Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.stecf.org/software/ASTROsoft/DER_SNR/der_snr.pro Дата изменения: Mon Oct 1 17:19:49 2007 Дата индексирования: Mon Oct 1 23:47:12 2012 Кодировка: Поисковые слова: halloween

;*****************************************************************************************
;+
;*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
;-----------------------------------------------------------------------------------------