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; ~dph/libidl/tg_extract.pro
; hack to "extract" a spectrum - distance from zero-order within
; some distance perpendicular from dispersion line
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PRO tg_extract, hml, maxdist, y, z, y0, z0, dd, dxd, list_selected

; hml = 'h' or 'm' or 'l' (or uppercase) = HEG or MEG or LEG;
;       specifies which arm of the 'x' to use for distance from
;       dispersion line calculation.

; maxdist = mm max distance from dispersion line (h or m) to allow.

; y,z = input coordinates in focal plane

;; y0,z0 - the Origin - hopefully the zero-order coord
;;      any point on the line will do, but w/ arbitrary offset in result.

; dd = mm, output vector of photon distances from zero order parallel to
;      dispersion. 

; dxd = mm, output vector of photon distances from dispersion line in
;       perpendicular direction.

; list_selected = optional output vector of indices within input
;                 vectors of selected photons

;; NOTE: To get a list of all photons distances when treated as either all MEG
;; or HEG, use a maxdist larger than a CCD, say 50mm.  Otherwise, the
;; dd,dxd pairs no longer correspond to the input vectors or
;; associated vectors (such as nord).  (added optional output of list_selected)


   IF n_params() EQ 0 THEN BEGIN
      print,'tg_extract, hml, maxdist, y, z, y0, z0, dd, dxd, [list_selected]'
      return
   ENDIF

;;;;;;;;; establish variables and commons ;;;;;;;;;;;;;;;; 
;
;;; these need to be tied to MARX parameters
;
;X_TG = 8635.0d                  ; mm focal distance, from HETG plate
X_TG = 8782.8d                  ; mm focal distance, from HETG plate, XRCF
;X_TG = 5366.55d                  ; mm focal distance, from TOGA plate

                                ; establish HEG and MEG specifics
TG_Period = 2000.95d            ; Angstrom, period
;TG_alpha = -5.9974d*!dtor     ; dispersion direction angle [radians]
;TG_alpha = -5.0d*!dtor     ; dispersion direction angle [radians]
;TG_alpha = -5.1908347d*!dtor     ; dispersion direction angle [radians]
TG_alpha = -5.69d*!dtor     ; dispersion direction angle [radians]

P_inner = TG_Period          ; set common variable
alpha_inner = TG_alpha       ; set common variable

;@MEG.dat
TG_Period = 4000.77d                 ; Angstrom, period
;TG_alpha = 3.9489d*!dtor    ; dispersion direction angle[radians]
;TG_alpha = 4.7411532d*!dtor
TG_alpha = 4.2*!dtor


P_outer = TG_Period          ; set common variable
alpha_outer = TG_alpha       ; set common variable

; LEG.dat
TG_Period = 9903.90
;TG_alpha = -0.9073
TG_alpha = 0.0059298040*!dtor

P_middle = TG_Period          ; set common variable
alpha_middle = TG_alpha       ; set common variable

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; select photons within specified distance perp. from dispersion line:
; (we don't have to select by HRMA shell, since HEG or MEG dispersion
; does this)  alpha_dispersion is the angle of the dispersion line
; relative to the -S array's long axis (the axaf y coordinate)
;
IF strupcase(hml) EQ 'M' THEN  BEGIN
   alpha_dispersion = alpha_outer
ENDIF ELSE IF strupcase(hml) EQ 'H' THEN BEGIN
   alpha_dispersion = alpha_inner
ENDIF ELSE IF strupcase(hml) EQ 'L' THEN BEGIN 
   alpha_dispersion = alpha_middle
ENDIF 

      
; the dispersion line by definition goes through (0,0) - NOT.
;

   dxd=-(y-y0)*sin(alpha_dispersion)+(z-z0)*cos(alpha_dispersion)
   list_selected = where(abs(dxd) LT maxdist)
   
; again, since we have defined the origin at (0,0) as the position of
; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;   NOT!
; the zero-order (not true for flight in general when there is aspect
; motion or an offset in pointing, but let's not consider it now),
; then the distance of the point from the zero order parallel to the
; dispersion is:
   

   dd = sqrt((float(y)-y0)^2+(float(z)-z0)^2 - float(dxd)^2)
;   dd = sqrt((float(y)-y0)^2+(float(z)-z0)^2 + float(dxd)^2)

; adjust sign:
  ;;; (may not want to do this for toga hacks.
   
   neg_ords =  where((y-y0) LT 0)
   IF (size(neg_ords))(0) NE 0 THEN dd(neg_ords) = -dd(neg_ords)
   

;  select photons only within the specified distance:
   
   dxd = dxd(list_selected)
   dd = dd(list_selected)
   
END




   ; A (crude) diagram of the geometry:
   ;
   ;                                                                 o P4
   ;                              o(y,z)=P1 = photon position
   ;                               \
   ;                                \
   ;                                 \  
   ;                                  o P2= intersection of perp from
   ;                                        P1 to dispersion line
   ;                                 

   ;
   ;
   ; o P0=(0,0) = origin == zero-order position;                     o P3
   ;                         dispersion direction is from P0 to P2
   ;
   ; X = P0_P1
   ; dxd = P1_P2
   ; dd = P0_P2
   ; P0_P3 = y-axis of detector array
   ; P3_P4 = z-direction

   ; X^2 = y^2 + z^2
   ; X^2 + dxd^2 = dd^2