MOPEX Online Manual

Overlap Pipeline

The main processing stages of the Overlap pipeine are as follows:

Zodiacal Background Subtraction (optional)

The subtraction is triggered by the namelist switch: apply_zodiacal_subtraction = 1 (or on). Default is off. The estimated zodiacal background level is given by the FITS image header keyword ZODY_EST (MJy/sr). The script will also check for the IRAC keyword SKYDRKZB (MJy/sr). This is the estimate of zodiacal level in the IRAC skydark. The value subtracted for IRAC will be ZODY_EST - SKYDRKZB, as described in the IRAC Data Handbook.

Bright Object Masking (optional)

A preprocessing step can be run to mask bright objects that can bias the background levels of the images in which they are present. Two modules are run: Med Filter and Detect (Outlier). The former produces background subtracted images, which are then input into Detect (Outlier). The latter detects bright objects in the background subtracted images and creates a set of mask images, one per input image, in which pixels corresponding to the detected objects (or radhits) are set to positive values. The background-subtracted images created by MedFilter are only used for bright object detection, not for the actual overlap correction.

Interpolation

Before background matching, the input images are interpolated onto the output grid defined by the Fiucial Image Frame (FIF) table. If no table exists, it needs to be created by the FIF module. If bright object masking is selected, masked pixels are not used in the interpolation.

Background Matching

The images interpolated to a common grid can be subtracted pixel-by-pixel in order to match their backgrounds. The metric to be minimized is the combined, uncertainty weighted difference between the overlapping parts of each pair of input images. It is minimized simultaneously with respect to the constant offsets in question. More information on the algorithm used can be found on the Basic Concepts: Background Matching Algorithm page.

NOTE: Background matching does not perform well in cases where data are divided evenly into two background levels. An example of this would be AORs taken in different epochs such that the zodical background has changed. In such cases, background can be brought into closer agreement by subtracting the estimated zodical background. This estimate is written to the IRAC and MIPS FITS headers in the ZODI_EST keyword.

Quicklook Mosaic

Since the interpolated images have already been made, it is only a matter of applying the computed offsets to the interpolated images and coadding them into a quicklook mosaic image. This is a quick-and-dirty way to examine the results of overlap correction. For greater control of the final product, use the Mosaic Pipeline.