Class ObsGeometry

We need to setup the proper object for the GeometryModel
 based on the format of the provided idctab.

We need to trap attempts to address values of 'chip' that
are not part of the observation; such as in sub-arrays.

This method applies the model to a pixel position
 to calculate the new position.
 Depending on the value of direction, this could mean
 going from distorted/raw to a corrected positions or
 the other way around.

 If a specific pixel scale is provided, this will be
 used to determine the final output position.

Correct the distortion coefficients for the effects of velocity aberration, IF the VAFACTOR value is present. This method relies on RA_TARG and DEC_TARG to provide the information on where the telescope was pointing, the point which serves as the center of the radial velocity aberration.

Converts input pixel position 'pixpos' into an X,Y position in WCS. Made this function compatible with list input, as well as single tuple input..apply

This method is the reverse of "apply" - it finds a position which, when distorted, maps to "pixpos". The method is iterative and is modelled on that in "tranback" in the dither package.

pixpos is an x,y pixel position tuple.

pixpos is assumed to be in the appropriate sub-array coordinates.

Richard Hook, ST-ECF/STScI, January 2003

This method applies the distortion to a 1x1 box at the reference position and updates the WCS based on the results.

This method is based directly on the 'drizzle' subroutine 'UPWCS' written by R. Hook.

 This method applies the model to an RA/Dec
  and calculates the pixel position.
  RA and Dec need to be in decimal form!

This needs to be expanded to include full distortion model
as well, i.e. inverse distortion solution.

This method will compute a new shape based on the positions of
the corners AFTER applying the geometry model.

These new position for each corner should be calculated by calling
self.geometry.apply() on each corner position.
This should also take into account the output scale as well.

Values for the corners must go from 0, not 1, since it is a Python array.
    WJH, 17-Mar-2005