Name
strehl - Strehl ratio computation
Synopsis
strehl [options] <cubes...>
Description
strehl computes the Strehl ratio for every plane in every cube given on
command line. See ALGORITHM to know how it is computed. strehl needs
to know some parameters about the mirror and observation wavelength to
generate the ideal PSF. Defaults for these parameters are taken for
NACO, use the options to feed other values. strehl accepts jokers on
command line :
> strehl *.fits
ATTENTION! The input image must be square (same size in x and y), and
this size shall be a power of 2! This to allow a correct PSF genera-
tion.
Algorithm
For each image in each cube (candidate image), the following algorithm
is applied:
1. Generate a PSF for a given telescope configuration and observation
wavelength.
2. Compute the PSF flux and the PSF peak.
3. Detect the brightest star (if not provided on the command line).
4. Compute the star peak and the star flux. This step requires a very
precise background estimation. A tiny error on the background can have
a strong effect on the computed strehl.
5. Strehl = (star_peak/star_flux) / (psf_peak/pasf_flux)
6. Compute the error on the Strehl
Attention: the theoretical PSF is generated only once per call! When
giving several cubes on command-line, it is up to the user to check
that the same parameters are valid for all files (check the wave-
length).
Options
-g An ideal PSF is generated for Strehl ratio computation. Using
this option, a FITS file named ’psf1.fits’ is generated in the
current directory, containing the generated ideal PSF (of Strehl
ratio equal to 1). Use it to cross-check that the generated PSF
fits your computation.
-p diameter
Primary mirror diameter in meters. Default to 8. This value is
actually 8.2 meters, corrected to take into account the central
obscuration ratio.
-d diameter
Secondary mirror diameter in meters. Default to 1.1.
-l ’lambda0 dlambda’
Do not forget the simple quotes around the 2 values. First value
is the central wavelength of observation. Second value is the
filter width. Values are given in microns.
Defaults to 5 and 0.1.
-s pixelscale
Pixel Scale in arc seconds. Defaults to 0.05.
-P ’x y’
x and y are the pixel position of the star used to compute the
strehl. If not provided, an automatic bright star detection pro-
cedure is applied.
-r radius
radius is the star radius in arcsecond used to compute the star
flux. default is 2.0 arcsec.
-R ’rad_in rad_out’
These radii define a ring around the star. This ring is used to
compute the background. If these values are not provided, a dif-
ferent background estimation method will be used. As the back-
ground estimation accuracy is an issue in the strehl ration com-
putation, it is far better to use this option.
-b bg Used to provide the backgound value.
-S size
Size in pixels of the square used to compute the PSF flux.
Default is 64 pixels. -T tel Used to define defaults for sup-
ported telescopes. Supportes ones are "3.60" and "ut4". Default
is "ut4".
Example
To compute the strehl ratio over 10 cubes named S001.fits to S010.fits,
on an 8 meter telescope with a 1.5 meter secondary mirror, a central
wavelength of 5.0 um and filter width of 0.1 um, a pixel scale of 0.05,
use:
strehl -p 8 -d 1.5 -l ’5 0.1’ -s 0.05 S*.fits
Files
Files shall all comply with FITS format
See Also
stcube