Quickstart¶

Once installed, astrogrism can be imported and used in an interactive Python session or in a Jupyter notebook. The main class used to read in grism observation FITS files and expose the functionality is the astrogrism.GrismObs class.

To load your data, simply run the following in a Python interpreter or Jupyter notebook cell, replacing the filename with your file:

from astrogrism import GrismObs
import astropy.units as u
g_obs = GrismObs("sample_grism_flt.fits")

This object makes available the tranforms between the world, detector (i.e. the undispersed direct image), and grism_detector frames. To get the transform between, for example, the world and grism detector frames, you can call:

world_to_grism = g_obs.geometric_transforms.get_transform("world", "grism_detector")

For instruments with two chips (WFC3 UVIS and ACS), you must specify the chip for which you want the transform, e.g.:

world_to_grism = g_obs.geometric_transforms["CCD1"].get_transform("world", "grism_detector")

This transform would then allow you to calculate the coordinates on the dispersed image given a right ascension and declination (currently required to be in degrees), the wavelength of interest (specified as a Quantity), and the order of the dispersed spectrum:

world_to_grism(264.10183, -32.90802, 0.7*u.um, 1.0)

This returns a tuple of values (x, y, x0, y0, order), where (x0, y0) denotes coordinates on the direct image, and (x, y) denotes the dispersed coordinates on the grism image. For more detail about the available geometric frames and the inputs and outputs of the transforms between them, see Geometric Transforms.

In addition to the geometric transforms, the GrismObs object stores the contents of the input FITS file as an astropy HDUList in the grism_image attribute. The direct image can also be loaded, e.g:

g_obs = GrismObs("sample_grism_flt.fits", direct_image="sample_direct.fits")

and is stored in the direct_image attribute in the same way as the grism_image:

GrismObs.direct_image
>>> [<astropy.io.fits.hdu.image.PrimaryHDU object at 0x7f9240ca8880>,
     <astropy.io.fits.hdu.image.ImageHDU object at 0x7f9201bcfd60>, ...]

GrismObs.grism_image
>>> [<astropy.io.fits.hdu.image.PrimaryHDU object at 0x7f9240ca8880>,
     <astropy.io.fits.hdu.image.ImageHDU object at 0x7f9201bcfd60>, ...]