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# How to acces airy disc in ZOS-API

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Hi,

I’m using ZOS-API in python to analyse PSFs using the Huygens and FFT methods. Is there a way to extract the airy radius and return it to Python?

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Best answer by David.Nguyen 17 January 2023, 17:37

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Thanks David, that works!

Userlevel 7
+2

Hi Charlotte,

I could be wrong but the Airy radius should be:

To solve this formula you need the wavelength (lambda), and the numerical aperture (NA). Both can be retrieved through the ZOSAPI. Specifically, the ISNA (image-space NA) operand gives you the NA of your system through the Merit Function Editor. In the ZOSAPI you can evaluate a single operand without calculating the entire Merit Function using the Merit Function Editor (MFE) method:

``GetOperandValue(MeritOperandType type, int srf, int wave, double Hx, double Hy, double Px, double Py, double Ex, double Ey) ``

So, a piece of code like below should return the Airy radius in Python:

``# Wavenumber in OpticStudiowavenumber = 1# Wavelength [um]wavelength = TheSystem.SystemData.Wavelengths.GetWavelength(wavenumber).Wavelength# Retrieve Merit Function Editor (MFE)mfe = TheSystem.MFE# Evaluate ISNA (Image-Space NA)isna = mfe.GetOperandValue(ZOSAPI.Editors.MFE.MeritOperandType.ISNA, 0, 0, 0, 0, 0, 0, 0, 0)# Calculate Airy radiusairy = 1.22 * wavelength / (2 * isna)# Display the resultprint(airy)``

I hope this helps.

Take care,

David