MOPEX Online Manual

Why MOPEX?

MOPEX provides a general purpose tool to generate mosaics of FITS images and to perform point source extraction. While both of these tasks are needed for many astronomical datasets, MOPEX addresses several complications specific to Spitzer data:

Geometric Distortion and Fast Projection: Spitzer images have significant geometric distortion, described in the FITS header using the World Coordinate System (WCS). Mosaicking and photometry must accurately interpret this information, which is not yet standard in all packages. Moreover, large Spitzer mosaics include many images, so a fast method is required when performing the projection.




Outlier Rejection: Spitzer data suffer from cosmic rays and other transient artifacts. While some of these effects are mitigated in the BCD pipeline, further outlier rejection is still required while mosaicking. The best algorithm for rejecting these outliers depends upon the observing mode. For example, temporal outlier rejection performs well in the case of highly redundant sky coverage, but it fails in other cases. A combination of temporal and spatial outlier rejection ("dual outlier rejection") has been developed at SSC, together with other options for specific cases.




Profile Fitting: Spitzer data are difficult to extract using profile fiting. Traditional fitting software uses the Point Spread Fucntion (PSF) which describes the image of a point source created at the focal plane by the optics of the telescope. However, detector effects like intrapixel sensitivity for example, are hard to encode in the PSFs used by most packages. Spitzer data are better extracted using the Point Response Function (PRF), which includes detector response, rather than the PSF to solve this problem. Furthermore, MOPEX profile fitting can optionally be performed upon the stack of input images, rather than the final mosaic, in order to preserve the pixel-location-dependent information.

In addition to solving these specific problems, MOPEX provides the standard data analysis functions for mosaicking and source extraction. Mosaicking options include background matching, median filtering, subpixel interpolation (including drizzle), sigma or exposure time weighting, and error propagation. Photometry options include image segmentation (passive and active deblending), aperture photometry, and residual image generation. MOPEX is packaged with a versatile interface allowing either minimal interaction (command-line perl scripts which read input "namelists") or a fully interactive GUI, providing easy access to all parameters, helpfiles, and inspection of intermediate products.