MOPEX User's Guide - IRSA

MOPEX User’s Guide
different size with different offsets from the origin of the FIF. The offsets of the interpolated
images in x- and y- directions relative to the FIF, and their sizes (in integral numbers of pixels)
are given in the file interpolated_images.txt.tbl, and also written in the headers of the interpolated
images in the keywords MINTOFFX and MINTOFFY.
For maximum speed of operation, a special set of routines have been developed that allow direct
plane-to-plane coordinate transformations that bypass computing the sky coordinates. For details,
see the document “Fast Direct Plane-to-Plane Coordinate Transformations” (Makovoz 2004,
PASP, 116, 971; http://www.journals.uchicago.edu/cgi-bin/resolve?PASP204097).
8.4.2 Default: Pixel Overlap Integration
The value Oj of an output interpolated pixel j is equal to the weighted average of the input pixels
Ii overlapping the output pixel. It is weighted with the relative overlap areas as follows (Eq. 1):
a ji
O j = ∑ Ii, where
i
A j
Here aji is the area of the overlap of output pixel j with input pixe l i.
A j = a ji
Basic Concepts in MOPEX 200
Image Interpolation
∑
i
Equation 8.11
For this scheme (see Figure 8.16), each output pixel is projected onto the input image frame and
the flux from all the input pixels overlapping the output pixels is integrated and then normalized
by the output pixel area to yield the result in the units of surface brightness.
If the Fine Res (FINERES on the command line) keyword is specified in the Mosaic
Interpolate module, an additional bi-linear interpolation of the input pixels is performed before
projecting onto the output grid. A grid of fineres pixels with the size defined as
Input_Pixel_Size/Fine Res is introduced. In Figure 8.17 the input pixels are shown in thick black
lines, the intermediate fineres pixels are shown in thin black lines. The value of a fineres pixe l is a
linear interpolation of the four closest input pixels, with the interpolation coefficients being the
relative overlap area of the pixel, with the same size as the original input pixe ls, and centered
around the fineres pixel. i.e. the value FP of shaded fineres pixel in Figure 8.17 is equal to:
FP = A 1 * IP 1 + A 2 * IP 2 + A 4 * IP 4 + A 5 * IP 5
Equation 8.12