APEX is the inbuilt Astronomical Point source EXtractor for MOPEX. It takes either a list of input images and performs Multiframe point source extraction, or a single image (e.g. a mosaic) and carries out Single Frame point source extraction. The APEX User List pipelines allow users to input their own list of source positions rather than using the Detect module to automatically search for sources in the input frame(s). APEX QA can then be used to create residual mosaics by subtracting the fitted point sources from the input images to check the results of profile fitting. All APEX flows can do both profile fitting and aperture photometry.
6.1.1 APEX Multiframe (apex.pl)
For Multiframe point source extraction, the extraction is carried out simultaneously on a stack of input images to give a single position and flux density estimate for each detected source. Initial point source detection is carried out on the co-added images to ensure the best signal to noise ratio. The typical components of point source detection are non-linear matched filtering and image segmentation. They produce a detection list with the potential point source position estimates. Subsequent point source estimation is performed by Point Response Function fitting (PRF fitting; see §8.6) of the potential point sources from the detection list. The fitting is performed simultaneously in all the input images (hence the term “Multiframe”). Passive deblending, i.e. the separation of detected clusters into individual sources, is done by default. Active deblending based on the quality of the fit is available as an option. The advantage of APEX Multiframe over APEX Single Frame is seen in data with intra-pixel variability (e.g. IRAC data) or data where the PRF varies across the array (IRAC). Aperture photometry is always carried out on the mosaicked image, regardless of whether you run Multiframe or Single Frame.
6.1.2 APEX Single Frame (apex_1frame.pl)
For single-frame point source extraction, the detection and estimation as described above are performed on a single input image (e.g. a mosaic).
6.1.3 APEX User List Multiframe (apex_user_list.pl)
APEX User List allows the user to input lists of object positions instead of using the Detect module to automatically find sources in the field. The code is still experimental. With some limitations discussed below, APEX User List will do PRF fitting and aperture photometry on the input list. With the default APEX User List parameters, the positions of objects will be allowed to move within a small radius as part of the fitting. PRF-fitted and aperture fluxes will be for that (possibly shifted) position. Aperture photometry on fixed positions is available on the command line. Note that APEX User List will not search for additional objects, so if any are missing then proper deblending cannot be done, i.e if an observed source is comprised of two blended sources, you must include two sources at that point in the User List input table, else APEX will attempt to fit the blend as a single source.
In User List mode, the user's input table becomes the detection list. This is set in the APEX User List multiframe/single frame Settings module with the parameter User List Fixed Positions Table File Name. It is also possible to interactively create the user list table by loading an image into MOPEX and clicking on the sources that you wish to extract. See §8.10 for full instructions on how to interactively create a User List table.
Nearby sources for each input source are identified and fitted in the Source Estimate module using the parameter Angular Distance. In User List mode, the parameters Fitting Area X and Fitting Area Y, which let the user define the fitting region, are required. Positions will move to "best-fit" values. Sources that are successfully fit but drift outside Max Shift X, Max Shift Y data pixels from the input position are flagged with the letter “O” (for “outside”) in the deblend column of the output table. Typically, the Select module will include a condition to filter out sources flagged in this way. The format of the output extract table is otherwise the same as the usual APEX output.
For APEX User List Multiframe, the creation of a detection map is required: include the Detection Map module in the pipeline. This creates "detectionmap.tbl" in the output directory. Once it is created, this can be turned off for subsequent runs. Also note that in this case the Input Type in the Source Estimate module should be set to Detection Map. These steps are needed to track the input source positions through the image stack.
In the APEX namelists, both the Detect MedFilter and Gaussnoise should be turned on to provide the ability to use background RMS for the point-source SNR.
The input source positions are sorted first in Dec in ascending order. Bins are created in this sorted list using bin size = 2 * Angular Distance. The sources are again sorted in RA within each bin. Each input source position is then searched to find its distance from the other sources in the current bin and 2 adjacent bins on both sides. If this distance < Angular Distance, then the source is flagged as a nearby source and included in the fitting.
6.1.4 APEX User List Single Frame (apex_user_list_1_frame.pl)
For single frame mode, the point source extraction is carried out as above, only this time it is carried out on the mosaic rather than the stack of BCDs.
6.1.5 Aperture Photometry on Fixed Positions
This can be done on the command line in Single Frame mode on mosaics by using the namelists provided in the cdf directory with "aponly_fixed" in the filename.
6.1.6 Residual Image Creation (apex_qa.pl)
APEX QA creates residual images to test the quality of PRF fitting by subtracting the fitted point sources from the input image(s). Input requirements differ depending on whether you are running Multiframe or Single Frame mode; see §6.6.
