STScI Logo

fceval noao.twodspec.longslit



fceval -- Evaluate coordinates using the FITCOORDS solutions


fceval input output fitnames


Input text file of pixel coordinates. This may be "STDIN" to read coordinates from the terminal or pipe.
Output text file of pixel coordinates and fitted coordinates. This may be "STDOUT" to write coordinates to the terminal or pipe.
Names of the user coordinate maps to evaluate.
database = "database"
Database containing the coordinate maps.


This task transforms pixel coordinates to the world coordinates fit with FITCOORDS. When there is no map for an axis the identify transform is used. If there are more the one map for an axis the average of the mapped coordinates is output. This is the same behavior as TRANSFORM.

The input file consists of two columns giving the x and y pixel values in the frame of the untransformed image data. The output is a file with four columns giving the input x any y pixel values and the user coordinates fit by FITCOORDS.

Two typical uses for this task are to look up world coordinates for points in the untransformed data and to generate transformations using GEOMAP and GEOTRAN.


1. Evaluate a wavelength and slit position fit where the input pixel coordinates are entered interactively and the output is written to the terminal.

    cl> fceval STDIN STDOUT arcfit,std
    1 1
    1. 1. 20.60425149463117 4202.47202514205
    60 1
    60. 1. 79.60425149463118 4203.316616448186
    1 512
    1. 512. 19.15606081299484 7356.089801036373
    60 512
    60. 512. 78.15606081299485 7355.042495319318

In this case the first axis corresponds to the spatial dimension and the second to the dispersion dimension. The arcfit was created using Angstroms and so the units of the last column is Angstroms.

2. One use of this task is to generate the inverse transformation from that produced by TRANSFORM. The steps are: 1) produce a grid of coordinates using LISTPIX and FCEVAL, 2) convert the user coordinates to pixel coordinates in the transformed data using WCSCTRAN, 3) fit a transformation using GEOMAP, and 4) transform the data with GEOTRAN.

    cl> listpix orig[*:5,*:5] wcs=physical verb- |
    >>> fceval STDIN STDOUT arcfit,std |
    >>> wcsctran STDIN coords trans world logical columns="3 4"
    cl> geomap coords geomap.db 1 61 1 512
    cl> geotran trans origNEW geomap.db coords flux+

This example uses pipes to eliminate intermediate files. But these files can be useful for understanding the process. LIXTPIX is used to generate a grid of points with some subsampling. Be sure to use "physical" for the coordinate system otherwise the grid of x and y values will be for the subsection. The order of the columns will be appropriate for GEOMAP to compute the inverse transformation. By reversing the order of the columns one could generate a transformation similar to that produced by TRANSFORM in order to use features in GEOTRAN not provided by TRANSFORM. However, the world coordinate system information will not be automatically set.


fitcoords, transform, geomap, geotran

Search Form · STSDAS