IRAC Instrument Handbook - IRSA - California Institute of Technology
IRAC Instrument Handbook - IRSA - California Institute of Technology
IRAC Instrument Handbook - IRSA - California Institute of Technology
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8.1.4 Source Extraction<br />
<strong>IRAC</strong> <strong>Instrument</strong> <strong>Handbook</strong><br />
The Spitzer source extractor, APEX, may be used to fit point sources in <strong>IRAC</strong> data. This s<strong>of</strong>tware can be<br />
run in two modes, in a single frame mode (apex_1frame) which can be run on an individual BCD, CBCD<br />
or a mosaic, and in a multi-frame mode which uses the mosaic to detect sources, but the individual BCDs<br />
or CBCDs to measure their fluxes.<br />
8.1.4.1 Noise Estimation<br />
The accuracy <strong>of</strong> the fluxes from APEX is very sensitive to the noise estimates, as these affect the fitted<br />
background value. For crowded fields, it is essential to include an estimate <strong>of</strong> the confusion noise<br />
(currently not included in the BCD uncertainty image). This can be estimated by measuring the difference<br />
between the actual image RMS and the estimated RMS in the uncertainty image, and then either adding it<br />
to the uncertainty image, or using it as the confusion noise value when generating uncertainty images with<br />
the post-BCD s<strong>of</strong>tware (see above).<br />
8.1.4.2 PRF Estimation<br />
The PRFs released with MOPEX should be fairly good matches to the data and a significant improvement<br />
on the previous versions. We do not recommend using the prf estimate tool to generate PRFs from the<br />
mosaics.<br />
8.1.4.3 Background Estimation<br />
The namelist parameter Background_Fit controls the type <strong>of</strong> background used for PRF fitting. If you give<br />
Background_Fit = 0, a median background is computed for the whole frame. A more accurate background<br />
estimate for PRF fitting, local to the source, can be generated by setting Background_Fit = 1. Note that<br />
the aperture fluxes reported by APEX are always made using the median background, and hence may be<br />
inaccurate for faint sources.<br />
8.1.4.4 Source Extraction<br />
Source detection and extraction are controlled by the parameters Detection max/min area and detection<br />
threshold. APEX will frequently try to split bright sources into several components. This tendency can be<br />
controlled by setting the Max_Number_PS parameter in &SOURCESTIMATE to a low number (2–3).<br />
Two files are output, extract raw.tbl contains all detections, and extract.tbl, which is a subset <strong>of</strong> extract<br />
raw.tbl containing the objects and fields which are selected by select conditions and select columns.<br />
Source extraction from the BCDs or CBCDs (multiframe mode) is recommended for <strong>IRAC</strong> data.<br />
8.1.4.5 Outlier Rejection<br />
By default, APEX will not perform outlier rejection. This can be gotten around by running the mosaicker<br />
with outlier rejection turned on and keeping the intermediate products (delete_intermediate files = 0).<br />
Introduction to Data Analysis 137 Post-BCD Data Processing
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