IRAC Instrument Handbook - IRSA - California Institute of Technology

Point Source Fitting IRAC Images
with a PRF
166
IRAC Instrument Handbook
algorithms to find centroids in the (C)BCDs as you do to centroid the PRF. Note also that some of the
IDL centroiding functions perform poorly with the very undersampled IRAC data at 3.6 and 4.5 microns).
ii) Shift the PRF array (in the example above a shift by 27, -35 resampled pixels would move the desired
PRF to the center of the array).
iii) Extract the point source realization by sampling the PRF at intervals corresponding to native pixel
increments (every 100 oversampled pixels in this example), making sure to pick up the center of the
central pixe l.
In IDL the commands would be:
i) Use rebin on the 128 2 , 5x oversampled PRF to produce the magnified PRF:
magPRF = rebin(PRF,2560,2560).
ii) Re-center the PRF. We find that calculating the first moments is usually a robust way to find the
centroids. Set xmin, xmax and ymin and ymax to approximately the same values in native pixels as you
use to estimate the centroids in your data:
xx = float(lindgen(2560,2560) mod 2560)
yy = float(lindgen(2560,2560)/2560)
xcen =
total(xx[xmin:xmax,ymin:ymax]*magPRF[xmin:xmax,ymin:ymax])/total(magPRF[xmin:xmax,ymin:ym
ax])
ycen =
total(yy[xmin:xmax,ymin:ymax]*magPRF[xmin:xmax,ymin:ymax])/total(magPRF[xmin:xmax,ymin:ym
ax])
xcensh = nx/2 - round(xcen)
ycensh = ny/2 - round(ycen)
cenPRF = shift(magPRF,xcensh,ycensh)
ii) Shift the re-centered PRF to the center of the central PRF pixel and trim to an integer multiple of the
100x oversampling factor such that the central pixel (1280,1280) is moved to (1200,1200), the center of
the trimmed array:
shiftedPRF = shift(cenPRF,27,-35)
phasedPRF = fltarr(2500,2500)
trimmedPRF = shiftedPRF[80:2559,80:2559]
phasedPRF[0:2479,0:2479] = trimmedPRF
iii) Sample the trimmed PRF to produce the point source realization:
PSR = rebin(trimmedPRF,25,25,/SAMPLE)
The center of the zero phase PSR in this example should be IDL pixel (12,12).