Geometric Transforms

The main functionality provided by Astrogrism is currently the geometric transforms to go between the three reference frames relevant to grism observations: the world (sky) frame, the direct (undispersed) image frame, and the dispersed grism image frame. The inputs and outputs going to/from each frame are listed below - note that the input needed to go from e.g. the world to the direct detector frame is the same as the output of the transform in the other direction, e.g. from the direct detector to the world frame.

Astropy Quantity input is currently accepted for wavelength, which will be converted automatically internally to the necessary units in that case. Currently, sky coordinates (right ascension and declination) must be input as decimal degrees.

World

Input/Output: (right ascension, declination, wavelength, order)

The right ascension and declination are specified in degrees. The wavelength units depend on instrument: Micron for ACS and WFC3 IR, Angstrom for WFC3 UVIS. If the wavelength is input as an astropy Quantity, it will be automatically converted to the correct units, otherwise (if a float is input) it must be in the correct units. The dispersion order is a unitless integer.

Direct Detector

Input/Output: (x, y, wavelength, order)

x and y are unitless pixel positions on the detector; wavelength and order are the same as for the World frame.

Grism Detector

Input/Output: (x, y, x0, y0, order)

All variables are unitless. x0 and y0 are the pixel positions on the direct detector. x and y are the corresponding dispersed pixel locations on the grism detector.