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skyxymatch -- match input and reference image x-y coordinates using the celestial coordinate WCS
skyxymatch input reference output
- The list of input images containing the input celestial coordinate wcs.
- The list of reference images containing the reference celestial coordinate wcs. The number of reference images must be one or equal to the number of input images.
- The output matched coordinate lists containing: 1) the logical x-y pixel coordinates of a point in the reference image in columns 1 and 2, 2) the logical x-y pixel coordinates of the same point in the input image in columns 3 and 4, 3) the world coordinates of the point in the reference image in columns 5 and 6, and 4) the world coordinate of the point in the input image in columns 7 and 8. The output coordinate list can be input directly to the geomap task. The number of output files must be equal to the number of input images or be the standard output STDOUT.
- coords = "grid"
- The source of the coordinate list. The options are:
- Generate a list of nx * ny coordinates evenly spaced over the reference image, and beginning and ending at logical coordinates xmin and xmax in x and ymin and ymax in y.
- The name of the text file containing the world coordinates of a set of points in the reference image.
- xmin = INDEF, xmax = INDEF, ymin = INDEF, ymax = INDEF
- The minimum and maximum logical x and logical y coordinates used to generate the grid of control points if coords = "grid". Xmin, xmax, ymin, and ymax default to 1, the number of columns in the reference image, 1, and the number of lines in the reference image, respectively.
- nx = 10, ny = 10
- The number of points in x and y used to generate the coordinate grid if coords = "grid".
- wcs = "world"
- The world coordinate system of the coordinates. The options are:
- Physical coordinates are pixel coordinates which are invariant with respect to linear transformations of the physical image data. For example, if the reference image is a rotated section of a larger input image, the physical coordinates of an object in the reference image are equal to the physical coordinates of the same object in the input image, although the logical pixel coordinates are different.
- World coordinates are image coordinates which are invariant with respect to linear transformations of the physical image data and which are in decimal degrees for the celestial coordinate systems. Obviously if the wcs is correct the ra and dec of an object should remain the same no matter how the image is linearly transformed. The default world coordinate system is either 1) the value of the environment variable "defwcs" if set in the user's IRAF environment (normally it is undefined) and present in the image header, 2) the value of the "system" attribute in the image header keyword WAT0_001 if present in the image header or, 3) the "physical" coordinate system. Care must be taken that the wcs of the input and reference images are compatible, e.g. it makes no sense to match the coordinates of a 2D sky projection and a 2D spectral wcs.
- xcolumn = 1, ycolumn = 2
- The columns in the input coordinate list containing the x and y coordinate values if coords = <filename>.
- xunits = "", ls yunits = ""
- The units of the x and y coordinates in the input coordinate list
= <filename>, by default decimal degrees for celestial
coordinate systems, otherwise any units.
The options are:
- Input coordinates specified in hours are converted to decimal degrees by multiplying by 15.0.
- The internal units of the wcs. No conversions on the input coordinates are performed.
If the units are not specified the default is "native".
- xformat = "%10.3f", yformat = "%10.3f"
- The format of the output logical x and y reference and input pixel coordinates in columns 1 and 2 and 3 and 4 respectively. By default the coordinates are output right justified in a field of ten spaces with 3 digits following the decimal point.
- rwxformat = "", rwyformat = ""
- The format of the output world x and y reference image coordinates in columns 5 and 6 respectively. The internal default formats will give reasonable output formats and precision for sky projection coordinates.
- wxformat = "", wyformat = ""
- The format of the output world x and y input image coordinates in columns 7 and 8 respectively. The internal default formats will give reasonable output formats and precision for sky projection coordinates.
- min_sigdigits = 7
- The minimum precision of the output coordinates if, the formatting parameters are undefined, or the output world coordinate system is "world" and the wcs cannot be decoded.
- verbose = yes
- Print messages about the progress of the task?
SKYXYMATCH matches the logical x and y pixel coordinates of a set of points in the input image input with the logical x and y pixels coordinates of the same points in the reference image reference using world celestial coordinate information in the image headers. SKYXYMATCH writes its results to the coordinate file output which is suitable for input to the GEOMAP task. The input and reference images may be 1D or 2D but must both have the same dimensionality.
If coords = "grid", SKYXYMATCH computes a grid of nx * ny logical x and y pixel coordinates evenly distributed over the logical pixel space of the reference image defined by the xmin , xmax , ymin , ymax parameters. The logical x and y reference image pixel coordinates are transformed to reference image celestial coordinates using world coordinate information stored in the reference image header. The reference image celestial coordinates are transformed to input image celestial coordinates using world coordinate system information in both the reference and the input image headers. Finally the input image celestial coordinates are transformed to logical x and y input image pixel coordinates using world coordinate system information stored in the input image header. The transformation sequence looks like the following for an equatorial celestial coordinate system:
(x,y) reference -> (ra,dec) reference (reference image wcs) (ra,dec) reference -> (ra,dec) input (reference and input image wcs) (ra,dec) input -> (x,y) input (input image wcs)
The reference and input image celestial coordinate systems may be equatorial, ecliptic, galactic, or supergalactic. The equatorial systems may be one of: 1) the mean place pre-IAU 1976 (FK4) system, 2) the same as FK4 but without the E-terms (FK4-NO-E) system, 3) the mean place post-IAU 1976 (FK5) system, 4) or the geocentric apparent place in the post-IAU 1976 (GAPPT) system.
SKYXYMATCH assumes that the celestial coordinate system is specified by the FITS keywords CTYPE, CRPIX, CRVAL, CD (or alternatively CDELT / CROTA), RADECSYS, EQUINOX (or EPOCH), MJD-WCS (or MJD-OBS, or DATE-OBS). USERS SHOULD TAKE NOTE THAT MJD-WCS IS CURRENTLY NEITHER A STANDARD OR A PROPOSED STANDARD FITS KEYWORD. HOWEVER IT OR SOMETHING SIMILAR, IS REQUIRED TO SPECIFY THE EPOCH OF THE COORDINATE SYSTEM WHICH MAY BE DIFFERENT FROM THE EPOCH OF THE OBSERVATION.
The first four characters of the values of the ra / longitude and dec / latitude axis CTYPE keywords specify the celestial coordinate system. The currently permitted values of CTYPE[1:4] are RA-- / DEC- for equatorial coordinate systems, ELON / ELAT for the ecliptic coordinate system, GLON / GLAT for the galactic coordinate system, and SLON / SLAT for the supergalactic coordinate system.
The second four characters of the values of the ra / longitude and dec / latitude axis CTYPE keywords specify the sky projection geometry. IRAF currently supports the TAN, SIN, ARC, and GLS geometries, and consequently the currently permitted values of CTYPE[5:8] are -TAN, -ARC, -SIN, and -GLS. SKYXYMATCH fully supports the TAN, SIN, and ARC projections, but does not fully support the GLS projection.
If the image celestial coordinate systems are equatorial, the value of the RADECSYS keyword specifies which fundamental equatorial system is to be considered. The permitted values of RADECSYS are FK4, FK4-NO-E, FK5, and GAPPT. If the RADECSYS keyword is not present in the image header, the values of the EQUINOX / EPOCH keywords (in that order of precedence) are used to determine the fundamental equatorial coordinate system. EQUINOX or EPOCH contain the epoch of the mean place and equinox for the FK4, FK4-NO-E, and FK5 systems (e.g 1950.0 or 2000.0). The default equatorial system is FK4 if EQUINOX or EPOCH < 1984.0, FK5 if EQUINOX or EPOCH >= 1984.0, and FK5 if RADECSYS, EQUINOX, and EPOCH are undefined. If RADECSYS is defined but EQUINOX and EPOCH are not, the equinox defaults to 1950.0 for the FK4 and FK4-NO-E systems, and 2000.0 for the FK5 system. The equinox value is interpreted as a Besselian epoch for the FK4 and FK4-NO-E systems, and as a Julian epoch for the FK5 system. Users are strongly urged to use the EQUINOX keyword in preference to the EPOCH keyword, if they must enter their own equinox values into the image header. The FK4 and FK4-NO-E systems are not inertial and therefore also require the epoch of the observation (the time when the mean place was correct), in addition to the equinox. The epoch is specified, in order of precedence, by the values of the keywords MJD-WCS or MJD-OBS (which contain the modified Julian date, JD - 2400000.5, of the coordinate system), or the DATE-OBS keyword (which contains the date of the observation in the form DD/MM/YY, CCYY-MM-DD, CCYY-MM-DDTHH:MM:SS.S). As the latter quantity is only accurate to a day, the MJD-WCS or MJD-OBS specification is preferred. If all 3 keywords are absent the epoch defaults to the value of equinox. Equatorial coordinates in the GAPPT system require only the specification of the epoch of observation which is supplied via the MJD-WCS, MJD-OBS, or DATE-OBS keywords (in that order of precedence) as for the FK4 and FK4-NO-E system.
If the image celestial coordinate systems are ecliptic the mean ecliptic and equinox of date are required. These are read from the MJD-WCS, MJD-OBS, or DATE-OBS keywords (in that order or precedence) as for the equatorial FK4, FK4-NO-E, and GAPPT systems.
USERS NEED TO BE AWARE THAT THE IRAF IMAGE WORLD COORDINATE SYSTEM CURRENTLY (IRAF VERSIONS 2.10.4 PATCH 2 AND EARLIER) SUPPORTS ONLY THE EQUATORIAL SYSTEM (CTYPE<lngax> = "RA--XXXX" CTYPE<latax> = "DEC-XXXX") WHERE XXXX IS THE PROJECTION TYPE, EVEN THOUGH THE SKYXYMATCH TASK SUPPORTS GALACTIC, SUPERGALACTIC, AND ECLIPTIC coordinate systems.
If coords is a file name, SKYXYMATCH reads a list of x and y reference image world coordinates from columns xcolumn and ycolumn in the input coordinates file and transforms these coordinates to "native" coordinate units using the xunits and yunits parameters. The reference image world coordinates are transformed to logical reference and input image coordinates using the value of the wcs parameter and world coordinate information in the reference and input image headers.
SKYXYMATCH will terminate with an error if the reference and input images are not both either 1D or 2D. If the world coordinate system information cannot be read from either the reference or input image header, the requested transformations from the world <-> logical coordinate systems cannot be compiled for either or both images, or the world coordinate systems of the reference and input images are fundamentally incompatible in some way, the output logical reference and input image coordinates are both set to a grid of points spanning the logical pixel space of the input, not the reference image, and defining an identify transformation, is written to the output file.
The computed reference and input logical and world coordinates are written to the output file using the xformat and yformat , rwxformat , rwyformat , and the wxformat and wxformat parameters respectively. If these formats are undefined and, in the case of the world coordinates, a format attribute cannot be read from either the reference or the input images reasonable defaults are chosen.
If the reference and input images are 1D then the output logical and world y coordinates are set to 1.
If verbose is "yes" then a title section is written to the output file for each set of computed coordinates, along with messages about what if anything went wrong with the computation.
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:
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
Additional information on IRAF world coordinate systems including more detailed descriptions of the "logical", "physical", and "world" coordinate systems can be found in the help pages for the WCSEDIT and WCRESET tasks. Detailed documentation for the IRAF world coordinate system interface MWCS can be found in the file "iraf$sys/mwcs/MWCS.hlp". This file can be formatted and printed with the command "help iraf$sys/mwcs/MWCS.hlp fi+ | lprint".
Details of the FITS header world coordinate system interface can be found in the draft paper "World Coordinate Systems Representations Within the FITS Format" by Hanisch and Wells, available from the iraf anonymous ftp archive and the draft paper which supersedes it "Representations of Celestial Coordinates in FITS" by Greisen and Calabretta available from the nrao anonymous ftp archives.
The spherical astronomy routines employed here are derived from the Starlink SLALIB library provided courtesy of Patrick Wallace. These routines are very well documented internally with extensive references provided where appropriate. Interested users are encouraged to examine the routines for this information. Type "help slalib" to get a listing of the SLALIB routines, "help slalib opt=sys" to get a concise summary of the library, and "help <routine>" to get a description of each routine's calling sequence, required input and output, etc. An overview of the library can be found in the paper "SLALIB - A Library of Subprograms", Starlink User Note 67.7 by P.T. Wallace, available from the Starlink archives.
1. Compute a matched list of 100 logical x and y coordinates for an X-ray and radio image of the same field, both of which have accurate sky projection world coordinate systems with different equinoxes. Print the output world coordinates in hh:mm:ss.ss and dd:mm:ss.s format
cl> skyxymatch image refimage coords rwxformat=%12.2H \ rwyformat=%12.1h wxformat=%12.2H wyformat=%12.1h
2. Given a list of ras and decs of objects in the reference image, compute a list of matched logical x and y coordinates for the two images, both of which have a accurate sky projection wcss, although the reference wcs is in equatorial coordinates and the input wcs is in galactic coordinates. The ras and decs are in columns 3 and 4 of the input coordinate file and are in hh:mm:ss.ss and dd:mm:ss.s format respectively. Print the output world coordinates in the same units as the input.
cl> skyxymatch image refimage coords coords=radecs \ xcolumn=3 ycolumn=4 xunits=hours rwxformat=%12.2H \ rwyformat=%12.1h wxformat=%12.2H wyformat=%12.1h