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FWHM too big??? - Printable Version

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FWHM too big??? - lino - 08-03-2007 11:48

Hi Emmanuel,

I did a very simple test: SEEING_FWHM 0.65 , TRACKERROR_TYPE NONE ed all the abberations set to zero. For a gaussian profile the FWHM and the 50% ee diameter have the same values but I found that the measured FWHM (obtained from the FLUX_RADIUS with -PHOT_FLUXFRAC 0.5) is 0.79". This value seems to me too big, isn't??


Other params used:
SEEING_FWHM 0.65 # FWHM of seeing in arcsec (incl. motion)
AUREOLE_RADIUS 200 # Range covered by aureole (pix) 0=no aureole
AUREOLE_SB 16.0 # SB (mag/arcsec2) at 1' from a 0-mag star
PSF_OVERSAMP 5 # Oversampling factor / final resolution
PSF_MAPSIZE 1024 # PSF mask size (pixels): must be a power of 2
M1_DIAMETER 2.65 # Diameter of the primary mirror (in meters)
M2_DIAMETER 0.98 # Obstruction diam. from the 2nd mirror in m.
WAVELENGTH 0.65 # average wavelength analysed (microns)

RE: FWHM too big??? - Emmanuel Bertin - 08-03-2007 12:15

No this is what you would expect. It is indeed true that the FWHM is equivalent to 2xFLUX_RADIUS for Gaussian profiles. But the SkyMaker PSF is not a Gaussian: it is a realistic model that takes into account the atmospheric MTF (exp(-f^5/3)), the instrumental diffusion (inverse square law in the wings), and convolution by the intrapixel response function (door function). In the end, the profile is far from a Gaussian, but it is realistic: to give you an example, for a 0.60" seeing on real MEGACAM images, 2xFLUX_RADIUS is measured at 0.72".
- Emmanuel.