The IDL code

The IDL code

The StarFinder algorithm has been provided with a collection of auxiliary routines for semi-automatic PSF extraction, noise estimation, basic image processing, in order to be able to fully analyze a stellar field and produce an output list of stars.

A widget-based Graphical User Interface has been created, which allows one to use StarFinder without knowing IDL. The main widget which appears on the screen  is nothing more than an interface to call different widget-based applications, in order to perform various operations on the stellar field image. The main widget defines a memory area to store some 'global' variables, whose name and meaning is listed below:

• image: the stellar field
• PSF: image of the Point Spread Function
• noise: array of noise standard deviation for each image pixel
• background: array, with an estimate of the background emission. Available after PSF extraction or stellar field analysis
• detected stars: synthetic image, containing one replica of the PSF for each detected star in field. Available after stars detection
• synthetic field: sum of background and detected stars

IDL users might wish to run the StarFinder routines directly from the IDL command line. Complete documentation on each module can be retrieved with the IDL procedure doc_library.

The more interesting programs to analyze a stellar field are:

• circ_mask: apply a circular mask to a 2D array, for instance the PSF
• click_on_max: select local maxima by mouse click
• compare_lists: find coincidences between two sets of points on a plane
• crosses: mark interesting points in an image with crosses
• display_image: display an image according to a set of options
• fitstars: multi-component PSF-fitting of one or more stars
• fwhm: compute the FWHM of a peak or stellar image
• gaussian_noise_std: compute the standard deviation of normally distributed noise
• halo_smooth: smooth the halo of a stellar image by means of a variable box size median filtering technique
• image_background: estimate the background emission
• image_core: extract the component connected to a specified starting position and above a pre-fixed threshold
• image_model: create a synthetic image given by a sum of shifted scaled replicas of the PSF
• match_coord: given two sets of coordinates on a plane, representing the same points in re-ciprocally translated and rotated reference frames, map one of them onto the other
• psf_extract: estimate the PSF by combination of a set of stars
• repair_saturated: repair saturated stars
• replace_pix: replace known bad pixels by local median
• search_objects: search meaningful maxima
• starfinder: stars detection, astrometry and photometry

Analysis of a stellar field

The analysis of a typical stellar field with the XStarFinder GUI-based application includes the following steps:

• Load the image (see the 'Load' task in the 'Image' menu, indicated briefly as 'Image.Load')
• Estimate the noise ('Image.Noise')
• Estimate the PSF ('PSF.Extract from Image')
• Post-process the retrieved PSF, by support masking, halo smoothing, normalization (see the tasks in the 'PSF' pull-down menu)
• Run the stars detection algorithm ('Astrometry and photometry' button).

When the field is very crowded, the first estimate of the PSF might not be satisfactory. Visual inspection of the retrieved PSF and of the synthetic stellar field model after stars detection (see the 'Display' pull-down menu for this) should enable the user to understand it: abrupt intensity changes at the edge of the PSF support suggest that the analysis should be repeated. In these cases it is possible to repeat automatically the PSF extraction procedure, by means of the 'Repeat extraction' task in the 'PSF' pull-down menu; after PSF refinement and post-processing, as described above, the stars detection algorithm should be run again. When the analysis is iterated this way, the result of the first reduction should be rejected, its only goal being to properly subtract the contaminating sources and the diffuse background emission to improve the PSF estimate. After analyzing different observations (e.g. at different wavelengths) of the same field, it is possible to compare the results and create a color-magnitude diagram with the 'Compare Lists' task. Complete documentation on supported facilities and operations can be found in the on-line help.

A "tutorial" with a complete description on how to use the code is available:
-for PostScript text press here,
-for PDF text press here.

Installation

StarFinder needs no particular installation procedure. StarFinder requires IDL version 5.2 or higher. Only the path of the StarFinder directory must be correctly set, by modifying the value of the IDL system variable !Path. This can be done very easily within the IDLDE environment, by means of the 'Preferences' sub-menu in the 'File' menu. Once the IDL session is open and the path for the StarFinder library is set, it is sufficient to enter the following at the IDL command line:

IDL>xstarfinder

The XStarFinder widget will appear on the screen.

How to obtain the code

StarFinder is a public domain IDL code. StarFinder can be obtained here .

Please send comments, suggestions, and bug reports to Emiliano Diolaiti

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