable, column name, quoted
+string constants, numeric constants, and functions.  Values given as
+sexagesimal strings are automatically converted to decimal numbers.  The
+operators are arithmetic, logical, and string.  The expression syntax is
+equivalent to that used in the CL and SPP languages.
+
+Additional information may be found in the help for \fBhedit\fR except that
+all unquoted nonnumeric strings are considered to be keywords or variables
+ and so the '(', ')' operators are not used.  The "field" references are
+not needed so the references "." and  "$" are not used and are not legal
+variable names in this task.
+
+operators:
+
+The following operators are recognized in expressions.  With the exception
+of the operators "?", "?=", and "@", the operator set is equivalent to that
+available in the CL and SPP languages.
+
+
+.nf
+        +  -  *  /              arithmetic operators
+        **                      exponentiation
+        //                      string concatenation
+        !  -                    boolean not, unary negation
+        <  <= >  >=             order comparison (works for strings)
+        == != && ||             equals, not equals, and, or
+        ?=                      string equals pattern
+        ? :                     conditional expression
+        @                       reference a variable
+.fi
+
+
+The operators "==", "&&", and "||" may be abbreviated as "=", "&", and "|"
+if desired.  The ?= operator performs pattern matching upon strings.
+The @ operator is required to reference keywords with
+one of the operator characters.  This is most like to be used as:
+
+        @"date-obs"
+
+A point to be aware of is that in the ?: conditional expression both
+possible result values are evaluated though the result of the expression
+is only one of them.  This means that one should not use this to
+call I/O functions that one wants to be executed only if a certain
+condition holds.
+
+intrinsic functions:
+
+A number of standard intrinsic functions are recognized within expressions.
+The set of functions currently supported is shown below.
+
+
+.nf
+	abs     atan2   deg     log     min     real    sqrt
+	acos    bool    double  log10   mod     short   str
+	asin    cos     exp     long    nint    sin     tan
+	atan    cosh    int     max     rad     sinh    tanh
+.fi
+
+
+The trigonometric functions operate in units of radians.
+The \fImin\fR and \fImax\fR functions may have any number of arguments up
+to a maximum of sixteen or so (configurable).  The arguments need not all
+be of the same datatype.
+
+A function call may take either of the following forms:
+
+.nf
+        <identifier> '(' arglist ')'
+or
+        <string_expr> '(' arglist ')'
+.fi
+
+The first form is the conventional form found in all programming languages.
+The second permits the generation of function names by string valued
+expressions and might be useful on rare occasions.
+
+special functions:
+
+In addition to the above intrinsic functions there are a number of
+astronomical functions. More will be added in time.  These are:
+
+.nf
+     sexstr - convert a number to a sexagesimal string (xx:mm:ss.ss)
+      epoch - compute an epoch given a date and time
+     julday - compute a Julian day given a date and time
+        mst - compute a mean sidereal time given a date, time, and longitude
+ ra_precess - precess ra from one epoch to another
+dec_precess - precess dec from one epoch to another
+    airmass - compute airmass given ra, dec, sidereal time, and latitude
+   eairmass - compute effective airmass given
+		ra, dec, sidereal time, exposure time, and latitude
+      obsdb - get parameters from the observatory database
+.fi
+
+.ls sexstr (number), sexstr (number, digits)
+Convert a number to a sexagesimal string in the format X:MM:SS.SS.  There
+is an optional second argument (the default is 0) which is the number of
+decimal digits in the seconds field.
+.le
+.ls epoch (date[, ut])
+Compute an epoch given a date and time.  The date is a string in the
+format DD/MM/YY, YYYY-MM-DD, or YYYY-MM-DDTHH:MM:SS.
+Typically this argument will be the standard FITS
+keyword DATE-OBS.  Because of possible confusion of the hyphen with
+subtraction this keyword would be specified as @"date-obs".  The time
+argument is optional.  If it is not given the time from the date
+string will be used and if absent a time of 0h is used.
+.le
+.ls julday (date[, ut])
+Compute a Julian day given a date and time.  The date and time are
+specified as described previously.
+.le
+.ls mst (date[, ut], longitude)
+Compute a mean sidereal time given a date, time, and longitude in degrees.  The
+date and (optional) time are specified as described previously.  The longitude
+may be given as a constant or using the observatory database function
+as shown in the examples.  The returned value is a sexagesimal
+string with two decimals in the seconds.
+.le
+.ls precess (ra, dec, epoch1, epoch2)
+Precess coordinates from one epoch to another.  The ra is the
+right ascension in hours, the dec in the declination in degrees,
+and the epochs are in years.  This function returns a formatted string with
+the precessed right ascension, declination, and epoch.  Numerical
+values for the right ascension and declination are obtained with the
+functions ra_precess and dec_precess.
+.le
+.ls ra_precess (ra, dec, epoch1, epoch2)
+Precess a right ascension from one epoch to another.  The ra is the
+input right ascension in hours, the dec is the declination in degrees,
+and the epochs are in years.  Because a function can return only one
+value there is a second function to return the precessed declination.
+The returned value is a sexagesimal string with two decimals in the seconds.
+.le
+.ls dec_precess (ra1, dec1, epoch1, epoch2)
+Precess a declination from one epoch to another.  The ra is the
+input right ascension in hours, the dec is the declination in degrees,
+and the epochs are in years.  Because a function can return only one
+value there is a second function to return the precessed right ascension.
+The returned value is a sexagesimal string with two decimals in the seconds.
+.le
+.ls arcsep (ra1, dec1, ra2, dec2)
+Compute the separation between two spherical coordinates.  The parameters
+ra1 and ra2 are coordinates in hours (right ascension, longitude, etc.)
+and the dec1 and dec2 parameters are coordinates in degrees (declination,
+latitude, etc.).  The computed value is returned in seconds of arc.
+.le
+.ls airmass (ra, dec, st, latitude)
+Compute an airmass given right ascension in hours, declination in
+degrees, sidereal time in hours, and latitude in degrees.  The latitude
+is often specified using the observatory database function as shown
+in the examples.
+.le
+.ls eairmass (ra, dec, st, exptime, latitude)
+Compute an "effective" airmass given right ascension in hours, declination
+in degrees, beginning sidereal time in hours, exposure time in seconds, and
+latitude in degrees.  The The latitude is often specified using the
+observatory database function as shown in the examples.  The effective
+airmass is based on a Simpson's rule weighting of the beginning, middle,
+and ending airmass (with no provision for paused exposure).  The weights
+are:
+
+.nf
+    effective = beginning + 4 * middle + ending
+.fi
+.le
+.ls obsdb (observatory, parameter)
+Return a value from the observatory database.  The observatory parameter is
+a observatory identification string as defined in the database.  Often this
+is the value stored in the OBSERVAT keyword.  Another special value is
+"observatory" which then follows a name resolution scheme.  The observatory
+database mechanism is described by the help topic \fBobservatory\fR.  The
+parameter is a string given the quantity desired.  Typically this would be
+"longitude" or "latitude" but there are other possible parameters.
+.le
+
+input/output functions:
+
+There are special functions for formatting, printing, error aborts,
+reading, writing, and deleting image header keywords, reading a text file,
+and reading and writing CL parameters.  Note that in \fBasthedit\fR
+one would not normally use the image input/output functions or
+the text file scanning function since any keyword reference reads or
+writes to the image header and one line of the text file is scanned
+automatically for each image.
+
+.nf
+     print  - print a set of arguments with default format
+     printf - print a set arguments with specified format
+     format - format a string
+     error  - print an error message and abort
+     clget  - get a value from a CL parameter
+     clput  - put a value to a CL parameter
+     scan   - scan a string and parse into keywords or variables
+     fscan  - scan a line of a text file
+     imget  - get the value of an image header keyword
+     imput  - put (add or modify) the value of an image header keyword
+     imdel  - delete an image header keyword
+.fi
+
+.ls print ([argument, ...])
+Print the arguments with default formats based on the type of value ending
+with a newline.  There may be zero or more arguments.  With zero arguments
+only a newline will be printed.
+.le
+.ls printf (fmt [, argument, ...])
+Print a list of arguments using the formatting syntax described later.
+Parameters to be formatted are given by the % fields and the values are
+passed as further arguments in the order in which they are referenced.
+There is no automatic newline so the format must include "\n" to
+produce newlines.
+.le
+.ls error (message)
+Print the "message", which can be any string variable such as might
+be produced by "format", and abort the task.  This is useful in
+conjunction with the conditional operator to abort if a variable
+takes an inappropriate value.
+.le
+.ls clget (parameter)
+Get the value of a CL parameter.  The argument must be a string.  The
+function value is the value of the parameter.
+.le
+.ls clput (parameter, value)
+Put a value into a CL parameter.  The parameter argument must be a
+string and the value can be anything.  The function returns a string
+of the form "clput: parameter = value" where parameter and value are
+the actual values.
+.le
+.ls scan (string, var, ...)
+Parse a string of whitespace separated words into a list of
+keywords or variables.  The number of variables assigned is
+the returned value of the function.
+.le
+.ls fscan (var, ...)
+Scan a line of a text file into a list of keywords or variables.  The arguments
+are zero or more variable names to which to assign the values of
+the whitespace separated fields.  The number of variables assigned
+is the returned value of the function.
+.le
+.ls imget (parameter)
+Get the value of an image header keyword from the current image.  The
+argument must be a string.  The function value is the value of the keyword.
+.le
+.ls imput (parameter, value)
+Put a value into an image header keyword for the current image.  The
+parameter argument must be a string and the value can be anything.  If the
+keyword exists it will be modified and if it does not exist it will be
+added.  The function returns a string of the form "imput: parameter =
+value" for new keywords or "imput: parameter = old_value -> value" for
+modified keywords where parameter and value are the actual values.
+.le
+.ls imdel (parameter)
+Delete an image header keyword.  The parameter argument must be a string.
+The returned values are the strings "imdel: parameter not found"
+or "imdel: parameter = value (DELETED)" where parameter is the parameter
+name and value is the old value.
+.le
+
+.ih
+FORMATS
+A  format  specification has the form "%w.dCn", where w is the field
+width, d is the number of decimal places or the number of digits  of
+precision,  C  is  the  format  code,  and  n is radix character for
+format code "r" only.  The w and d fields are optional.  The  format
+codes C are as follows:
+    
+.nf
+b       boolean (YES or NO)
+c       single character (c or '\c' or '\0nnn')
+d       decimal integer
+e       exponential format (D specifies the precision)
+f       fixed format (D specifies the number of decimal places)
+g       general format (D specifies the precision)
+h       hms format (hh:mm:ss.ss, D = no. decimal places)
+m       minutes, seconds (or hours, minutes) (mm:ss.ss)
+o       octal integer
+rN      convert integer in any radix N
+s       string (D field specifies max chars to print)
+t       advance To column given as field W
+u       unsigned decimal integer 
+w       output the number of spaces given by field W
+x       hexadecimal integer
+z       complex format (r,r) (D = precision)
+    
+    
+Conventions for w (field width) specification:
+    
+    W =  n      right justify in field of N characters, blank fill
+        -n      left justify in field of N characters, blank fill
+        0n      zero fill at left (only if right justified)
+absent, 0       use as much space as needed (D field sets precision)
+    
+    
+Escape sequences (e.g. "\n" for newline):
+    
+\b      backspace   (not implemented)
+\f      formfeed
+\n      newline (crlf)
+\r      carriage return
+\t      tab
+\"      string delimiter character
+\'      character constant delimiter character
+\\      backslash character
+\nnn    octal value of character
+    
+Examples
+    
+%s          format a string using as much space as required
+%-10s       left justify a string in a field of 10 characters
+%-10.10s    left justify and truncate a string in a field of 10 characters
+%10s        right justify a string in a field of 10 characters
+%10.10s     right justify and truncate a string in a field of 10 characters
+    
+%7.3f       print a real number right justified in floating point format
+%-7.3f      same as above but left justified
+%15.7e      print a real number right justified in exponential format
+%-15.7e     same as above but left justified
+%12.5g      print a real number right justified in general format
+%-12.5g     same as above but left justified
+
+%h	    format as nn:nn:nn.n
+%15h	    right justify nn:nn:nn.n in field of 15 characters
+%-15h	    left justify nn:nn:nn.n in a field of 15 characters
+%12.2h	    right justify nn:nn:nn.nn
+%-12.2h	    left justify nn:nn:nn.nn
+    
+%H	    / by 15 and format as nn:nn:nn.n
+%15H	    / by 15 and right justify nn:nn:nn.n in field of 15 characters
+%-15H	    / by 15 and left justify nn:nn:nn.n in field of 15 characters
+%12.2H	    / by 15 and right justify nn:nn:nn.nn
+%-12.2H	    / by 15 and left justify nn:nn:nn.nn
+
+\n          insert a newline
+.fi
+
+.ih
+EXAMPLES
+1.  The following command file exercises the astronomical functions:
+
+.nf
+    cl> type cmds
+    observat = "kpno"
+    time = sexstr (1.2345)
+    epoch = epoch (@'date-obs', ut)
+    jd = julday (@'date-obs', ut)
+    mst = mst (@'date-obs', ut, obsdb (observat, "longitude"))
+    rap = ra_precess (ra, dec, epoch, 1950)
+    dap = dec_precess (ra, dec, epoch, 1950)
+    airmass =  airmass (ra, dec, mst, obsdb (observat, "latitude"))
+    airmass
+    airmass = " "
+    airmass = eairmass (ra, dec, mst, itime, obsdb (observat, "latitude"))
+    cl> imhead obj001 l+
+        ...
+	DATE-OBS= '05/04/87'            /  DATE DD/MM/YY
+	RA      = '13:29:24.00'         /  RIGHT ASCENSION
+	DEC     = '47:15:34.00'         /  DECLINATION
+	UT      = ' 9:27:27.00'         /  UNIVERSAL TIME
+	ITIME   =                  600  /  REQUESTED INTEGRATION TIME (SECS)
+        ...
+    cl> asthedit obj001 cmds table="" verbose+
+    obj001:
+      $I = pix
+      $D = 22/01/96
+      $T = 19:14:41
+      observat = kpno
+      time = 1:14:04
+      epoch = 1987.257752395672
+      jd = 2446890.894062519
+      mst = 14:53:39.81
+      rap = 13:27:49.84
+      dap = 47:27:05.72
+      airmass = 1.079684154511483
+      airmass = 1.07968415451148 -> DELETED
+      airmass =  
+      airmass =  -> 1.08519059292424
+.fi
+
+Note the use of the keyword deletion and syntax for adding an empty
+value.
+
+2.  The following command file shows computing a mid-ut and using a table
+of values.
+
+.nf
+    cl> type cmds
+    midut = sexstr ($ut + $itime/3600./2.)
+    imagetyp = $imagetyp
+    cl> type table
+    object	9:27:27		600
+    comp	9:48:00		10
+    object	9:49:00		600
+    flat	12:00:00	2
+    cl> asthedit obj* cmds table=table colnames="imagetyp ut itime" verbose+
+    obj001.imh:
+      $I = obj001.imh
+      $D = 22/01/96
+      $T = 20:38:39
+      midut = 9:32:27
+      imagetyp = object
+    obj002.imh:
+      $I = obj002.imh
+      midut = 9:48:05
+      imagetyp = comp
+    ...
+.fi
+
+3.  The following example computes quantities used by some NOAO tasks from
+a minimal ESO/IHAP header.
+
+.nf
+    cl> type eso.dat
+    observat = "eso"
+    ut = sexstr ((@'tm-start'+0.1) / 3600.)
+    utend = sexstr ((@'tm-end'+0.1) / 3600.)
+    epoch = epoch (@'date-obs', ut)
+    st = mst (@'date-obs', ut, obsdb (observat, "longitude"))
+    exptime = (utend>ut)?(utend-ut)*3600.:(utend+24-ut)*3600.
+    ra = sexstr (@'postn-ra' / 15)
+    dec = sexstr (@'postn-dec')
+    airmass = airmass (ra, dec, st, obsdb (observat, "latitude"))
+    imagetyp =  $imagetyp
+    filter = $filter
+    cl> type table.dat
+    object	V
+    as> imhead eso
+        ....
+	DATE-OBS= '12/12/92'            / Date this data created dd/mm/yy
+	TM-START=             84854.    / '23:34:14' measurement start time
+	TM-END  =             84974.    / '23:36:14' measurement end time (U
+	TIME-SID=                 1.    / '00:00:01' sidereal start time
+	POSTN-RA=           354.0709    / '23:36:17' tel. position right-asc
+	POSTN-DE=           6.556945    /'+06:33:25' tel. position declinati
+        ....
+    as> asthedit eso eso.dat table=table.dat col="imagetyp filter" verbose+
+    eso:
+      $I = eso
+      $D = 23/01/96
+      $T = 09:02:55
+      observat = eso
+      ut = 23:34:14
+      utend = 23:36:14
+      epoch = 1992.948616307863
+      st = 0:18:56.76
+      exptime = 120.000000000006
+      ra = 23:36:17
+      dec = 6:33:25
+      airmass = 1.255875187126549
+      imagetyp = object
+      filter = V
+    as> imhead eso
+	...
+	DATE-OBS= '12/12/92'            / Date this data created dd/mm/yy
+	TM-START=             84854.    / '23:34:14' measurement start time
+	TM-END  =             84974.    / '23:36:14' measurement end time (U
+	TIME-SID=                 1.    / '00:00:01' sidereal start time
+	POSTN-RA=           354.0709    / '23:36:17' tel. position right-asc
+	POSTN-DE=           6.556945    /'+06:33:25' tel. position declinati
+	OBSERVAT= 'eso     '
+	UT      = '23:34:14'
+	UTEND   = '23:36:14'
+	EPOCH   =     1992.94861630786
+	ST      = '0:18:56.76'
+	EXPTIME =     120.000000000006
+	RA      = '23:36:17'
+	DEC     = '6:33:25 '
+	AIRMASS =     1.25587518712655
+	IMAGETYP= 'object  '
+	FILTER  = 'V       '
+	...
+.fi
+
+The 0.1 in the UT calculation are to account for round-off.
+Note the use of the conditional expression for the exposure time.
+
+4.  The following example is for a case where there was no telescope
+information but there is date and time information.  This example is
+relevant to data from the Kitt Peak Schmidt telescope circa 1993.
+A table is prepared with the RA, Dec, and Epoch of each observation
+and all other information is derived from the date, ut, and observatory
+database. 
+
+.nf
+    cl> type table.dat
+    12:45:32  +49:34:12   1950
+    13:12:02  -01:12:05   1950
+    cl> type cmds.hast
+    epoch   = epoch (@'date-obs', ut)
+    ra      = ra_precess ($ra, $dec, $epoch, epoch)
+    dec     = dec_precess ($ra, $dec, $epoch, epoch)
+    st      = mst (@'date-obs', ut, obsdb (observat, "longitude"))
+    airmass = eairmass (ra, dec, st, exptime, obsdb (observat, "latitude"))
+    midut   = sexstr (ut + exptime/3600./2.)
+    cl> asthedit *.imh cmds.hast table=table.dat colnames="ra dec epoch" ver+
+    sbs0119.imh:
+      $I = sbs0119.imh
+      $D = 23/01/96
+      $T = 10:38:32
+      epoch = 1987.257752395672
+      ra = 12:47:14.84
+      dec = 49:22:00.39
+      st = 14:53:39.81
+      airmass = 1.154765212092646
+      midut = 9:32:27
+    sbs0120.imh:
+      $I = sbs0120.imh
+      epoch = 1987.257752395672
+      ra = 13:13:56.90
+      dec = -1:23:54.30
+      st = 14:53:39.81
+      airmass = 1.336016291162518
+      midut = 9:32:27
+.fi
+
+Note the use of the table and image header epochs in the precession.
+
+5.  The following example shows the use of the printf function,
+and a null image name, and interactive command input.
+
+.nf
+    cl> asthedit "" ""
+    astcalc> ra = 12:20:30
+    astcalc> dec = 45:00:10
+    astcalc> ep1 = 1950
+    astcalc> ep2 = 2000
+    astcalc> ra1 = ra_precess (ra, dec, ep1, ep2)
+    astcalc> printf ("ra=%h dec=%h\n", ra1, dec_precess (ra, dec, ep1, ep2))
+    ra=12:22:57.4 dec=44:43:32.25
+.fi
+
+.ih
+REVISIONS
+.ls ASTHEDIT V2.11.2
+Y2K update:  The epoch, julday, and mst functions now take either the old
+or new FITS style date strings.  The time argument is optional and if
+it is not specified the time from the date string is used and if neither
+time is present a value of 0h is used.  New internal variables $GMD,
+$GMT, and $GMDT for the current time Greenwich time are defined.
+.le
+.ls ASTHEDIT V2.11
+There are new astronomical functions and input/output functions.
+
+The command syntax may now use "=" as a delimiter as well as the whitespace.
+
+A new parameter "update" allows protecting images and accessing read-only
+images for the purpose of calculating and printing quantities.
+
+The special variable name "$I" has the value of the image name, $D
+the current date, and $T the current time.
+
+The case of no image name creates and deletes a temporary image so the
+task can be used purely as a calculator (but see \fBastcalc\fR).
+.le
+.ih
+SEE ALSO
+astcalc, hedit, hfix, mkheader, setairmass, setjd, asttimes, precess,
+observatory
+.endhelp