Help for Photometry Widget

The idp3_phot widget computes photometric values for a circular 
or square aperture designated in the ROI.  These values include
the total flux in the aperture, the number of pixels in the aperture,
the median flux, and standard deviation of the mean, as well as 
sharpness and target corrected flux.  The sharpness is defined as
the sum of the squares of the flux divided by the sum of the flux
squared.  The target corrected flux is defined as 
  (total background subtracted flux * (1. + (bf * number of pixels))) / ac
where bf is the PSF background fraction of flux per pixel and ac is the
aperture correction.

For photometry on an object in a single image, a grid of objects
in a single image, or an object in a stack of images the user must
specify the following:

  Aperture Shape - circular or square
  Sharpness - compute:yes/no
  Center - may be specified in x and y coordinates or right
	   ascension and declination.  May load the centroid
	   solution for the reference image or the target RA
	   and Dec.  RA and Dec positions are converted to
	   X and Y using the WCS of the reference image.
  Target Radius - radius of target aperture in pixels
  Background Inner Radius - inner radius of background in pixels
  Background Outer Radius - outer radius of background in pixels
  Median Threshold - the percentage of a border aperture pixel  
		     required in order for its flux to contribute
		     to the median computation.
  Aperture Correction - aperture correction applied for computation
			of the true flux
  PSF Background Fraction of Flux per Pixel - the fraction of background
		     flux per pixel in the PSF, used in the computation
		     of the true flux.

  The user may define the above parameters by loading an Aperture
  text file.  See example file below. 

  Mark Aperture will cause the circular radius or square box of the 
  target aperture to be drawn in the ROI.

  Compute is used to compute the following output values at the bottom of
  the widget if all of the above parameters are defined:
    target total flux, number of pixels used, median flux, and RMS 
    background subtracted target total flux, pixels, median flux, and RMS
    background total flux, pixels, median flux, and RMS
    target corrected flux 
    the sharpness and the background subtracted sharpness.

  Save writes the above output with input parameters to the file specified
  in Output File.  Any comment in the Comment field is added to the text
  record.

  Computations for multiple objects in the ROI may be performed with Grid
  or one object in multiple images of the ROI with Stack. When Grid is
  selected the user is prompted for the name of a text file identifying
  the object coordinates.  See the sample GRID text file below.

  Additional information regarding the location of the photometric center
  for each image in the stack is needed for the Stack option.  The options
  are:

    0.  Center = the x,y location of the reference centroid.  This assumes
	that all of the images are aligned.

    1.  Center = the x,y location of each of the image centroids.  

    2.  Center = the Right Ascension and Declination from the widget to 
	compute the x,y position in each of the images in the stack using
	their defined WCS.

    3.  Center = the Right Ascension and Declination (RA_TARG,DEC_TARG 
	for HST or RA_REF,DEC_REF for Spitzer data) to determine the x,y 
	location using the WCS.

    4.  Center = the x,y position defined in the widget.


Sample text file for aperture photometry parameters:

   xcen       = 38.3862            ; aperture x center
   ycen       = 79.5495            ; aperture y center
   racen      = '16:00:00.027'     ; aperture right ascension
   deccen     = '44:59:58.84'      ; aperture declination - cannot supply both
				   ; pixel center and ra/dec center
   tradius    = 2.5                ; target radius
   biradius   = 3.5                ; background inner radius
   boradius   = 4.5                ; background outer radius
   sharp      = 1                  ; compute sharpness - 0=no, 1=yes
   ap_corr    = 0.99               ; aperture correction
   bkg_fract  = 0.015              ; PSF Background Fraction of Flux per Pixel
   med_thresh = 0.90               ; threshold for including pixels in median
   cntr_def   = 0                  ; definition of the photometric center -
				   ; used for stack photmetry, see values above
   comment    = 'this is a test'   ; comment associated with this data
   outname    = 'test.phot'        ; name of output file


Sample text file for defining grid parameters, single line for each object
where position (xc/yc or ra/dec) must be defined but target radius (tr),
bir (background inner radius), and bor (background outer radius) are optional:

 xc=120.926   yc=129.891  tr=2.1  bir=3.5  bor=4.8
 xc=183.856   yc=111.273  tr=1.9  bir=2.7  bor=3.9
 xc=232.980   yc=54.078   tr=2.3  bir=4.1  bor=6.0
 xc=192.007   yc=25.755