Grism S/N: Measured vs. Model

This plot shows the S/N per resolution element (about 25 to 28 Angstroms) in the NUV band for various exposure times and NUV direct magnitudes: s2n_per_res_elem.ps .

Here are more plots of the S/N per resolution element for NUV and FUV .


These plots show the median (formal) signal-to-noise per 3.5 Angstrom pixel for the NUV bandpass vs. an idealized model. The values are taken from the "median_s2n" column in the -xg-gsp.fits files.

median_s2n_06466.ps (ELAISS1_07: 29,000 seconds)
median_s2n_06451.ps (ELAISS1_00: 87,000 seconds)
median_s2n_06051.ps (CDFS_00: 184,000 seconds)



Parameters:

nuv_zpmag  = 20.082375;     /* mag = ( -2.5 * log10(flux) ) + nuv_zpmag */

exptime    = 29103.;        /* ELAISS1_07 */

exptime    = 86557.;        /* ELAISS1_00 */

exptime    = 154000.;       /* CDFS_00 */

respdip    = 0.75;          /* Average NUV response dip (average effective exposure). */

backrate   = 0.00128;       /* Average NUV grism background photons per second per square arcsecond */
                            /* (includes response dip and grism EA) */

eacorr     = 0.60;          /* Effective Area grism(NUV 1st)-to-direct ratio */

aperfrac   = 0.90;          /* Aperture fraction. */

objwidth   = 15.0;          /* Object width in arcseconds. */

psf_fwhm   = 7.0;           /* Average NUV PSF FWHM in arcseconds (1 res.elem.). */

disp       = 4.0;           /* Average NUV Angstroms per arcsecond. */

wave_range = 1250.;         /* NUV order 1 band wavelength range in Angstroms. */




Computed values:

nuv_direct = pow(10.,((mag - nuv_zpmag)/(-2.5)));                 /* true photons per second */

nuv        = nuv_direct * aperfrac * eacorr * respdip * exptime;  /* photons NUV grism 1st order */

signal     = nuv * 3.5 / wave_range;                              /* photons per 3.5 Angstroms. */

background = backrate * exptime * objwidth * (3.5 / disp);        /* background per 3.5 Angstroms. */

noise      = sqrt(( signal + background ));                       /* poisson noise. */

s2n        = signal / noise;


........Compute S/N per resolution element..........

res_elem_per_nuvband = wave_range / ( psf_fwhm * disp ); /* Res. elements in nuv 1st order band */

signal     = ( nuv / res_elem_per_nuvband );             /* photons per res.elem. */

background = backrate * exptime * objwidth * psf_fwhm;   /* background photons per res.elem. */

noise      = sqrt(( signal + background ));              / * poisson noise. */

s2n        = signal / noise;