Detailed Description

This module provides functions for computing the Strehl ratio of point spread functions (PSF) from IFU observations, typically using PSF calibrator stars.

What is the Strehl Ratio?

The Strehl ratio is a measure of optical system quality, defined as the ratio of the peak intensity of the actual PSF to the peak intensity of a theoretical diffraction-limited PSF. It ranges from 0 (poor) to 1 (perfect diffraction limit).

For adaptive optics (AO) systems like ERIS, the Strehl ratio is a key performance metric indicating:

• AO correction quality
• Atmospheric turbulence effects
• Overall image sharpness

Computation Method:

The Strehl ratio computation uses the HDRL library and follows these steps:

1. Load collapsed cube image (wavelength-averaged) from:
   • PSF_CALIBRATOR observations (preferred)
   • STD star observations (alternative)
   • STD_FLUX observations (alternative)
2. Determine measurement parameters from FWHM QC parameters:
   • flux_radius: aperture for stellar flux integration (default: 3σ of PSF)
   • bkg_radius_low: inner radius for background annulus
   • bkg_radius_high: outer radius for background annulus
3. Locate PSF center within image (avoiding edges)
4. Compute Strehl ratio using HDRL:
   • Integrates flux within aperture
   • Estimates background from annulus
   • Compares to theoretical Airy disk for given telescope parameters
5. Save QC parameters:
   • ESO QC STREHL: Strehl ratio value
   • ESO QC STREHL ERROR: Uncertainty estimate

Telescope Parameters:

• Primary mirror radius (M1): 8.15 m (VLT)
• Secondary mirror radius (M2): 0.9 m
• Wavelength: Central wavelength of observation band
• Pixel scale: Depends on SPIFFIER pre-optics (25, 100, or 250 mas)

Adaptive Parameter Selection:

The module automatically adjusts measurement radii if:

• PSF is near image edge (reduces radii to fit)
• FWHM information is available (uses 3σ rule)
• User provides -1 for parameters (triggers automatic mode)

Note

This module depends on the HDRL (High-level Data Reduction Library)

Computation requires collapsed cube images (mean or median along wavelength)

Error images are generated using detector model if not provided