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IRAC: PSF/PRF |
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Point source fitting to IRAC data has proven problematic as the point spread function (PSF) is undersampled, and, in channels 1 and 2, there is a significant variation in sensitivity within pixels. To deal with these problems, we have developed Point Response Functions (PRFs) for IRAC. A PRF is a table (not an image, though for convenience it is stored as a 2D FITS image file) which combines the information on the PSF, the detector sampling and the intrapixel sensitivity variation. By sampling this table at regular intervals corresponding to single detector pixel increments, an estimate of the detector point source response can be obtained for a source centered at any given subpixel position.
In-flight PRF FITS files (August 2008)
The IRAC point response functions (PRFs) at 3.6, 4.5, 5.8 and 8.0 microns. The PRFs were generated from models refined with in-flight calibration test data involving a bright calibration star observed at several epochs. Central PRFs for each channel are shown above with a logarithmic scaling to help display the entire dynamic range. The PRFs are shown as they appear with 1/5th the native IRAC pixel sampling of 1.2 arcseconds to highlight the core structure. Core PRFsThe FITS files of the core PRFs can be obtained by clicking on the links listed below. These core PRFs can be used for PRF-fitting photometry and source extraction in BCDs for all but the brightest sources. The IRAC IST still recommends aperture photometry in all instances except in crowded fields and regions with a strongly varying background.(includes details of the creation, testing, and proper use of the PRFs supplied on this page in IRAC images). The PRFs are provided in two different samplings, 1/5th and 1/100th native pixels. The 1/100th native pixel sampling have been created by interpolating the 1/5th sampled PRFs onto a finer grid. These PRFs are designed to work with the SSC-provided photometry extraction software APEX. The 1/5th pixel sampling versions are the originally derived versions and are appropriate for use with custom prf-fitting software, but not APEX. For both versions of sampling, the PRFs are provided for 25 positions in a 5x5 grid upon the array for each channel. The PRFs are normalized such that the flux is unity in a 10 arcsecond radius annulus around each point source with the zero pixel phase instance (centered on a pixel).
 The extended IRAC point response functions (PRFs) at 3.6, 4.5, 5.8 and 8.0 microns with high signal to noise out to the edge of the array. The extended PRFs are displayed with a logarithmic scaling to reveal the whole dynamic range.
Extended PRFsThe FITS files of the above images can be obtained by clicking on the links listed below. In order to gain high signal-to-noise out to the edge of the arrays, PRFs were generated from a combination of on-board calibration and science observations of stars with different brightness, joined together to produce extended high dynamic range (HDR) observational PRFs. These PRFs have two main components: a core HDR PRF created by the observations of a reference star, and the extended region from observations of a set of bright stars that saturated the IRAC array. They can be used to perform source extraction and PRF-fitting photometry of bright, highly saturated stars with extended wings. The core of the extended PRF was generated using the prf_estimate module of Mopex which has been shown to be inadequate for making good PRFs for IRAC. As a result, the extended PRF should not be used for PRF-fitting photometry and source extraction of non-saturated point sources. Instead, the above core PRF is more appropriate. Also, note that the detailed structure of the center of saturated sources fitted using the extended PRF will not be correct in detail.These extended HDR PRFs have a pixel size of 0.2 IRAC pixels, or ~0.24 arcsec. The size of each PRF image is 1281x1281 pixels, covering an area of ~5.1 arcmin x 5.1 arcmin. The PRFs are centered within each image. The PRFs are calibrated in MJy/sr. The PRFs represent an unsaturated, very high S/N image of Vega, and the flux density contained within a 10 native IRAC pixel aperture radius (50 HDR PRF pixels), with the sky level estimated in a radial annulus from 10 to 20 native IRAC pixels, is equal to the flux density of Vega. The pedestal level of each image is set to zero in the corners of each PRF. To produce the core portion of the HDR PRF, 300 HDR observations of a calibration star were obtained during three separate epochs, each observation consisting of short exposures (0.6 sec/1.2 sec) and long exposures (12 sec/30 sec). The HDR PRFs were generated by first combining short-exposure frames and long-exposure frames separately. The short frames enabled the cores to be constructed without a saturation problem, while the long exposures allowed the construction of a higher signal-to-noise PRF in the wings out to 15 arcseconds. The assembly required the replacement of any saturated areas in the long-exposure frames with unsaturated data from the same pixel area of the short-exposure frames. It also required the replacement of a few pixels in the long-exposure frames by the corresponding pixels in the short-exposure frames to mitigate the non-linear bandwidth effect in channels 3 and 4. The "stitching" of the two components of the HDR PRF was completed using a 1/r masking algorithm requiring a percentage of each frame to be added together over a small annulus two IRAC pixels in width just outside the saturated area. Each epoch was treated separately and then all three epochs were aligned and a median was taken to remove background stars. 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 PRFs by matching their overlapping area. 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 arcsec wide, between the core (contributing where the extended PRF data were missing due to saturation cutoff), and the extended PRF. The merged extended 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. For these PRFs, the pixel size is 1/5th that of the native IRAC pixel size.
Point Source Fitting PhotometryThe 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. Please refer to the PRF photometry page for more details on the comparison of aperture and PRF-fitting photometry.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 use of 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. The majority of the IRAC wavefront error is a lateral chromatic aberration that is most severe at the corners of the IRAC field. The aberration is due to the difficulty of producing an achromatic design with a doublet lens over the large bandpasses being used. The effect is small, with the total lateral chromatic dispersion less than a pixel in the worst case. The sky coordinates of each pixel have been accurately measured in flight using astrometric observations of an open cluster, resulting in distortion coefficients that are in the world coordinate system of each image. The main effect is that the PRF and distortion may be slightly color-dependent, which may be detectable for sources with extreme color variations across the IRAC bands (please see the IRAC Data Handbook or array location dependent photometric corrections for more details).
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