In astronomical telescopes, stray light can significantly degrade the detection of weak extraterrestrial signals. Accurate modeling of surface scattering is essential, but can be computationally expensive, especially when considering the low intensity of the desired signals. To mitigate this, importance sampling can be used to more efficiently measure stray light reaching the detector, improving both accuracy and computational efficiency.
Understanding Stray Light in Telescopes
Astronomical telescopes, such as the Maksutov telescope in this example, often face the challenge of stray light caused by scattering off the inside of the barrel. This scattered light can travel through the system and reach the detector, contaminating the weak extraterrestrial signals. For the purposes of this analysis, we will focus on scattering from the barrel, assuming that the barrel is designed to minimize specular reflection and has a Lambertian scattering profile with 100% of the rays scattering.
Ray Tracing and Stray Light Measurement
This example models an off-axis light source, which represents the main source of stray light. Light entering the telescope scatters off the inside surface of the barrel and can ultimately reach the detector.
Refer to the design as below:
If we perform a ray trace, the detector viewer reports the following statistics.
The detector shows that about 4% of the source rays representing 0.53% 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 perform importance sampling on a target sphere located at the second corrector lens. Detector cannot be used 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.
If an object is listed in the importance sampling list, OpticStudio will scatter rays towards a target sphere centered at this object. To account for the scatter profile, OpticStudio will weight the power in these rays so that the flux seen at the object is realistic; the signal to noise ratio is increased. The size of the target sphere and the maximum solid angle it may subtend are specified by the user.
After performing a ray trace, we get the following detector statistics.
Using importance sampling we achieve over 6 times as many rays on the detector and we can also see more structure to the scatter intensity.
Conclusion
By applying importance sampling in OpticStudio, we can significantly enhance the precision of stray light measurements, improving the signal-to-noise ratio and reducing computational time. This technique enables a more accurate evaluation of the stray light contamination, helping optical designers decide whether additional measures, like baffles, are required to meet system specifications.