Strehl

The most commonly used metric for evaluating the AO correction is the Strehl ratio. The Strehl ratio is defined as the ratio of the peak image intensity from a point source compared to the maximum attainable intensity using an ideal optical system limited only by diffraction over the telescope aperture. The Strehl ratio is very frequently used to perform the quality control of the scientific data obtained with the AO assisted instrumentation.

In PyHDRL this is hdrl.func.Strehl.

Algorithm

The implemented function assumes that the input raw image contains a single object and it is already pre-processed. This means that the instrument signature (bad pixels, CRH detection, bias level and dark current subtraction, flat fielding etc.) has been removed and the contribute from the sky background subtracted.

This function also allows the user to correct a residual local sky background evaluated in an annular region centred on the peak of the (expected single) object PSF, by setting bkg_radius_low and bkg_radius_high (in arcsec).

The PSF is identified and its integrated flux, whose determination is controlled by flux_radius (arcsec), is normalized to 1.

Next the PSF barycentre is computed and used to generate the theoretical normalised PSF. This depends on:

• the telescope pupil characteristics (m1, m2, in metres)

• the wavelength (wavelength, in metres)

• the detector pixel scale on sky (pixel_scale_x, pixel_scale_y, in arcsec)

Then the Strehl ratio is obtained by dividing the maximum intensity of the image PSF by the maximum intensity of the ideal PSF. The associated error is finally computed.

Compute

compute() takes a hdrl.core.Image.

strehl = hdrl.func.Strehl(
    wavelength=1.635e-6,
    m1=5.08 / 2,
    m2=5.08 / 2 * 0.36,
    pixel_scale_x=0.0331932 / 2.0,
    pixel_scale_y=0.0331932 / 2.0,
    flux_radius=1.5,
    bkg_radius_low=1.5,
    bkg_radius_high=2.0,
)
result = strehl.compute(himg)

result is a hdrl.func.StrehlResult with strehl_value, strehl_error, star_x, star_y, star_peak, star_peak_error, star_flux, star_flux_error, star_background, star_background_error, computed_background_error and nbackground_pixels.