Spitzer Documentation & Tools

Purpose: Match background levels for an ensemble of IRAC or MIPS BCDs by assigning to each a scalar offset, calculated by a least-squares minimization of the overlap differences between BCDs. This code can handle large datasets and may work where MOPEX Overlap fails.

Author: Don Mizuno (Boston College)
Date Contributed: 25 Mar 2008
System Requirements: IDL v5.4 or higher; astrolib IDL library

afrl_bcd_overlap was written by D. R. Mizuno (Boston College) and is provided "as is" for testing and usage by other observers. This software is not considered part of the pipeline and no installation support from IRSA should be expected. We are however very interested in your feedback in order to improve or replace the algorithm and are glad to work with you in test cases and analysis. Please send your comments to carey{@}ipac.caltech.edu (remove the {} from the email address first).

Information and Download

Download afrl_bcd_overlap.tar (Tar File, 272 KB)

Overlapping BCDs can have differing "DC" levels in the overlapping regions, due to various causes: BCDs containing bright sources may have "droop" error; BCDs in overlapping AORs may have differing Zodiacal levels, or errors in Zodiacal correction; and BCDs at the start of scan legs can have anomalous background levels. The solution is to add a scalar offset to each BCD, to minimize in the least-squares sense the level differences in the overlapping regions.

The MOPEX package from the SSC includes an overlap correction facility, but it has two major shortcomings: its use is practical only for a limited total number of BCDs (at most a few tens of thousands), and it offers only an "exact" overlap solution which is subject to a spurious ramping effect in the resulting offsets. The overlap correction presented here uses sparse-matrix operations, which allow for up to about 10^6 BCDs on current systems, and features an adjustable damping effect in the solution, which corrects local level deviations but generally preserves global background levels, at the cost of some residual overlap level differences. See Mizuno et al. (2008, PASP 120, 1028) for a discussion of the overlap correction procedure.

This routine should work with any IRAC or MIPS BCD data, although it has been extensively used and tested only on MIPS 24 mu data. It has been used with Sun/Unix and Linux systems. It has not been used on PC versions of IDL.