# evalxy stsdas.hst_calib.foc.focgeom

NAME · USAGE · DESCRIPTION · PARAMETERS · EXAMPLES · BUGS · REFERENCES
SEE_ALSO

## NAME

evalxy -- Evaluate four-parameter fit to get V2,V3.

evalxy x y

## DESCRIPTION

This task applies a linear transformation specified by four parameters to a pair of X & Y coordinates and prints the input X,Y coordinates and the transformed coordinates. The result of the transformation is also saved in parameters v2 and v3. You can run fitxy in order to compute the fit and write the coefficients to the evalxy parameters parity, scale, angle, v2_0 and v3_0.

The coefficients of the fit for image X,Y coordinates are defined as follows:

```	v2 = sign * x * scale * cos(angle)
- y * scale * sin(angle) + v2_0

v3 = sign * x * scale * sin(angle) +
y * scale * cos(angle) + v3_0

where sign is +1 or -1 depending on whether
the parity is "same" or "opposite"
```

## PARAMETERS

x = 512. [real]
x and y are the X,Y pixel coordinates at which the fit is to be evaluated. See also parity.
y = 512. [real]
See the description of x.
(parity = "opposite") [string, allowed values: same | opposite]
The parity of the input coordinate system with respect to the output coordinates. If parity = "same" the parity of the X,Y coordinates is the same as that of the V2,V3 coordinates. If parity = "opposite" the quantity that is changed is the sign of the X coordinate.
(scale) [real]
The scale of the output coordinates with respect to input X,Y. In the case of fitting from FOC image pixel coordinates to HST V2,V3 coordinates, the scale will be in arcseconds per pixel.
(angle) [real]
For the case of fitting FOC image coordinates to V2,V3 coordinates, the angle is in degrees from the -X direction (i.e., the photocathode sample direction) to the +V2 direction. This case has parity = "opposite". When fitting between data sets with the same parity, angle is the counterclockwise rotation of points in the input (X,Y) coordinate system required to align with corresponding points in the output (V2,V3) coordinate system.
(v2_0) [real]
The zero point offset (v2_0,v3_0) is the point in V2,V3 corresponding to (X,Y) = (0,0).
(v3_0) [real]
See the description of v2_0.
(v2) [real]
This is a task output parameter. The fit evaluated at x and y gives V2,V3 coordinates which are saved as task parameters v2 and v3.
(v3) [real]
This is a task output parameter. See the description of v2.

## EXAMPLES

1. After running fitxy, evaluate the fit at x=257, y=513.

```	evalxy 257 513
```

## REFERENCES

This task was written by Phil Hodge.