Frequently asked questions (FAQ) about the Photon Simulator (PhoSim):


Why photons?


Light is composed of particles called photons.  By simulating light as photons, we are properly representing the physics of light.  Then, we can deal with the physics of photons in a number of different ways appropriate for the situation (raytracing, diffraction, quantum mechanical interaction).  This ultimately allows the code to predict accurately the response of light to the system in the same way it occurs in Nature.

Why a simulator?


A simulator is a powerful tool that lets the user obtain synthetic data with perfect knowledge of the inputs.  This allows algorithms to be tested or experiments to be done that are impossible in real life.  Having a simulator along side traditional analysis techniques can be quite helpful.

Why a Monte Carlo?


A Monte Carlo is a set of algorithms that use probabilistic sampling to arrive at computationally efficient answer.  In this case, the Monte Carlo is over the angular, time, and wavelength-dependent light pattern incident on an optical system.  This is the most computationally efficient simulation method, and it also makes the calculation easily amenable to parallelism.

Can PhoSim capture the complexity of images using this approach?

Yes.  An ab initio physics approach can reproduce most known features in images.  PhoSim has reproduced:  1) the typical ellipticity from atmospheric turbulence by comparing with existing telescope short exposure data, 2) the typical ellipticity de-correlation patterns due to the different wedges of atmospheric turbulence, 3) the typical astrometric jitter on small spatial scales, 4) the spot diagrams produced through optical designs from alternative benchmark raytrace codes, 5) charge diffusion patterns similar to actual laboratory measurements, 6) PSF patterns with self-consistent opto-mechanical physics of a telescope with an active optic system, 7) astrometry, PSF, and flat patterns due to doping variations in sensors matching measured images, 8) intensity-dependent PSF and sub-linear flat variation due to self-consistent electron physics.  See the Technical page for more details.


Is PhoSim too complicated for my application?


Most likely, no.  PhoSim can simulate complex interactions in the atmosphere and optical and sensor instrumentation, but can also simulate a very basic optical system in an idealized manner.  PhoSim is highly modular and the user can control the complexity.  Consider the most simplest method of constructing an image without using a photon Monte Carlo.  To produce even the simplest image with one source, you would need to know:  1) the intensity of a source, 2) the background level, 3) the angular resolution or blurring, 4) the source position on the image, and 5) the number of pixels.  With PhoSim simulating a source using the default “generic” telescope,  the input would be defined by the source properties:  1) angular position, and 2) spectral energy distribution and the physical description of the “generic” optical system:  1) aperture size, 2) a filter curve (at least a central wavelength and a width), 3) the sensor thickness, 4) the focal length, 5) pixel size, and 6) the number of pixels.  Thus, the input complexity is similar, but the PhoSim simulation is physically meaningful.


Isn’t PhoSim very slow?


No.  PhoSim is faster than other codes except those that only use basic gaussian image convolution methods.  There are a variety of computational innovations that have been made to achieve this including numerical algorithms and multi-threading.


Is PhoSim just one possible model of many for very complex phenomena?


Not really.  The goal of PhoSim is to have the entire code based on pure ab initio physics of the response of photons (and eventually electrons) to the system.  The numerical implementation of this is carefully studied, continually improved, and extensively tested.  The “model” part of PhoSim is the instrument and site characteristics.  This is actually entirely part of the input files to PhoSim and is therefore exposed to the user.      There are some parts of PhoSim in the past that resembled a model for a specific effect, but for the most part those aspects of PhoSim have been removed and replaced with pure ab initio physics.  There are assumptions and simplifications, however, that are explained in the technical references.


Is PhoSim hard to use?


Hopefully not.  There is a variety of documentation including the tutorials designed to onboard a new user quickly.  The installation procedure should only take a few minutes.

How can I simulate my favorite astronomical source?

Many different ways.  The interface is quite flexible in allowing generic sources, truth images, moving sources, complex spectral energy distributions, parameterized spatial models, etc.   See the tutorials for more information.

Can PhoSim be used for simulations not related to astronomy?