![]() Generally speaking, the Spot Diagram is useful if the aberrations are large compared to the diffraction limited performance of the system. To compute a more accurate PSF both on and off axis, a consideration of diffraction is required. This indicates the Spot Diagram is a useful and reasonable indicator of performance off axis only. On-axis, the spot is much smaller than the Airy Disk, while off axis the spot is much larger than the Airy Disk. Now the Spot Diagram will indicate the size of the Airy Disk relative to the geometric spot distribution: Is this an accurate representation of the optical system performance? To answer this question for the Spot Diagram results, we need to compare the spot distribution to the diffraction limited response.Ī quick way to compare the geometric aberrations to the diffraction limit is to add an Airy Disk reference ellipse to the Spot Diagram. Open the Settings and select Show Airy Disk. On-axis, however, the Spot Diagram predicts perfect imagery. The off axis geometric PSF clearly shows the coma and astigmatism of the system. The Spot Diagram is very effective at showing the effects of the geometric, or ray aberrations of the telescope. There is no interaction or interference between the rays. Note the Spot Diagram is a collection of points, with each point representing a single ray. The Spot Diagram for two field points, one on-axis and the other at an angle of 2 degrees, is shown below. Here is what the optical system looks like: This system is a simplified Newtonian telescope, and the included sample file is PSF_Newtonian.ZMX. The sample optical system used here is a single parabolic F/5 mirror with a focal length of 50 mm. In this way, the Spot Diagram is a geometric PSF. This feature launches many rays from a single source point in object space, traces all the rays through the optical system, and plots the (x, y) coordinates of all the rays relative to some common reference. One of the most basic analysis features in OpticStudio is the Spot Diagram. This article will discuss the basic theory and provide some guidance as to the proper use of each type of PSF. ![]() OpticStudio has three basic types of PSF calculations: a geometric (no diffraction) Spot Diagram, the diffraction-based FFT and Huygens PSF. Second, diffraction effects will also spread the image, even in a system that has no aberrations. First, aberrations in the system will spread the image over a finite area. (A telescope forming an image of a distant star is a good example.) Although the source may be a point, the image is not. The Point Spread Function (PSF) of an optical system is the irradiance distribution that results from a single point source. The merits of each tool will be discussed, as will useful feature settings for the most accurate analysis. The analysis features used are the Spot Diagtam, the FFT PSF, and the Huygens PSF. This article discusses how the Point Spread Function is modelled and interpreted in OpticStudio. ![]()
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