metric -- Translate WF/PC or WFPC2 pixel coordinate to RA and Dec with geometric corrections and optional centroiding.
metric infile[group] x y
This task corrects for geometric distortions of the specified CCD chip (i.e., the chip corresponding to the specified file group) and transforms to the reference chip's coordinate. The reference chip is always at group 2, i.e., for WF (or WFPC2) it is WF2 and for PC it is PC6. It then uses the reference chip's group parameters CRVAL, CRPIX, and CD matrix coefficients to translate the corrected pixel coordinate to RA and Dec. Thus the epoch of RA and Dec is the same as these group parameters, usually J2000 for HST data.
This task can take three kinds of inputs: (1) a single pair of (X,Y) coordinate, specified by using the x and y input parameters; (2) X and Y in table(s), specified by passing the table and column name (separated by a space) for the x and y parameters; and (3) interactive cursor input, specified by setting x to "", which will cause the image to be displayed. You can then move the cursor to the desired position and press a key to get the coordinates. Pressing "q" will end the task; pressing "c" will enable centroiding; any other key will return the X,Y coordinate at the cursor position.
Centroiding is used to calculate a refined center position of a point source in the image by using the given position as an approximate initial value. Choosing a proper box size for centroiding is important: for example, if there is another star close to the target star, a large box will make the centroid lie between these two stars, instead of at the center of the target star. The algorithm used here is identical to the IRAF task imcntr. (Refer to the help file of imcntr for details of the algorithm.)
Results are sent to the terminal. The output will contain the following columns:
* original input pixel coordinates * pixel coordinates after centroiding (if applicable) * coordinates after geometric corrections * coordinates after transformation to the reference chip * RA and Dec
1) X can be a number representing the X coordinate, and the y parameter is passed a number representing the corresponding Y coordinate.
2) A null value ("") can be passed to x, meaning that pixel coordinates (both X and Y) are taken from interactive cursor location on the displayed image. In this case, any value passed to y is ignored.
3) x can be passed a table name (in either ASCII text or STSDAS format) with a space between the table name and column name, for example, x = "file.tbl COLNAME". In this case, you can specify a column name for the Y coordinates using the y parameter. If separate tables are passed to x and y and the two tables have different numbers of rows, the task will quit at the end of the shorter table. You can also enter values interactively by passing "STDIN" as the table name; in this case, you would end input by typing an EOF character (usually Control-Z or Control-D).
If this parameter is set to "no", output is printed in decimal degrees for RA and Dec.
This parameter is ignored if cursor input is used, as centroiding in cursor input is controlled by the "c" keystroke (see DESCRIPTION above).
1. Single pair of (X,Y) input, with decimal degree output for RA and Dec:
cl> metric w1234567t.d0h 19 53 hms-
2. Input positions from a text table where X are in the 3rd column and Y in the fifth column, send result to the file coord.result:
cl> metric w1234567t.d0h "coord.txt c3" "c5" > coord.result
3. Input X from an STSDAS table coord.tab and Y from a text table coord.txt. X's are in the column called "xname" and Y's in the 1st column:
cl> metric w1234567t.d0h "coord.tab xname" "coord.txt c1"
4. Input X's and Y's from the terminal (each line has an X,Y pair, first column is X), do the centroiding:
cl> metric w1234567t.d0h "STDIN c1" "c2" cen+
5. Input position(s) from the cursor:
cl> metric u1234567t.d0h ""
Geometric distortion and camera offset/rotation coefficients for WF/PC were supplied by Roberto Gilmozzi and Shawn Ewald at STScI in November 1992. Coefficients for WFPC2 were supplied by Roberto Gilmozzi, Ellyne Kinney, and Shawn Ewald in December 1994.
xy2rd, imcntr, wmosaic, invmetric