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precess -- Precess images, tables, or lists of coordinates.


precess input output fromdate todate


The task will precess one of the following: 1) the coordinate system of images, 2) the right ascension and declination columns of tables, or 3) a list of right ascension and declination values from text files. The IAU (1976) precession constants are used (see Lieske, Jay H., Astron. Astrophys., 73, 282-284 (1979)).

The following standards apply to all types of precession. Dates can be entered either in years with a fractional component or as a Julian day number. Any date value that is greater than 3000 will be considered a Julian day number. A year number can be prefixed with either j, J, b or B, where the J stands for a Julian year and the B for a Besselian year. A year number without a prefix is assumed to be a Julian year. The only exception to these rules is the EQUINOX header keyword in an image. The keyword value is normally a floating-point number rather than a character string and therefore does not have a character prefix. If the value of EQUINOX is exactly 1950.0, then B1950 is assumed. This can be overridden with the fromdate parameter. Actually, the EQUINOX keyword value is gotten as a character string, and it is permitted to have a prefix. This is because the EQUINOX keyword in HST images is in fact a character string equal to "J2000". When the todate is written to the output header, however, no prefix will be included.

If text files are used, then each line must contain the right ascension and declination in that order. If the parameters fromdate or todate are undefined ("") then they must be given following the RA and DEC. The input values for RA & DEC may be in sexagesimal notation. Output will be in sexagesimal notation if the hms_format parameter is set to yes. The number of digits of precision that will be printed is determined by the precision of the input RA & DEC, except that the minimum is four decimals. Blank lines and lines beginning with # are ignored.

Image precession is done by modifying the parameters that describe the transformation between pixel coordinates and RA & DEC. Precession shifts and rotates the coordinate system, so this task precesses the coordinates (CRVAL1 & CRVAL2) at the reference pixel and rotates the CD matrix by the appropriate amount. If the fromdate parameter is undefined (i.e., "") then the EQUINOX keyword of the image is used as the fromdate. If EQUINOX is not found then EPOCH will be used. An error will occur if fromdate is undefined and the image contains neither EQUINOX nor EPOCH keyword.

Certain limitation checks are made before the image is precessed. The first two axes of the image must be either RA or DEC, as specified by the CTYPE1 & CTYPE2 keywords. The task will not handle RA or DEC being the third axis. Images that include either the north or south pole are processed correctly, however, if the reference pixel (either before or after precession) is at a pole, the image orientation will be poorly defined and the image rejected. A pixel is considered too close to the pole if "cos(DEC) <= ~1e-15". If your image is rejected because the reference pixel is too close to a pole, you can change the reference coordinate slightly.

Images with Aitoff or Mercator projections will not be precessed. MWCS requires zero declination for the reference pixel for these projections, but precession changes the declination of the reference pixel.

If an input file is a table, then the RA and DEC in a pair of columns in the table will be precessed. The column names for RA, DEC, fromdate and todate are gotten from the parameters namcol1, namcol2, namcol3, and namcol4, respectively. If the fromdate or todate parameters are undefined (i.e., ""), the dates must come from table columns.

If an output table is created, it will differ from the input table only in that RA and DEC coordinates will have been precessed. If the input table is modified in-place, new columns will be created by prepending "p_" to the names of the existing RA and DEC columns. The units will be taken from the column units (case insensitive) if they can be interpreted; otherwise, the ra_units parameter is used to specify the units. Because this is true of each column, you should make sure that column units are defined for both RA and DEC, or that both are undefined and the ra_units parameter is correct. If the column units for RA begin with "hour" or "hr", units of hours will be assumed. Units of degrees are specified by column units beginning with "deg", and radians are specified by column units beginning with "rad". In contrast to text I/O, the units for RA and for DEC can be quite different, e.g., radians for RA and degrees for DEC.


input [file name template]
Name(s) of file to be precessed. The task will determine whether each input file is an image, table, or text file. Wild cards and file lists are accepted, e.g., "*.tab" or "@file.lst".
output [file name template]
Output file name(s).

For text files an output file name must be specified.

If an output image or table name is the same as the input name, or if output is null or blank, the input image or table will be modified in-place.

For a table that is modified in-place, new column names will be created for the precessed right ascension and declination be prepending "p_" to the input column names.

Note that IRAF can build new file names from existing file names. For example, if the input parameter was passed a wildcard such as "*.tab" to process all tables, the output parameter can be set to "new_//*.tab" to prefix all of the output files with the string "new_". This is described in more detail by typing "help files".

fromdate = "b1950" [string]
This is the master date of the input coordinates to precess; it is independent of whether the image contains an EQUINOX keyword, or the table or text file contains a fromdate column. The EQUINOX keyword or fromdate column is used only if fromdate is not specified.

This value will be used for all files specified by input.

todate = "j2000" [string]
This is the date to which the coordinates should be precessed. It works like fromdate. For an image, however, todate must be specified. That is, there is no header keyword (like EQUINOX) that is used if todate is not specified.
(ra_units = "hours") [string, allowed values: hours | degrees | radians]

If an ASCII text file is used as input, this parameter defines the units used for right ascension, and implies units for declination. If hours or degrees are used for right ascension, the task assumes declination is in degrees. If radians are used for RA, then they are assumed for DEC as well.

Units used in processing tables are taken from the columns, unless either RA or DEC column units is null or cannot be interpreted, in which case, this parameter is used.

All units are in degrees for images.

(hms_format = yes) [boolean]
Use HH:MM:SS.d format for ASCII output?

If hms_format = yes, the output right ascension and declination will be in HH:MM:SS.d, DD:MM:SS.d format (or DD:MM:SS.d, DD:MM:SS.d if ra_units="degrees"). You will get garbage if you set hms_format = yes and ra_units = "radians".

(namcol= "ra") [string]
Column name for right ascension.
(namcol= "dec") [string]
Column name for declination.
(namcol= "fjy") [string]
Column name for the date from which to precess.
(namcol= "tjy") [string]
Column name for the date to which to precess.
(verbose = yes) [boolean]
The input and output file names will be printed to the standard output if verbose is set to yes.


1. Precess coordinates typed in from the keyboard from b1900 to b1950, displaying the results on the terminal screen. The output values are in HH:MM:SS.d, DD:MM:SS.d format by default. Alternate lines of the example are printed by the user and by the task.

	to> tprecess STDIN STDOUT b1900 b1950
	         --RA--        --DEC--
	0   0
	      0:02:33.7          0:16:42
	0   0.0000000
	   0:02:33.6760      0:16:42.433
	0.000000000   90
	 12:01:16.84413    89:43:17.5513
	0.000000000  -90
	  0:01:16.84413   -89:43:17.5513
	18.25   37:12:05.4
	    18:16:43.05       37:13:14.7
	18:15   37:12:05.4
	    18:16:43.05       37:13:14.7
	3:18:06.423   42:57:57.19
	    3:21:27.893      43:08:42.27

2. Precess columns "ra" and "dec" in a table "" from b1900 to j2000. Suppose that we know that the units for those columns are not given in the table, so we specify the units explicitly.

	to> tprecess b1900 j2000 ra_units=degrees

3. Precess the coordinate parameters of an image in-place from j2000 to b1950. images.hedit is used to display the coordinate parameters before and after precessing.

	im> hedit x0dw0503t.d0h crval*,cd* .
	x0dw0503t.d0h,CRVAL1 = 187.706042553463
	x0dw0503t.d0h,CRVAL2 = 12.391055609997
	x0dw0503t.d0h,CD1_1 = -4.594922E-6
	x0dw0503t.d0h,CD1_2 = -4.173452E-6
	x0dw0503t.d0h,CD2_1 = -4.125204E-6
	x0dw0503t.d0h,CD2_2 = 4.648664E-6

	to> tprecess x0dw0503t.d0h "" j2000 b1950

	im> hedit x0dw0503t.d0h crval*,cd* .
	x0dw0503t.d0h,CRVAL1 = 187.073476992675
	x0dw0503t.d0h,CRVAL2 = 12.6671491006909
	x0dw0503t.d0h,CD1_1 = -4.597562E-6
	x0dw0503t.d0h,CD1_2 = -4.170475E-6
	x0dw0503t.d0h,CD2_1 = -4.122262E-6
	x0dw0503t.d0h,CD2_2 = 4.651335E-6

4. Precess all four groups of w4.hhh, putting the results in t4.hhh.

	to> tprecess  w4[1],w4[2],w4[3],w4[4]  t4[1/4],t4[2],t4[3],t4[4]


A date entered with trailing blanks is considered uninterpretable. For instance the string "j2000 " is not interpretable because of the trailing blank.



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