The astronomical telescope is one of the most well noted optical systems requiring stray light analysis. The reason for this is because the desired signal (extraterrestrial sources) is often so low that any noise in the form of stray light is very detrimental.

In this example, we will measure the amount of stray light that scatters off the inside of the telescope barrel, ultimately reaching the detector. We will see that by using importance sampling, we increase the number of rays hitting the detector, and get a more accurate measurement of stray light reaching the detector. Open the file “IS.zmx” found on the last page of this article.



The file models a Maksutov telescope with an off-axis light source representing the main source of noise in the system. The light enters the telescope and reflects/scatters from the surface of the barrel. Note: some scattering would also take place at the optical surfaces, but we will concern ourselves with the barrel’s contribution for the purposes of this analysis. We model the telescope barrel with a lambertian scattering profile with 100% of the rays scattering (assume the barrel is machined to stifle specular reflection). If we perform a ray trace, the detector viewer reports the following statistics.



The detector shows that about 4% of the source rays representing 0.6% of the energy actually make it to the detector. To accurately measure the power on the detector from scattered light, we want as many rays hitting the detector as possible. This is where importance sampling plays a useful role.

We will importance sample a target sphere located at the second corrector lens; we cannot use the detector because it doesn’t receive light directly from any scatter points. Note: the size parameter defines the radius of the target sphere. This is intentionally set slightly larger than the primary mirror aperture to ensure all rays are included that hit the detector. We will leave the subtended solid angle limit to the default value. Enter data in the Scatter To tab as shown.



After performing a ray trace, we get the following detector statistics.



Using importance sampling we achieve over twice as many rays on the detector and we can also see more structure to the scatter intensity. The total power at the detector has also increased indicating that our previous scatter model was under-sampled.

Now that we have measured the power reaching the detector, we could determine whether further measures are needed to stifle stray light. If the signal to noise ratio was still high enough to suit our purposes, we may decide to avoid the time and cost of implementing baffles in the telescope. If it is determined that further noise suppression is required to meet the system specifications, baffles might need to be instituted.