Can’t you just use a Lambertian source with uniform irradiance that has the same shape and size as your entrance aperture (say a Source Ellipse), located at the aperture, shooting rays directly into your optical system?  Following Smith, if the source has a uniform radiance of N (watts per unit area per steradian) and an area of A, then the power associated with the source is simply pi*N*A (watts).  You can start with the night sky radiance in whatever units you like (or luminance, converted into radiance), convert this to say watts per cm^2 per steradian, then take your entrance aperture area in cm^2, and easily find the required source power.

Here’s a snippet from Smith, Modern Optical Engineering:

Regards,

Jeff

Hi and thanks, Jeff.  My question involves the relationship between the number of Lambertian analysis rays and the power collected at the focal plane. If I assign 1 watt to 1E6 rays, shouldn't each ray carry 1 microwatt of power? That will deliver P watts of Lambertian power to the focal plane. If I increase the number of rays to 1E7 but keep the power at 1 watt, shouldn't each ray now carry 0.1 microwatt of power?  And shouldn't there still only be approximately P watts collected at the sensor due to the Lambertian source?  I'm not seeing that happen; the collected power seems to be connected to the number of rays.  Have you observed this as well?

The total detected power shouldn’t depend on the number of source rays unless you happen to have the Minimum Absolute Ray Intensity parameter set to a value that causes weaker rays to terminate before reaching the detector.  A value of 0 turns this intensity threshold feature off.

If that’s not the problem, then I would need to see an example file to take a closer look.