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MIPS : PSF |
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IMPORTANT: This page discusses and includes examples of the MIPS Point Spread Function (PSF), not the Point Response Function (PRF) used by APEX for point source extraction. Please read this document for a detailed explanation of the difference between PSFs and PRFs. For example PRFs, please click here.
Point Spread Function (PSF) Models for MIPS ImagingThe theoretical PSFs below are close to what is seen on-orbit, and should be adequate for most observation planning.MIPS delivers diffraction-limited imaging capability at all three wavelengths due to the matching of the pixel scales to the width of the Spitzer PSF. In the wide field of view mode at 70 microns the pixels are slightly too large to achieve lambda/2D sampling of the PSF, so observers who are particularly interested in getting the highest possible spatial resolution at 70 microns should utilize the narrow field of view ("super resolution") mode. In addition, image quality at the corners of the 24 micron field of view will be slightly degraded.
Knowledge of the appearance of point sources in MIPS imaging data can be very helpful in planning or determining the feasibility of a number of potential observational programs. While some uncertainty should be expected the use of robust modeling techniques can provide the kind of information necessary and has proven to reproduce the actual data very well. Using the Spitzer version of the Tiny Tim Point Spread Function modeling program (developed for the Spitzer Science Center by John Krist; STScI), a number of representative images of point sources have been generated. The modeling code has been developed using Spitzer and MIPS design specifications and measured instrument and telescope parameters. The point spread function (PSF) in the MIPS bands is not expected to be highly sensitive to small changes or differences in telescope and instrument optical parameters. The MIPS PSF images are dominated by the telescope optics, not the internal instrument optics. For a more complete discussion of the MIPS imaging capabilities and image quality, see the MIPS chapter of the SOM. We provide here the model PSF images in FITS image format files that can be displayed and manipulated with most astronomical imaging analysis software (e.g. IRAF). These images can be used to plan and determine the feasibility of observations that might be sensitive to the Spitzer PSF. These PSFs are very close to the measured in-orbit PSFs.
The Theoretical PSF FITS FilesThe naming convention of the FITS image files is mips_lambda_temppos.fits, where lambda is the MIPS band modeled - 24, 70, f70 (for 70 micron fine scale) or 160, temp is the source blackbody temperature and is equal to 50K, 500K, or 5000K, and pos is the x,y pixel number of the source position on the array such that x,y = 1,1 is the upper left corner of the array - c for center, or ll = 118, 10 (lower left), lr = 118, 118 (lower right), ur = 10, 118 (upper right), and ul = 10, 10 (upper left). The off-center positions only apply to the 24 micron band models, and not to the longer wavelength bands. Variations of the PSF over the array in the 70 and 160 micron model images appear to be negligible. All the files have images that are approximately 5 arcminutes in diameter, and they are over-sampled by 4 times. It has been found from modeling HST/NICMOS images using Tiny Tim that creating over-sampled PSFs, and then demagnifying them, works well (display with xmag=0.25 and ymag=0.25 in the IRAF display command parameters achieves this, for example).The model PSF FITS Image files are HERE.
Example 24 micron PSFBelow are examples of the contents of the FITS image files, in this case a 24 micron image at the array center (T = 5000 K). The PSF is strongly centrally peaked with a well defined first Airy ring. For bright astronomical sources with a very high signal-to-noise ratio, the full extent of the PSF might be visible in the MIPS images. The telescope secondary mirror support "spiders" result in the radially extending artifacts in the PSF. Observing programs that might be searching for faint sources closely associated with very bright objects likely need to take these low level PSF image artifacts into account when determining the detectability of such sources.Click on any image to retrieve a larger version.
The model PSF FITS Image files are HERE.
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