Source Extraction

imcore performs source detection on the FITS image input_file, optionally using a CASU-style confidence map confidence_map formed from a combination of flatfield images and a bad pixel/columns list. If no confidence map is to be used, confidence_map should be specified as 'noconf'.

Single or multi-extension input FITS images may be processed. A separate source catalogue is generated for each extension of the input FITS image, and stored into a new binary table extension in the output catalogue file. These files have a standard blank (dummy) primary HDU.  Existing catalogue files of the same name are automatically clobbered and overwritten. FITS header information from the image PHU is concatenated with the image extension headers such that each table extension contains all the relevant header information needed for interpretation and calibration.

For each extension processed, a regions file for the DS9 image display tool is optionally produced (these can be loaded from `Load regions' in the `Regions' menu), containing commands to plot an ellipse for each of the detected sources. The files are named by appending _n_cat.ell to the filename of the input image (after removing any .fit extension), where n is the number of the corresponding extension in the output catalogue file (numbered from 1).

Processing proceeds for each 2D image by first estimating an overall grid of background values on a scale size of ~64 pixels (default). The raw background values are derived using an algorithm based on a robust (MAD) iterative k-sigma clipped median. These are then (lightly) filtered to effectively produce a low resolution background image which, using bilinear interpolation, defines a local background for each pixel in the original map. After ``removing'' the varying component of the background a global robust sky level and noise level are estimated using the same robust estimator as before.

Image detection is then based on searching for connected series of contiguous pixels sticking above a user-specified threshold and above a specified minimum size ipix.

Pixels flagged in the confidence map as having zero weight, or having NULL pixel values in the image, are ignored during the background estimation phase. The confidence map is used to fully weight pixels for image detection (based on their predicted noise properties), and thereby automatically compensates for varying noise levels (eg. due to different exposures in stacked dither images) within the image.

imcore_list is the list driven version of imcore. It does essentially the same analysis as the latter, but with the addition of an input list of equatorial positions which tells the program where to put down its apertures.

For more information and general references on the methodology see the image analysis documents on our publications page.

SYNOPSIS

imcore input_file confidence_map output_table ipix threshold

imcore_list input_file confidence_map listfile output_file threshold