MOPEX Online Manual

Overlap Modules: Mosaic Interpolate

PURPOSE

This module performs a projection of input images onto a 2D plane defined by the FIF table, and an interpolation of the input pixel values to the output array of pixels of the user-defined pixel size. It corrects for the optical distortion in the input images, using the WCS distortion parameters in the input FITS headers. If Bright Object Masking is turned on (mask_bright = 1) then any pixels in the Detection Map with bits 0-14 set are masked out and the masked pixels are not used in the interpolation. The process is intended to, but not restricted to, accept images measuring surface brightness and to yield images in the same units. If the input FITS header does not contain an allowed string specifying units of MJy/sr or microJy/arcsec, then it will assume the input units are counts.

PARAMETER BLOCK

&MOSAICINTIN
 INTERP_METHOD = 3,
 GRID_RATIO = 2,
 INTEG_TIME_KWD = 'EXPTIME',
&END

INPUTS

INTERP_METHOD (int): Four interpolation options are available: 1 (default), 2 (drizzle), 3 (grid) and 4 (bicubic). Each is controlled by a single parameter, which can be set when that method is selected.

1. FINERES (float): Input pixels are interpolated onto a grid of fineres pixels with the size defined as Input_Pixel_Size/FINERES before projecting onto the output grid.

2. DRIZ_FAC (float): The drizzle interpolation algorithm is similar to that used by HST. Each input pixel is shrunk by DRIZ_FAC. The values of the shrunken pixels are equal to the values of the original pixels. The shrunken pixels are then projected onto the output image frame. DRIZ_FAC is greater than 0. The smaller it is, the more times the resampling of the original pixel.

3. GRID_RATIO (int): This method is intended to create a first-look mosaic very quickly using a grid interpolation. Each input pixel is filled with GRID_RATIO squared grid points. Each grid point is assigned the value of the pixel it belongs to. Each grid point is projected onto the output frame and the flux associated with the grid point is added to the output image pixel into which the grid point was projected. The gain in speed being up to 10 times as fast as the other interpolation methods. The price you pay is the fidelity of the interpolated images. Do not use the output of this option for science.

4. ALPHA (float): The parameter of the bicubic interpolation. The default value is -0.5.

OUTPUTS

FIF Output Table (mosaic_fif.tbl): The FIF table describing the final coordinate frame.

Geometry Output Table (interp_bcd_list.txt.tbl): The interpolated images' offsets in x- and y-direction relative to the FIF and their sizes (in integral numbers of pixels) are specified in this file.

Interp Stack (interp_*fits): the output images, interpolated, distortion corrected, and projected onto the ouput FIF. Because of the interpolation, you will notice that the noise of the original BCD image has been smoothed a little bit.

Coverage Stack (interp_*covg.fits): The corresponding coverage map for each output image, taking into account the omission of bad pixels in the input.

DISCUSSION

The module performs projection of input images onto the FIF, including interpolation of the input pixel values to the output array of pixels of the user-defined pixel size. It corrects for the optical distortion in the input images. The process is intended, but not restricted, to accept images measuring surface brightness and to yield images in the same units. If the input FITS header does not contain an allowed string specifying units of MJy/sr or microJy/arcsec, then it will assume the input units are counts.

If the module Detect (Outlier) has been included in the namelist, then it will have been run prior to this module to identify bright sources in the image. All of the detections will have been flagged in the Detection Mask by setting bits 0-14. and you have the option to ignore these pixels by setting mask_bright = 1 in the namelist. Image pixels corresponding to masked pixels in the Detection Mask will be ignored during the interpolation.

In general, a projection onto the reference frame of an input image with optical distortions will not be a simple rectangle. Each interpolated image occupies a part of the FIF and is, in general, of different size, with different offests from the origin of the FIF. The interpolated images' offsets in x- and y- directions relative to the FIF, and their sizes (in intergral numbers of pixels) are given in the file interpolated_images.tbl, and also written in the headers of the interpolated images in the keywords MINTOFFX and MINTOFFY.

When choosing your interpolation scheme, bear in mind that each frame is corrected by a single value for the entire frame. If speed or memory are a factor when running MOPEX on your machine, then you may want to consider a simple interpolation scheme here, and reserve the more complicated interpolation for the Mosaic script.