;+
; NAME:
;	FOVPIX
; PURPOSE:
;       To determine the pixels in a Polar Camera image that
;       approximate the (circular) field of view of a narrow-field
;	photometer.
; CATEGORY:
;	
; CALLING SEQUENCE:
;	result = FOVPIX( YEAR, MONTH, DAY, CAM, ZA, AZ, HALFANG [, DEGREES=DEGREES])
; INPUTS:
;	YEAR,
;	MONTH,
;	DAY:	Date (in Universal Time) of the image.
;	CAM:	Camera number (0/1).
;	ZA:	Zenith angle of the center of the field of view.
;	AZ:	Azimuth angle of the center of the field of view.
;       HALFANG: Angle between the center and an edge of the field
;		of view.
; OPTIONAL INPUTS:
;	None.
; KEYWORD PARAMETERS:
;       /DEGREES: Set if the angle inputs are expressed in degrees.
;	The default is radians.
; OUTPUTS:
;       The function returns a vector of indices into a standard
;       256x256 Polar Camera image, giving the pixels whose view
;	directions lie within HALFANG of the direction (ZA,AZ).
; OPTIONAL OUTPUTS:
;	None.
; COMMON BLOCKS:
;	None.
; SIDE EFFECTS:
;       A file of image transformation parameters is read to set
;	up the (COL,ROW) -> (AZ,ZA) conversion.
; RESTRICTIONS:
;       No warning is issued if the requested field of view lies
;	outside the imaged portion of the sky.
; PROCEDURE:
;       The transformation parameters for the specified date and
;       camera are read.  Arrays of azimuth and zenith angle are
;       generated for the pixels of the specified camera image,
;       and angular distances from the input direction are calculated.
;       The indices of pixels lying within the specified half-angle
;	are returned.
; EXAMPLE:
;       east30=FOVPIX(1993,12,8,0,30,90,1.0,/DEGREES)   ; returns
;       ; the pixel indices for a 1-deg half-angle field of view
;	; at zenith angle 30 deg toward the East (azimuth 90 deg).
; SEE ALSO:
;	CR2AZ.PRO
; MODIFICATION HISTORY:
; 	Written by:	D P Steele, ISR, 18 Oct 1994
;-

FUNCTION fovpix,year,month,day,cam,za,az,halfang $
	       ,degrees=degrees,debug=debug

IF N_PARAMS() LT 7 THEN BEGIN
    doc_library,'fovpix'
    RETURN,-1
    ENDIF

yr=year

IF KEYWORD_SET(degrees) THEN f=!DTOR ELSE f=1.

zenang=f*za		; convert input angles
azim=f*az		; to radians
hlfang=f*halfang	; for computations

col=REBIN(INDGEN(256),256,256)	; generate column and
row=TRANSPOSE(col)		; row number arrays

cr2az,yr,month,day,cam,0,col,row,camaz,camzen $	; convert to azimuth and
     ,degrees=degrees,bin=2,site=PCsite		; zenith angle arrays

angdist=ACOS(COS(zenang)*COS(camzen*f)+ $	; from desired (AZ,ZA)
	     COS(azim-camaz*f)*SIN(zenang)*SIN(camzen*f))

IF KEYWORD_SET(debug) THEN STOP

RETURN,WHERE(angdist LT hlfang)			; returned indices in FOV

END