IRAC Instrument Handbook - IRSA - California Institute of Technology

Point Source Fitting IRAC Images
with a PRF
155
IRAC Instrument Handbook
The IRAC PRFs are centered relative to the optical axis, so they are slightly off center in array
coordinates due to array distortion. APEX assumes that the PRF is centered on its array, so to use the
PRFs with APEX requires them to be re-centered. APEX also requires odd-valued axes.
APEX performs PRF fitting by varying the position and flux of a source using a modified simplex
technique (see the APEX manual). However, for IRAC data, particularly in channels 1 and 2, where the
PRF is undersampled, the default 5x sampling of the PRF is insufficient to obtain a sufficiently accurate
position for fitting.
Therefore the following transformations were applied to the PRFs:
i) The PRFs were magnified (using linear interpolation) by a factor of 20 (so the resultant PRF
sampling is x100).
ii) The last row and column were removed to give odd-valued axes.
iii) The PRF was recentered on a first-moment centroid measured using the array values within a
250 (resampled) pixel border.
iv) The PRF was zeroed out in a 50 (resampled) pixel border (to avoid wrapping problems).
v) Information describing the PRFs and their modifications was added to the headers.
C.3 Results of Tests with PRF fitting
C.3.1 Test on Calibration Stars
One sample observation (AOR) was selected for each of the nine brightest IRAC calibration stars (Reach
et al. 2005, [23]). The selected AORs were from 2005 June 05 to 2006 September. Photometry was
performed on the five BCDs in each AOR and the results averaged. (C)BCD uncertainties and imasks
were used. The pipeline versions were S14.0−S14.4. The central PRF, modified for APEX use as
described above, was used as the stars were close to the center of the array in each of the images.
APEX_1frame was used with current default parameters in the namelists provided in the cdf/ subdirectory
of the MOPEX distribution, e.g., apex_1frame_I1.nl etc, with one change. A Normalization
Radius for the PRF is needed to correspond to the IRAC calibration radius of 10 pixels. This was placed
in the parameter block for sourcestimate: Normalization_Radius = 1000 (since it is in units of PRF pixels,
and the sampling is 100x).
We performed aperture photometry using a 10 pixel (calibration) radius for IRAC channels 1 and 2, and a
3 pixel radius for IRAC channels 3 and 4, and a 12−20 pixel background annulus for all. Aperture
corrections from this Handbook were applied to IRAC channels 3 and 4. The use of smaller apertures at
longer wavelengths is not critical but reduces the effect of background noise. No aperture corrections
were needed for IRAC channels 1 and 2 for this aperture/annulus combination as it is used to define the
flux calibration. The IRAC channel 1 aperture photometry was divided by the empirical pixel-phase flux
correction from Chapter 4 in this Handbook: