Calibration Access and Data Handbook

Procedure

The routine converts a raw input count rate from the instrumental into the natural system. The raw input count rate is the total (background and source) count rate extracted within the aperture defined by the radius used in the photometric calibration analysis. This aperture radius can be queried by CAL_omGetApertureRadius.

The routine corrects the raw count rate for CCD deadtime and detector non-linearity.

The detector non-linearity correction (coincidence loss correction) is not applied when the source extent flag is set. The count rates are corrected for deadtime after the coincidence loss correction was applied.

It is possible to select between two different versions of the linearity (=coincidence loss) correction. The user can select between a theoretical coincidence loss correction and an empirical linearity correction using the optional input parameter linearity_type. The empirical linearity correction is derived by default.
While the theoretical linearity correction is a hard coded formula (see pseudo code below), the empirical linearity applies an additional parameterized correction function to the theoretical formula. The coefficients of the empirical linearity correction function are stored in the extension COINCIDENCE of the tt PHOTTONAT CCF constituent.

The empirical coincidence correction multiplies the theoretical coincidence correction formula with an additional parameterized function f(x), where x is the count to framerate ratio:

$$f(x)=\sum_{i=1}^n a_i\cdot x^{(i-1)}$$ (37)

where

x	is the count to framerate ratio, i.e. cts_detected * frametime
a$_i$	parameters stored in column PLINFUNC of extension COINCIDENCE of
	PhotToNat CCF file
n	number of elements stored in column PLINFUNC of extension COINCIDENCE

;
function f_omraw2nat,eflag,frametime,deadfrac,cts_detected,$
 linearity_type=linearity_type
;
; apply deadtime and coincidence correction to the raw OM count rates 
; 16/12/97 initial version
; 24/02/98 rename variables to make code more intuitive
; 03/02/00 swap coincidence loss and deadtime correction 
;          set eflag to excluded extended sources from coincidence loss
;          correction

; 19/01/01 prepare for empirical linearity correction
; 12/02/01 include empirical linearity correction
;

;INPUT:
; eflag           logical to flag extended sources
; cts_detected    measured photon rate per second
; deadfrac        frametransfer time expressed as fraction of frametime   
; frametime       CCD frametime in units of seconds

; optional INPUT:
; linearity_type  optional integer parameter selecting type of 
;                 linearity correction (0=Igor's empirical, 1=theoretical)

;OUTPUT:
; photons_in      in-falling photon rate (not corrected for phd-fraction)

if eflag then  cts=cts_detected $

else begin

 ; correct for coincidence losses
 if cts_detected lt 1./frametime then begin

    ; calculate theoretical correction, which is needed anyhow
    cts=alog(1.-cts_detected*frametime)/frametime*(-1.)

    if linearity_type eq '0' then begin

      ; use empirical correction!!!
      ; apply additional correction term: Igor's parameterized function
      ; parameters are stored in column PLINFUNC of extension COINCIDENCE
      ; in PhotToNat CCF file
     x=cts_detected*frametime
     f_x=0.D0
     idim=n_elements(a_i)
     for i=0,idim-1 do f_x= f_x + a_i[i]* x^i
     cts=cts*f_x

    endif else if linearity_type eq '1' then begin

     ; no further correction for theoretical curve required
     cts=cts

    endelse
  endif else begin

  ; more counts than frames, either source is extended or very very bright

  cts=infinity ; (i.e. largest possible number*(1-deadfrac) )
  print,'warning: count rate gt frametime, rate set to largest possible number'

 endelse

endelse

   ;correct for frametransfer time
   photons_in=cts/(1.-deadfrac)


return,photons_in
end

A correction for the global loss of detector efficiency is applied optionally using the value of the PHDFRAC keyword in the PhotToNat CCF file. The value stored in PHDFRAC describes the fraction of detected events as measured at the time of the most recent photometric calibration update. The correction for the average detector ageing or change in the threshold setting is achieved by:

$$photons=photons_{in}/(1-eventLoss)*PHDFRAC$$ (38)