[Last Change: 24 Feb 2012 (rev 8)]

Specifying limits on the number of rays

trace-nest has a number of different methods of controlling the number of rays which traverse the optical system. Its fundamental limitation is that it can only truly control the number of rays which enter the telescope, not the number which exit.

Specifying the input number of rays

trace-nest limits the number of generated rays in one of two ways:

• It can generate rays for a span of time; the number of rays is determined by the source fluxes; or
• It can generate a specified density of rays at the entrance aperture

The limit_type parameter is used to choose which of these ways is used, and can take one of the following possible values:

ksec	kiloseconds
sec	seconds
r/cm2	rays per square centimeter
r/mm2	rays per square millimeter

The limit parameter in turn specifies the numerical value of the limit. Source fluxes are specified during source definition, which is covered elsewhere.

Specifying the output number of photons

The number of output photons may be set by the throttle parameter.

The distinction between a ray and a photon is subtle but important. SAOTrace assigns a weight to each ray corresponding to its probability of reaching the exit aperture (based upon the probability of reflecting off of the optics). A photon is a ray with unit weight. For most users it's photons which matter and SAOTrace can be directed to randomly select rays based upon their weight to create photons The suggested base trace-nest settings for the throttle parameter do this without specifying a limit on the number of output photons.

Photons generated in excess of the throttle limit are discarded, but if fewer photons then the specified limit reach the exit aperture of the telescope there's no way to generate extra rays to reach the specified limit. So, the best way to generate the expected amount is to iterate; start small, determine the system throughput, and adjust the input limit accordingly. The value of the random number seed will slightly affect the number of output photons.