Extending POPPY by defining your own optics and instruments

POPPY is designed to make it straightforward to implement your own custom optics classes, which will interoperate with all the built-in classes. Conceptually all that is needed is defining the get_transmission and/or get_opd functions for each new class.

Many examples of this can be found in poppy/optics.py

Defining a custom optic from an analytic function

The complex phasor of each optic is calculated automatically from that optic’s transmission (i.e. the throughput for the amplitude of the electromagnetic field) and optical path difference (i.e. the propagation delay in the phase of the electromagnetic field). Both of these quantities may vary as a function of position across the optic, and as a function of wavelength.

AnalyticOpticalElement subclasses must implement either or both of the functions get_transmission() and get_opd(). Each takes a Wavefront as its sole argument besides self. All other necessary parameters should be set up as part of the __init__ function defining your optic.


This is new in version 0.5 of poppy; prior versions used a single function getPhasor to handle computing the entire complex phasor in one step including both the transmission and OPD components. Version 0.5 now provides better flexibility and extensibility by allowing the transmission and OPD components to be defined in separate functions, and automatically takes care of combining them to produce the complex phasor behind the scenes.

Example skeleton code:

class myCustomOptic(poppy.AnalyticOpticalElement):
    def __init__(self, *args, **kwargs):
        """ If your optic has adjustible parameters, then save them as attributes here """

    def get_opd(self,wave):
        y, x = self.get_coordinates(wave)
        opd = some_function(x,y, wave.wavelength, self)
        return opd

    def get_transmission(self, wave):
        y, x = self.get_coordinates(wave)
        transmission = other_function(x,y, wave.wavelength, self)
        return transmission

    # behind the scenes poppy  will calculate:
    #    phasor = transmission = np.exp(1.j * 2 * np.pi / wave.wavelength * opd)

Note the use of the self.get_coordinates() helper function, which returns y and x arrays giving the coordinates as appopriate for the sampling of the supplied wave object (by default in units of meters for most optics such as pupil planes, in arcseconds for image plane optics). You can use these coordinates to calculate the transmission and path delay appropriate for your optic. If your optic has wavelength dependent properties, access the wave.wavelength property to determine the the appropriate wavelength; this will be in units of meters.

The get_coordinates() function automatically includes support for offset shifts and rotations for any analytic optic: just add a shift_x, shift_y or rotation attribute for your optic object, and the coordinates will be shifted accordingly. These parameters should be passed to poppy.AnalyticOpticalElement.__init__ via the **kwargs mechanism.

Defining a custom optic from a FITS file

Of course, any arbitrary optic can be represented in discrete form in 2D arrays and then read into poppy using the FITSOpticalElement class.

The transmission array should contain floating point values between 0.0 and 1.0. These represent the local transmission of the electric field amplitude, not the total intensity.

The OPD array should contain floating point numbers (positive and negative) representing a path delay in some physical units. The unit must be specified using the BUNIT keyword; allowed BUNITs are ‘meter’, ‘micron’, ‘nanometer’ and their standard metric abbreviations.

If you are using both an OPD and transmission together to define your optics, the arrays must have the same size.

The spatial or angular scale of these arrays must also be indicated by a FITS header keyword. By default, poppy checks for the keyword PIXSCALE for image plane pixel scale in arcseconds/pixel or PUPLSCAL for pupil plane scale in meters/pixel. However if your FITS file uses some alternate keyword, you can specify that keyword name with the pupilscale= argument in the call to the FITSOpticalElement constructor, i.e.:

myoptic = poppy.FITSOpticalElement(transmission='transfile.fits', opd='opdfile.fits', pupilscale="PIXELSCL")

Lastly if there is no such keyword available, you can specify the numerical scale directly via the same keyword by providing a float instead of a string:

myoptic = poppy.FITSOpticalElement(transmission='transfile.fits', opd='opdfile.fits', pupilscale=0.020)

Creating a custom instrument

POPPY provides an Instrument class to simplify certain types of calculations. For example, the WebbPSF project uses Instrument subclasses to provide selectable filters, pupil masks, and image masks for the instruments on JWST.

Any calculation you can set up with a bare POPPY OpticalSystem can be wrapped with an Instrument to present a friendlier API to end users. The Instrument will hold the selected instrument configuration and calculation options, passing them to a private method _getOpticalSystem() which implementors must override to build the OpticalSystem for the PSF calculation.

The general notion of an Instrument is that it consists of both

  1. An optical system implemented in the usual fashion, optionally with several configurations such as selectable image plane or pupil plane stops or other adjustable properties, and
  2. Some defined spectral bandpass(es) such as selectable filters. If the pysynphot module is available, it will be used to perform careful synthetic photometry of targets with a given spectrum observed in the given bandpass. If pysynphot is not installed, the code will fall back to a much simpler model assuming constant number of counts vs wavelength.

Configurable options such as optical masks and filters are specified as properties of the instrument instance; an appropriate OpticalSystem will be generated when the calcPSF() method is called.

The Instrument is fairly complex, and has a lot of internal submethods used to modularize the calculation and allow subclassing and customization. For developing your own instrument classes, it may be useful to start with the instrument classes in WebbPSF as worked examples.

You will at a minimum want to override the following class methods:

  • _getOpticalSystem
  • _getFilterList
  • _getDefaultNLambda
  • _getDefaultFOV
  • _getFITSHeader

For more complicated systems you may also want to override:

  • _validateConfig
  • _getSynphotBandpass
  • _applyJitter

An Instrument will get its configuration from three places:

  1. The __init__ method of the Instrument subclass

    During __init__, the subclass can set important attributes like pixelscale, add a custom pupil optic and OPD map, and set a default filter. (n.b. The current implementation may not do what you expect if you are accustomed to calling the superclass’ __init__ at the end of your subclass’ __init__ method. Look at the implementation in poppy/instrument.py for guidance.)

  2. The options dictionary attribute on the Instrument subclass

    The options dictionary allows you to set a subset of options that are loosely considered to be independent of the instrument configuration (e.g. filter wheels) and of the particular calculation. This includes offsetting the source from the center of the FOV, shifting the pupil, applying jitter to the final image, or forcing the parity of the final output array.

    Users are free to introduce new options by documenting an option name and retrieving the value at an appropriate point in their implementation of _getOpticalSystem() (to which the options dictionary is passed as keyword argument options).

  3. The calcPSF() method of the Instrument subclass

    For interoperability, it’s not recommended to change the function signature of calcPSF(). However, it is an additional way that users will pass configuration information into the calculation, and a starting point for more involved customization that cannot be achieved by overriding one of the private methods above.

Be warned that the poppy.Instrument API evolved in tandem with WebbPSF, and certain things are subject to change as we extend it to use cases beyond the requirements of WebbPSF.