IRAC Instrument Handbook - IRSA - California Institute of Technology

IRAC Instrument Handbook
Observations of the stars Vega, Epsilon Eridani, Fomalhaut, Epsilon Indi and Sirius were used in the
construction of the extended portion of the PRF. Each star was observed with a sequence of 12 sec IRAC
full frames, using a 12-point Reuleaux dither pattern with repeats to obtain the required total integration
time (the stars were typically observed for 20 − 60 minutes during each epoch). The images were aligned,
rescaled to the observation of Vega, and then averaged together with a sigma-clipping algorithm to reject
background stars.
The core HDR PRFs were aligned and rescaled to the extended portion of the PRF by matching their
overlapping areas. The alignment was done at best to an accuracy of ~ 0.1 arcsec. The rescaling was made
by forcing the cores to have the same flux density, that of Vega, within a 10 native IRAC pixel radius
aperture. The stitching was made using a mask with a smooth 1/r transition zone, 2.4 arcseconds wide,
between the core (contributing where the extended portion PRF data were missing due to saturation
cutoff), and the extended portion of the PRF. The merged PRFs were then cropped to a final 5.1 arcmin x
5.1 arcmin size, and a pedestal level was removed in order to have a surface brightness as close as
possible to zero in the corners of the images.
4.7.3 Point Source Fitting Photometry
The PRF is not an oversampled representation of a point source. Rather it is a map of the appearance of a
point source imaged by the detector array at a sampling of pixel phases (positions of the source centroid
relative to the pixel center). For that reason, performing aperture photometry directly on the PRF is not
strictly correct.
IRAC provides diffraction-limited imaging internally. The image quality is limited primarily by the
Spitzer telescope. The core PRFs are provided for 25 positions in a 5x5 grid on the array for each channel.
Interpolating to the nearest position is needed. The extended PRFs have been created at the center of the
array. Therefore these PRFs degrade as a function of distance from the center. The PRFs will vary with
position on the array, including, but not limited to, the relative position of the optical ghosts in channels 1
and 2, and the diffraction spikes in all channels.
A step-by-step description of IRAC PRF-fitting photometry is given in Appendix C.
4.8 Calculation of IRAC Zmags
Some software packages, such as IRAF's "phot" task, require specifying "zmag". For IRAC data, you
need to know the pixel size of the IRAC image being analyzed in order to convert surface brightness to
flux density. The zmag can be evaluated from 2.5xlog(F0/C), where F0 is the zero magnitude flux density
in Jy for the relevant channel, tabulated in Table 4.1, and C is the conversion factor from MJy/sr to
µJy/pixel, e.g., 8.461595 for 0.6″ x 0.6″ pixels (the value of C will be different depending on the pixel
size).
Calibration 50 Calculation of IRAC Zmags