;***************************************************************************************** ;+ ;*NAME: ; DER_SNR ;*CLASS: ;*CATEGORY: ; spectra ;*AUTHOR: ; Felix Stoehr [ST-ECF] ;*PURPOSE: ; Estimate the derived signal to noise ratio from the flux. ;*CALLING SEQUENCE: ; snr = der_snr(flux) ;*PARAMETERS: ; flux (REQ) (I) (N) (F) ; array containing the flux values (the computation is unit independent) ;*EXAMPLES: ;*SYSTEM VARIABLES USED: ;*INTERACTIVE INPUT: ;*SUBROUTINES CALLED: ; medianvalue like the IDL median but for pair numbers of values in the array ; the average value of the closest values is returned. In the median ; function of IDL, the higher of the two values is returned. ;*FILES USED: ;*SIDE EFFECTS: ;*RESTRICTIONS: ;*NOTES: ; The DER_SNR algorithm is an unbiased estimator describing the spectrum as a whole as long as ; * the noise is uncorrelated in wavelength bins spaced two pixels apart ; * the noise is Normal distributed ; * for large wavelength regions, the signal over the scale of 5 or more pixels can ; be approximated by a straight line ; ; For most spectra, these conditions are met. ; ;*PROCEDURE: ; This function computes the signal to noise ratio DER_SNR following the ; definition set forth by the Spectral Container Working Group of ST-ECF, ; MAST and CADC. ; ; signal = median(flux) ; noise = 1.482602 / sqrt(6) median(abs(2 flux_i - flux_i-2 - flux_i+2)) ; snr = signal / noise ; values with padded zeros are skipped ;*REFERENCES: ; * ST-ECF Newsletter, Issue #42: ; www.spacetelescope.org/about/further_information/newsletters/html/newsletter_42.html ; * Software: ; www.stecf.org/software/ASTROsoft/DER_SNR/ ;*I_HELP nn: ;*MODIFICATION HISTORY: ; 24.05.2007, fst, Initial Import ; 01.01.2007, fst, added more help text, switched the functions, added a missing ' ;- ; ;***************************************************************************************** function medianvalue, xx ;***************************************************************************************** ; Like the IDL median but for pair numbers of values in the array ; the average value of the closest values is returned. In the median ; function of IDL, the higher of the two values is returned. ; ; Felix Stoehr, [ST-ECF] xxx = xx(sort(xx)) n = n_elements(xxx) if ((n mod 2) eq 0) then begin return, 0.5 * (max([0,xxx(n/2-1)])+xxx(n/2)) endif else begin return, xxx((n-1)/2) endelse end ;----------------------------------------------------------------------------------------- ; ;***************************************************************************************** function DER_SNR, flux ;***************************************************************************************** if (n_params(0) eq 0) then begin print, 'usage: der_snr, flux' print, 'flux: array containing the fluxes' print, 'return value: estimated signal to noise ratio' retall endif select = where(flux ne 0.0) selectflux = flux(select) n = n_elements(selectflux)-1 if (n gt 4) then begin signal = medianvalue(selectflux) noise = 0.6052697 * medianvalue(abs(2.0 * selectflux(2:n-2) \$ - selectflux(0:n-4) - selectflux(4:n))) return, signal/noise endif else begin return, 0 endelse end ;-----------------------------------------------------------------------------------------